ammonia measure total ammonia‐nitrogen (TAN) which Water Quality 77
includes both NH3‐N and NH4+‐N. Decimal fractions of
TAN concentrations occurring as NH3‐N at different of nitrite entering the blood (and thus the amount of
temperature and pH values (Table 4.5) may be used to methaemoglobin formed) depends on the ratio of chlo-
estimate NH3‐N from TAN. ride to nitrite in the water. Either raising the chloride
concentration or lowering the nitrite concentration will
‘Ammonia toxicity’ to fish and other aquatic creatures decrease nitrite uptake by competitive exclusion of ion
is attributed primarily to NH3. As ammonia concentra- uptake. The simplest procedure for counteracting nitrite
tion in water increases, ammonia excretion by aquatic toxicity in fish is to treat water with common salt (NaCl)
organisms diminishes, and levels of ammonia in blood to increase reduce the ratio of chloride to nitrite. A Cl:
and other tissues increase. The result is an elevation in NO2−‐N ratio of 20:1 prevents negative effects of high
blood pH and adverse effects on enzyme‐catalyzed NO2−‐N concentration on channel catfish and seems to
reactions and membrane stability. Ammonia increases have broad application to other nitrite‐sensitive species.
oxygen consumption by tissues, damages gills and Water exchange or replacement also can be effective in
reduces the ability of blood to transport oxygen. Its reducing NO2−‐N concentrations.
toxicity is usually expressed by reduced growth rate
instead of mortality. However, disease susceptibility also Determination of the highest permissible NO2‐N
increases in organisms exposed to sub‐lethal concentra- concentration for pond waters is difficult because of
tions of ammonia. Tolerance to ammonia varies with interactions with chloride and because nitrite toxicity is
species, physiological condition and environmental closely related to DO concentrations. Generally, pond
factors. Lethal concentrations to warmwater fish and managers should be concerned when NO2−‐N concen-
crustaceans for 24–96 hr exposure are between 0.4 and trations exceed 2 or 3 mg/L.
2.0 mg/L NH3‐N.
4.3.8 Hydrogen Sulphide
Risk of ammonia intoxication is difficult to evaluate for Sulphide is an ionisation product of hydrogen sulphide
pond‐grown animals because daily fluctuation in pH and participates in the following equilibria:
causes NH3 concentrations to change continuously.
Although afternoon pH (and, therefore, NH3 levels) may H2S HS H ,K 10 7.01
be high in ponds, it seldom remains at its greatest level
for more than a few hours. When pH declines at night,
NH3 concentrations decrease and ammonia that may
have accumulated in the blood during daylight, when pH HS S2 H , K 10 13.89.
was high, can then be excreted across the gills. The pH regulates the distribution of total sulphide
among its forms (H2S, HS− and S2−). Un‐ionised hydro-
Nitrite may accumulate to concentrations of gen sulphide (H2S) is toxic to aquatic organisms; the
1–10 mg/L NO2−‐N, or more, in water of aquaculture ionic forms have no appreciable toxicity. Analytical pro-
systems under certain conditions. When nitrite is cedures measure total sulphide. The proportions (deci-
absorbed by fish, it oxidises ferrous iron in haemoglobin mal fractions) of total sulphide‐sulphur occurring as
to ferric iron to form methaemoglobin that is not capa- un‐ionised hydrogen sulphide at different pH values at
ble of combining with oxygen. Blood containing sig- 28 °C are presented in Table 4.6. The proportion of
nificant amounts of methaemoglobin is brown, so hydrogen sulphide decreases as the pH increases. These
nitrite poisoning is commonly called ‘brown blood dis- decimal fractions may be multiplied by total sulphide‐
ease.’ Crustaceans contain haemocyanin, a compound sulphur concentrations to give the hydrogen sulphide
similar to haemoglobin but with copper instead of iron. concentration (as sulphur).
Reactions of nitrite with haemocyanin are poorly under-
stood, but nitrite can be toxic to crustaceans. Concentrations of 0.01–0.05 mg/L of hydrogen sulphide
may be lethal to aquatic organisms. Any detectable
Tolerance to nitrite varies greatly. Tilapia, carp, catfish, hydrogen sulphide is considered undesirable. The pres-
and trout and salmon are sensitive because nitrite is con- ence of hydrogen sulphide may be recognised without
centrated into the blood by the same gill anion‐uptake water analysis, for the ‘rotten-egg’ smell of hydrogen
mechanism responsible for uptake of chloride. sulphide is detectable at very low concentration.
Freshwater sunfishes (Centrarchidae), temperate basses
(Moronidae), and perhaps others exclude nitrite from 4.3.9 Total Alkalinity and Total Hardness
the bloodstream and these fish are relatively tolerant. Total alkalinity results primarily from bicarbonate in
Marine fish are also tolerant of nitrite because of the waters for aquaculture, and fish have no direct require-
high chloride concentration in seawater. ment for bicarbonate. There is evidence that crustaceans
Because chloride and nitrite are concentrated by the
same mechanism in nitrite‐sensitive species, the amount
78 Aquaculture
Table 4.6 Decimal fractions (proportions) of total sulphide‐sulphur existing as un‐ionised hydrogen sulfide in freshwater at different pH
values and temperatures. Note: multiply values by 0.9 for seawater.
pH 16 18 20 22 Temperature 26 28 30 32
5.0 0.993 0.992 0.992 0.991 24 0.990 0.989 0.989 0.989
5.5 0.977 0.976 0.974 0.973 0.969 0.967 0.965 0.963
6.0 0.932 0.928 0.923 0.920 0.991 0.908 0.903 0.897 0.891
6.5 0.812 0.802 0.792 0.781 0.971 0.758 0.746 0.734 0.721
7.0 0.577 0.562 0.546 0.530 0.914 0.497 0.482 0.466 0.450
7.5 0.301 0.289 0.275 0.263 0.770 0.238 0.227 0.216 0.206
8.0 0.120 0.114 0.107 0.101 0.514 0.090 0.085 0.080 0.076
8.5 0.041 0.039 0.037 0.034 0.250 0.030 0.029 0.027 0.025
9.0 0.013 0.013 0.012 0.011 0.096 0.010 0.009 0.009 0.008
0.032
Source: Adapted from Boyd and Tucker (2014). 0.010
need bicarbonate when molting, and 50–100 mg/L is con- 4.4 Pond Water‐Quality Management
sidered sufficient for this purpose. Waters with low total
alkalinity are poorly buffered and the availability of carbon Aquaculture water‐quality management usually has one
for photosynthesis is limited. Waters containing 50 mg/L of four goals:
or more total alkalinity have more stable water quality and 1) Correcting problems with the facility’s water supply.
are more productive than waters of lower total alkalinity. 2) Enhancing the productivity of ponds that rely on in‐pond
Waters with total alkalinity more than 200–300 mg/L may
have high pH and low availability of phosphorus. primary productivity to support aquaculture production.
3) Mitigating the negative consequences of using manu-
Groundwater from some aquifers is both supersatu-
rated with carbon dioxide and high in total alkalinity. factured feeds to support aquaculture production.
When such water is exposed to the air, carbon dioxide is 4) Reducing the impact of aquacultural effluents on
lost, and a portion of the bicarbonate is transformed to
carbonate and precipitates as calcium carbonate. This receiving water bodies.
phenomenon is usually not harmful in ponds or other Technologies exist for correcting any water‐quality prob-
large grow‐out units, but precipitation of calcium car- lem associated with source waters, but it is almost always
bonate on eggs or fry of fish or shrimp in hatcheries may too costly to attempt treatment except for small facilities
be harmful. or facilities raising high‐value animals. Problems with
water supplies are best addressed by carefully selecting
In freshwater aquaculture systems, total hardness sites with water of adequate volume and proper quality.
concentration will usually be similar to that of total alka-
linity. The most common aberration involves certain The remainder of this chapter will focus on some
well waters with hardnesses drastically lower than alka- common water‐quality management practices, with
linities. When used in ponds, such water may develop emphasis on enhancing pond productivity and prevent-
very high pH in response to photosynthesis. In brack- ing water‐quality deterioration in ponds with feeding.
ishwater and seawater, total hardness will be many times Emphasis on pond aquaculture is justified because ponds
greater than total alkalinity. This condition does not are far and away the most common aquaculture system
cause problems with culture species. in the world. Water‐quality management in other sys-
tems is briefly discussed in section 4.4.11. Additional
Calcium is a major contributor to total hardness in information on water‐quality management in ponds and
freshwater, and low hardness is an indicator of low other aquaculture systems is found in Boyd and Tucker
calcium concentration. Adequate calcium in water is (1998, 2014) and Tidwell (2012). Treating aquacultural
essential for good hatchability of fish eggs. In seawater, effluents is discussed later, in section 4.5.
magnesium contributes much more to hardness than
does calcium. 4.4.1 Liming of Acidic, Low‐Alkalinity Pond Waters
Ponds built on sulphidic soils, often called acid‐sulphate
Boyd et al. (2016) provide a detailed summary of alkalin- soils, may have waters with pH values so low that culture
ity and hardness and their management in aquaculture.
species will not survive, or if they survive, they will not Water Quality 79
grow well. Such situations may be corrected by heavy Table 4.7 Lime requirements (kg/ha) based on soil pH and soil
liming and management procedures designed to mini- texture.
mise the contact of sulphide‐bearing soil with the air to
prevent its oxidation and the formation of sulphuric acid. Soil Clayey Soil texture Sandy
However, it is best to avoid using sites with potential pH
acid‐sulphate soils for aquaculture. 5,000 Loamy 2,500
<4.5 4,000 2,000
The more common acidity problem occurs in ponds 4.5–5.0 3,000 4,000 1,500
constructed on soils with a high concentration of 5.1–5.5 2,000 3,000 1,000
exchangeable aluminium. Waters in such ponds have pH 5.6–6.0 1,500 2,000
of 5.5–6.5 and alkalinity below 20 mg/L. Fish will grow in 6.1–6.5 1,000 1,500 750
such waters, but natural productivity of fish food organ- 6.6–7.0 1,000 500
isms will be low, and waters will be poorly buffered 7.1–7.5 500 250
against pH change in response to phytoplankton photo- >7.5 0 750
synthesis. Liming materials increase the pH of bottom 375 0
soils and elevate alkalinity and hardness in water. These
changes improve conditions for microbial activity and 0
growth of benthic animals; increase the availability of
carbon dioxide, phosphorus and other nutrients; enhance total alkalinity after 3–4 wk. If the alkalinity is still low,
phytoplankton growth; and improve the survival and more agricultural limestone may be applied.
growth of the aquacultural crop.
Application of agricultural limestone to water nor-
The most common liming materials are agricultural mally increases total alkalinity and hardness by roughly
limestone, burnt lime and hydrated lime. These products equal amounts. The solubility of calcium carbonate in
are made from limestone, which is a relatively soft rock water in equilibrium with normal atmospheric concen-
composed of calcium carbonate (CaCO3 or calcite), trations of carbon dioxide is about 60 mg/L. In ponds,
calcium magnesium carbonate (CaCO3.MgCO3 or dolo- there is usually more carbon dioxide available from
mite) or a mixture of these carbonates. Burnt lime, CaO, organic matter decomposition and more calcium car-
is made by burning limestone in a furnace at high tem- bonate is dissolved. Nevertheless, if pond waters con-
perature. Hydrated lime, Ca(OH)2, is made by treating tain more than 80–100 mg/L alkalinity, agricultural
burnt lime with water. Both burnt lime and hydrated limestone will not usually dissolve.
lime are usually fine white powders.
It is important that agricultural limestone be spread
Agricultural limestone is the safest and least expen- over the entire pond. This can best be done when ponds
sive liming agent to use for routine management of are empty between crops, but limestone may also be
ponds with low alkalinity and acidic bottom soils. spread over the pond surface from a boat. Acidic ponds
Agricultural limestone dissolves slowly and does not will usually need to be limed after every crop if they are
cause rapid changes in water pH. Burnt lime and drained for harvest. Acidic ponds that are not drained
hydrated lime have the same overall effects as agricul- for harvest usually need to be limed at 3‐ to 4‐yr
tural limestone, but initially cause a much higher pH intervals.
and are not normally applied to pond water during the
culture period. These two materials, however, can be 4.4.2 Pond Fertilisation
applied to the bottoms of empty ponds at 2000–3000 kg/ Fertilisers are frequently used in pond aquaculture to
ha to raise pH and kill pathogens and other unwanted stimulate phytoplankton productivity and enhance the
organisms. Hydrated lime reacts with carbon dioxide availability of natural food organisms. Turbidity created
and is sometimes used as a treatment for high carbon by phytoplankton also shades pond bottoms to discour-
dioxide levels. age the growth of underwater aquatic weeds. Ponds may
be fertilised with organic materials such as animal excre-
Ponds with total alkalinity below 50–60 mg/L must be ment, agricultural byproducts, or grass. They may also
limed. The liming rate needed to increase soil pH and be fertilised with chemical fertilisers.
total alkalinity can be estimated by one of several lime
requirement tests which are described elsewhere (Boyd 4.4.2.1 Organic Fertilisers
and Tucker, 2014). An alternative is to measure pond Organic fertilisers, sometimes called ‘manures’ usually
soil pH in a 1:1 mixture of dry pulverised pond soil and consist of relatively low‐quality substances that might
distilled water. The liming rate can be selected based on otherwise be considered a waste product, such as poultry
bottom soil pH from Table 4.7. Another approach is to
apply agricultural limestone at 1,000 kg/ha and check
80 Aquaculture Table 4.8 Approximate grades of common commercial fertilisers.
manure, poultry litter, cowshed manure, plant byprod- Percentage
ucts and wastewater. Organic material added to ponds
provides food for animals in three ways, although in Fertiliser N P2O5 K2O
practice all three contribute to growth of the animal
under culture. Urea 45 0 0
Calcium nitrate 15 0 0
First, POM may be consumed directly by animals. This Sodium nitrate 16 0 0
is usually a minor contribution because most organic Ammonium nitrate 33–35 0 0
fertilisers are nutritionally inadequate, unpalatable or Ammonium sulphate 20–21 0 0
too difficult to ingest to support high food animal pro- Superphosphate 0 18–20 0
duction through direct consumption. Triple superphosphate 0 44–54 0
Monoammonium phosphate 11 48 0
Second, the organic material is partially decomposed Diammonium phosphate 18 48 0
by bacteria and fungi resulting in the production of high‐ Calcium metaphosphate 0 62–64 0
quality detritus consisting of microbial biomass and Potassium nitrate 13 0 44
partially decomposed organic matter. The detritus is Potassium sulphate 0 0 50
consumed by zooplankton and benthic macroinverte- Potassium chloride (muriate of potash) 0 0 60
brates. The detritus, as well as the zooplankton and mac-
roinvertebrate consumers, are then eaten by the animal base of food webs that ultimately culminate in the growth
under culture. of the animal under culture. Key nutrients for aquatic
plant growth are phosphorous and nitrogen. Potassium
The third food pathway originates when inorganic and other nutrients are sometimes important.
nutrients (nitrogen, phosphorus and others) and carbon
dioxide are released as the organic matter decomposes. The two most common fertilisers used worldwide in
These nutrients stimulate plant growth, which then aquaculture are triple superphosphate and urea. Other
serves as the base of complex food webs culminating in common fertilisers used in aquaculture are listed in
growth of the animal under culture. Table 4.8. Most fertilisers are packed in bags and sold as
dry granules or prills (spherical pellets), but ammonium
Organic fertilisers have their maximum benefit in polyphosphate and phosphoric acid are liquids.
developing countries where small‐scale farmers may
have low‐quality materials available from farm livestock Nitrogen is usually added as urea, ammonium or nitrate.
and cannot afford chemical fertilisers. Although it seems Phosphorus occurs as orthophosphate or polyphosphate,
to be an unsophisticated practice, fish production in while potassium is in its ionic form. Fertilisers dissolve in
organically fertilised ponds can be as high as or higher water to release their nutrients. Urea begins to hydrolyze
than that achieved with chemical fertilisers. There are, at once and is completely transformed to ammonia and
however, several disadvantages to manures: carbon dioxide within a few days. Polyphosphate also
●● most organic fertilisers have a low percentage of nutri- quickly hydrolyzes to orthophosphate.
ents and vary in composition; The grade of a fertiliser is usually reported as percent-
●● because of low nutrient quality, large application rates ages of nitrogen (N), phosphorus oxide (P2O5), and potas-
sium oxide (K2O). Thus, triple superphosphate is usually a
are necessary; 0‐46‐0 fertiliser, diammonium phosphate is a 18‐48‐0
●● large applications can cause DO depletion and accu- fertiliser and urea is a 45‐0‐0 fertiliser. Table 4.9 shows the
approximate grades of common commercial fertilisers.
mulation of partially decomposed manure in pond Note that some primary fertiliser sources contain one
bottoms: application rates should not exceed about primary nutrient, such as urea or triple superphosphate,
50 kg/ha of dry matter per day in un‐aerated ponds to while others may contain two primary nutrients such
avoid DO depression by microbial degradation of the as diammonium phosphate and potassium phosphate.
material; A mixed fertiliser is made by blending two or more pri-
●● organic fertilisers encourage growth of algal mats; mary fertiliser sources to provide two or three primary
●● organic fertilisers may contain antibiotics put into ani- nutrients. Mixed fertilisers representing a wide range of
mal feeds, and they have high concentrations of trace grades can be purchased in most countries.
metals; and
●● many consumers have negative attitudes about aqua-
culture products from organically fertilised ponds.
4.4.2.2 Chemical Fertilisers
The concept behind chemical fertilisation is that key
plant nutrients are added to the pond to stimulate plant
growth (primary production) which, in turn, serves as the
Table 4.9 Nitrogen and phosphorus budgets as percentages Water Quality 81
of the original input of the two elements in feed to catfish ponds.
Once blooms have been initiated, fertiliser can be
Pathway Nitrogen Phosphorus applied according to need as judged from the Secchi disk
visibility. A Secchi disk visibility of 25–40 cm is usually
Harvest in fish 31.5 31.0 considered adequate. As an example, where the normal
Ammonia volatilisation 12.5 — application rate is 2 kg N/ha and 8 kg P2O5/ha, the applica-
Denitrification 17.4 — tion rate might be adjusted for Secchi disk (SD) visibility
Sediment accumulation 22.6 57.6 as follows: SD ≤25 cm, 0 kg N and P2O5; SD = 26–30 cm,
Effluent 16.0 11.4 0.5 kg N and 2 kg P2O5/ha; SD = 31–35 cm, 1 kg N and 4 kg
P2O5/ha; SD = 36–40 cm, 1.5 kg N and 6 kg P2O5/ha; SD
Sometimes, fertilisers will be supplemented with the >40 cm, 2 kg N and 8 kg P2O5/ha.
secondary nutrients calcium, magnesium and sulphur.
The usual sources of these elements are calcium and Fertilisers containing ammonium or urea are potentially
magnesium sulphates. Supplements of trace elements, acidic because nitrifying bacteria in water can oxidise
iron, manganese, zinc, copper, boron and others may be ammonia to nitrate and yield hydrogen ion. The hydro-
added to fertilisers. gen ion will react in water to decrease total alkalinity and
pH. The adverse influence of nitrification on alkalinity
Fertiliser granules or prills are water soluble but settle and pH can be counteracted by routine applications of
to the pond bottom before completely dissolving. Much agricultural limestone to ponds where alkalinity is natu-
of the phosphorus may be adsorbed by the bottom soil rally low. In brackishwater, seawater and freshwater with
instead of remaining in the water. This problem can be alkalinity above 50 mg/L, acidity from nitrogen fertilisers
lessened by using liquid fertilisers. is not a problem.
Liquid fertilisers are denser than water and must be 4.4.3 Mechanical Aeration and Mixing
diluted 1:10 with pond water and splashed over pond
surfaces or released into the propeller wash of an out- Mechanical aeration is used widely in pond aquaculture
board motor while a boat is driven over the pond surface. to prevent low DO concentrations, especially at night;
If liquid fertilisers are not available or considered too improve the efficiency of feed use; and increase fish or
expensive, granular or prilled fertilisers may be placed in shrimp production. Mechanical aerators increase the
10 to 20 times their volume of pond water, pre‐dissolved rate of oxygen transfer from air by providing a greater
by vigorous stirring and splashed over the water surface. surface area between air and water. This is achieved by
splashing water into air or by releasing bubbles into
Typical application rates of N and P2O5 are 2–10 kg/ha water.
each per application. Many farmers apply excessive nitro-
gen. We recommend applications of 2 kg N/ha and 8 kg Aerators are most effective when DO concentrations
P2O5/ha for freshwater ponds and 8 kg/ha each for N and in water are low, because the driving force causing oxy-
P2O5 for ponds filled with brackishwater or seawater. gen to enter the water is the difference in pressure
between oxygen in air and oxygen in water. Likewise,
Diammonium phosphate or monoammonium phos- aerators transfer oxygen from water to air when water is
phate are good fertilisers for freshwater, because they supersaturated with DO.
have N:P2O5 ratios of about 1:3 and 1:4, respectively. For
brackishwater or seawater, a fertiliser with a N:P2O5 ratio In addition to transferring oxygen to water, aerators
of 1:1 is best, and a mixed fertiliser with a 20‐20‐0 grade circulate pond water. Water circulation is beneficial in
is a good choice. In brackishwater ponds for shrimp cul- moving oxygenated water away from aerators to prevent
ture, many farmers want to encourage diatom growth. localised saturation and loss of oxygen‐transfer effi-
This can be done by applying nitrogen fertilisers weekly ciency by the aerator. It also prevents ponds from ther-
at 2–3 kg N/ha. Nitrate is especially efficient in promot- mally stratifying. Aerator‐induced water currents
ing diatoms and sodium nitrate can be used as a diatom‐ transport oxygen throughout the pond for use by the cul-
promoting fertiliser. ture species. Circulation creates ripples on the water sur-
face that favor the outward diffusion of ammonia and
Fertiliser may need to be applied at intervals of 2‐ to other toxic gases. It also delivers water containing DO
4‐wk to maintain phytoplankton blooms. However, over the pond bottoms to avoid localised pockets of
more frequent applications may be necessary to initiate anaerobic sediment.
blooms. In ponds with feed applications, nutrients enter
the water from feed wastes and applications of fertilisers In ponds with a high animal biomass, aeration may be
often are unnecessary or only necessary until feeding necessary during daytime to provide circulation even
rates reach 15 or 20 kg/ha per day. though little or no oxygen transfer results. However, the
amount of aeration applied must be reduced during day-
time to minimise oxygen loss to the air and lessen energy
82 Aquaculture reduce risks, a safety factor of 1.5 must be applied, result-
ing in an aeration requirement of about 11 kW/ha in our
costs. At night, aeration must be increased to prevent example.
low DO concentration.
Few studies have examined the best ways of position-
There are many kinds of mechanical aerators. ing aerators in ponds. Fish move to aerated zones, so
Paddlewheel aerators and vertical turbine aerators positioning is less important in fish ponds than in shrimp
(Figures 4.9 and 4.10) are two common types of aerators. ponds. In fact, in channel catfish farming, all aerators are
Diffused‐air aeration systems release bubbles of air near located in one end of the pond, near the source of elec-
pond bottoms to rise through the water column. Another trical power. This practice allows the use of a single
type is the propeller‐aspirator‐pump aerator, which electrical service panel and greatly reduces the amount
releases tiny air bubbles into turbulent water created by a of electrical wire between the panel and aerators.
rapidly rotating impeller.
In shrimp ponds, the long‐standing practice of posi-
The standard aeration efficiency (SAE) is the quantity tioning aerators to create a circular movement of water is
of oxygen an aerator will transfer per kilowatt of power probably as good as any other alternative. To avoid ero-
in 1 hr when standardised to conditions of 20 °C, 0 mg/L sion, aerators must not be mounted in water less than
DO, and clean water. Typical SAE values range from 1 to 0.75‐m deep or near embankments. Water currents also
2 kg oxygen/kW per hr. Oxygen‐transfer rate decreases must not impinge on embankments for the same reason.
as ambient DO concentrations increase above 0 mg/L; Moreover, water currents from one aerator must not
during operation in a pond containing 3–4 mg/L DO, the collide with currents from another.
actual oxygen‐transfer rate will be 40–50% of the SAE.
Oxygen production in photosynthesis occurs in sur-
Experience and calculations based on pond oxygen face waters and if waters are thermally stratified, oxy-
budgets suggest that production can be increased by gen cannot be distributed to bottom waters. Mechanical
about 500 kg for each kW of aeration applied. For mixing can disrupt stratification and possibly reduce
example, suppose 1500 kg/ha can be produced without the need to aerate. Mixing may also benefit water qual-
mechanical aeration. To increase production by 3500 kg/ ity by increasing the volume of inhabitable water and
ha to 5000 kg/ha, the minimum aeration requirement will
be about 7 kW/ha (3500 kg/ha ÷ 500 kg/kW). However, to
Figure 4.9 Paddlewheel aerators in a catfish pond. Source: Reproduced with permission from Danny Oberle, 2017.
Water Quality 83
Figure 4.10 A vertical‐pump (sprayer‐type) aerator. Source: Reproduced with permission from Les Torrans, 2017.
possibly reducing occurrence of noxious bloom of blue‐ because of the large water volume in most ponds. Various
green algae (cyanobacteria). treatments, some based on accelerating natural pro-
cesses, have been proposed but most either do not work
Many mixing devices have been developed for use in or are too expensive (Boyd and Tucker, 2014). Ammonia
aquaculture, even though research has not provided con- can be removed by exchanging water and this is com-
vincing evidence of the benefits of mixing. But energy monly practiced in some pond cultures, although water
costs are rising worldwide, and the possibility for using exchange increases effluent discharge volume and cause
mechanical water mixers to lessen the requirements for downstream pollution.
mechanical aeration appears worthy of a second round
of scientific investigation and farm trials. Potential benefits Because ammonia is difficult to remove from pond
of mixing and description of mixing devices are detailed water, the key to ammonia management in ponds is to
in Boyd and Tucker (2014). minimise the probability of accumulation to toxic levels
by operating within the assimilative capacity of the pond
4.4.4 Ammonia Management ecosystem. If ammonia concentration is routinely high in
Methods of preventing ammonia toxicity vary among fertilised ponds, nitrogen is being supplied in wasteful
culture systems, but the common goal is to maintain excess of requirements for algal growth. In ponds receiv-
feeding rate within the limits of the system’s capacity to ing feed, the risk of ammonia toxicity is reduced by using
remove or detoxify the ammonia produced as a by‐product moderate stocking and feeding rates and using high‐
of feeding. In flow‐through systems and net pens, ammo- quality feeds and good feeding practices to maximise
nia is removed by water flow. In water‐recirculating nitrogen retention by the farmed animals. Other than
aquaculture systems, ammonia is treated by transform- these general rules and using experience to determine
ing it to nitrate in biological nitrification filters. the pond’s assimilative capacity, it is difficult to precisely
define nitrogen loading limits for ponds.
Ammonia is much more difficult to manage in ponds
than in other aquaculture systems because nitrogen Innovative pond culture systems have been developed
transformation rates vary daily and seasonally depending in the last 25 years that allow much higher feeding
on water temperature, sunlight, wind speed and other rates without risk of ammonia toxicity (Tidwell, 2012).
environmental variables that are impossible to control. Partitioned pond systems use water movement to
Also, removing ammonia from pond water is difficult enhance ammonia‐removal processes, and fish produc-
tion can be several times greater than in traditional ponds.
84 Aquaculture degree of supersaturation in near‐surface waters can
become high. Despite the frequent occurrence of DO
To provide even greater waste‐removal capacity, pond supersaturation in surface waters, gas bubble disease is
systems have been developed where oxygen is supplied rare in aquaculture ponds. Most ponds are relatively
entirely by aerators and wastes are removed by bioflocs, deep (>1 m), and deeper waters apparently provide refu-
which are suspended aggregations of algae, bacteria, pro- gia for the farmed animals because light attenuation with
tozoans, fecal matter, and uneaten feed. Intensive turbu- depth reduces oxygen production in photosynthesis (see
lent mixing is key to successful operation of biofloc sections 4.2.8 and 4.4.3) and hydrostatic pressure reduces
systems because flocs must be suspended to function ΔP (section 4.3.6). In most ponds, supersaturation will
properly. Biofloc systems work best for growing shrimp, only threaten eggs or fry restricted to the surface by lack
tilapia, and other animals that can feed directly on the of mobility.
flocs as a supplement to manufactured feed.
Gas bubble disease is not uncommon in intensive
4.4.5 Control of High pH culture systems because commonly used water sup-
High pH occurs in ponds as a result of sustained periods plies (such as ground waters) may be supersaturated
of high rates of photosynthesis. Problems are most com- and water distribution systems present multiple
mon in ponds where total alkalinity far exceeds total opportunities for supersaturation to develop, even if
hardness. the supply is not initially supersaturated. Most culture
tanks and raceways are also shallow, which does not
Several treatments have been used to lower pH (Boyd provide an opportunity for hydrostatic pressure to
and Tucker, 2014). In some cases, water exchange may be compensate for supersaturation. Using deeper culture
used to flush out phytoplankton and this will often lower tanks to avoid high ΔP is seldom a reasonable design
pH. Copper sulphate treatment will kill algae causing a option, so potential problems in the water supply must
decline in carbon dioxide use in photosynthesis and a be addressed by using good well drilling practices
resulting drop in pH. This practice, however, can cause (if ground waters are used) and properly installing the
low DO concentration when dead algae decay. Organic water supply system to minimise opportunities for air
amendments such as manure and molasses have been entrainment.
applied because carbon dioxide released when these
materials decay will depress pH. This treatment can also Gas supersaturation can be corrected by exposing
cause DO depletion. In emergencies, aluminium sul- water to air. The rate of degassing is highest when water
phate may be applied. Aluminium hydrolyzes to release is broken into fine droplets or thin films to increase the
hydrogen in and lower pH. ratio of air volume to water volume. Packed columns
(Figure 4.11) are simple, effective devices to manage gas
Waters with low total hardness and high total alkalin- supersaturation. Packed columns consist of a plastic or
ity concentration, e.g., 3 to 10 times more alkalinity than metal tube filled with a packing material that has a high
hardness may have pH values of 10 or more when photo- void volume (‘Pall rings’ are shown as the packing
synthesis is rapid. Application of calcium sulphate (gyp- material in Fig. 4.11). Water trickles through the col-
sum) to increase calcium concentration can be an umn as a film or in fine droplets. Gas exchange is
effective treatment. It is usually recommended that enhanced if air is forced through the column opposite
enough calcium sulphate be added to bring total hard- the direction of water flow. Packed columns often serve
ness concentration up to that of total alkalinity. The the dual roles of removing supersaturated gases and
treatment rate for gypsum in mg/L may be estimated as: adding oxygen. Colt and Bouck (1984) provide detailed
design information.
C aSO4 2H2O 2 total alkalinity total hardness .
This treatment has a residual effect that varies with the 4.4.7 Clearing Pond Waters of Clay Turbidity
flushing rate of the pond. Muddy ponds often have low‐alkalinity, acidic water.
The application of agricultural limestone and fertiliser to
4.4.6 Managing Gas Supersaturation increase alkalinity, phytoplankton abundance and pH
Photosynthesis by plants or algae can cause DO super- can cause clay particles to settle in some ponds. When
saturation in ponds. Photosynthesis is usually restricted liming and fertilisation are not effective, flocculating
to shallow waters because light is rapidly attenuated with agents may be applied to precipitate suspended clay
depth in aquaculture ponds. Wind or mechanical devices particles and clear water of turbidity. The most common
can mix supersaturated waters with deeper waters and agents are: organic matter, usually hay; calcium sulphate,
prevent supersaturation, but on calm, sunny days the commonly known as gypsum; and aluminium sulphate,
often called alum.
Water Quality 85
Figure 4.11 A packed‐column aerator used to water in off-gas
aerate water and remove supersaturated
gases. Source: Reproduced with permission
from Craig Tucker, 2017.
water
flow
gas packing media
flow water to culture facility
air or
oxygen in
The use of organic matter to treat turbidity is problem- concentration. For example, if the total alkalinity is
atic because of the large quantities needed. The treat- 25 mg/L, the maximum, safe alum treatment is 25 mg/L.
ment rates are usually 2–3 t/ha of hay or 500–1000 kg/ha
of animal manure. Such large applications of organic 4.4.8 Aquatic Weed and Phytoplankton Control
matter can cause depletion of DO and mortality of culture Plants are essential features of most pond aquaculture sys-
animals. tems. They are the base of the food web and they function
with other ecosystem components to maintain adequate
Gypsum treatment increases calcium concentration water quality. For example, oxygen produced in plant pho-
and encourages flocculation of clay particles by neutral- tosynthesis can be a major source of DO in ponds.
ising the negative charges on their surfaces. Gypsum
must be applied at rates of 200–1000 kg/ha to achieve Although plants are desirable in most aquaculture
good turbidity removal. Depending upon the hydraulic ponds, some species interfere with pond management,
retention time, calcium concentration will be elevated endanger the farmed animal, or impair product quality
for months or even years to provide future protection (Figure 4.12). These plants then become weeds, and
against turbidity from suspended clay particles. aquaculturists often try to prevent them from develop-
ing or to eradicate them once established.
Alum dissolves in water to temporarily increase the
concentration of aluminium ions. Aluminium ions are 4.4.8.1 Control of Vascular Plants and Filamentous
trivalent and more effective than divalent calcium ions in Macroalgae
flocculating clay particles. In most water, an application Ponds must be managed to reduce opportunities for
of 25–30 mg/L of alum effectively removes clay turbidity. weed growth rather than relying on herbicides to treat
Some clays are more difficult to flocculate, so it is advis- problems after they develop. The most common
able to conduct simple tests by dosing waters with differ- approach to weed control is fertilising ponds to encour-
ent alum concentrations to determine the effective rate. age phytoplankton growth at the expense of other plant
types. Deepening the edges of ponds also helps by dis-
Alum must be applied on a calm, clear day to prevent couraging the establishment of shoreline emergent or
wind action or rainfall from disturbing the floc. Usually, submersed weeds. Non‐toxic dyes can also be used to
particles of floc become obvious within 10–15 min inhibit the growth of submersed plants by shading the
after treatment, and the pond clears of turbidity within a water. Removing weeds mechanically or by hand may
few hours. reduce the possibility of having to use other control
measures but is possible only in small ponds and is
Because alum forms sulphuric acid in water, it reduces seldom an option in commercial aquaculture.
total alkalinity and lowers pH. Each milligram per liter
of alum consumes about 0.5 mg/L of total alkalinity.
To assure residual total alkalinity and a safe pH, alum
treatment rates must not exceed the total alkalinity in
86 Aquaculture
Figure 4.12 Floating mats of the filamentous macroalgae Pithophora sp. growing in a freshwater fish pond. Mats of algae and dense
stands of submersed plants make it difficult to harvest or feed pond‐grown fish. Source: Reproduced with permission from Craig Tucker,
2017.
Certain fish, most notably grass carp, have been used properly can result in poor weed control; risks to people,
for weed control. Grass carp feed almost exclusively on the aquaculture crop, or wildlife; or herbicide residue
larger plants and when used at low densities (25–50 fish/ problems in the aquaculture product. The label provides
ha) for weed control they do not compete with other fish information on the active ingredient, directions for
for feed. Grass carp are banned or tightly regulated in correct use on target plant species, warnings and use
some countries. Several species of tilapia consume mac- restrictions and safety and antidote information.
rophytic filamentous algae and submersed plants as
parts of their omnivorous diets. However, the tendency With a few exceptions (copper‐based herbicides, for
for tilapia species to overpopulate ponds is a major draw- example), most aquatic herbicides are not very toxic to
back to their use for weed control. Tilapias also do not aquatic animals. However, herbicide use is dangerous
tolerate cold water temperatures and must be restocked because treatment of plant‐infested ponds causes dramatic
each year if weeds persist in temperate ponds. deterioration of water quality. Decay of herbicide‐killed
plants may cause DO depletion and increased levels
Using chemical herbicides is usually the quickest way of dissolved carbon dioxide and ammonia. In many
to eradicate established weed communities. Many chem- instances, only a portion of the pond must be treated
icals have been used to control aquatic weeds but rela- at one time. Supplemental aeration must always be
tively few are legal to use in aquaculture ponds. Lists of available any time herbicides are used.
legal herbicides differ among countries and legal status
changes over time, so we will not discuss individual her- 4.4.8.2 Phytoplankton Bloom Management
bicides. Printed and online resources for aquatic plant Ponds are usually managed to promote phytoplankton
management are abundant. and discourage other plant growth. Phytoplankton can,
however, become a weed problem when community
Effective herbicide use depends on correctly identifying biomass becomes excessive or when certain species
the weed problem, selecting the most cost‐effective produce odors or toxins. Phytoplankton overabundance
chemical, proper application and managing the effects in fertilised ponds indicates that too much fertiliser is
of herbicide use on water quality. Correct weed identi- being applied and the solution is to reduce fertilisation.
fication is critical because herbicides effective on one In pond cultures with feeding, abundant phytoplankton
species may be ineffective on other species. Even if the blooms and noxious blue‐green algae are a natural
proper herbicide is selected, failure to use herbicides
consequence of large amounts of nutrients excreted into Water Quality 87
the water as metabolic waste.
water of greater pH, an effective copper concentration
Algicides are used to manage phytoplankton blooms can be achieved more easily by using chelated copper
for two distinctly different purposes. First, they are used products. The main disadvantage of chelated copper com-
to eliminate undesirable phytoplankton species from pounds is their high cost, which is usually several times
the community. This can be relatively successful when greater than copper sulphate per unit weight of copper.
managing algae‐related off‐flavors, but can be dangerous
when treating toxin‐producing algae. Toxins stored Chelated copper algicides can be mixed with water and
inside algal cells are released when cells are killed and sprayed over pond surfaces. Also, some workers mix
lyse, causing a sudden, and potentially dangerous, chelated copper algicides with water and drain the solu-
increase in waterborne toxin concentrations. The sec- tion from a boat‐mounted tank into the turbulence
ond use of algicides is to reduce overall phytoplankton caused by the propeller of an outboard motor while
biomass. This practice seems logical because excessive driving the boat over the pond surface.
phytoplankton biomass is related to poor water quality,
but using algicides to kill a portion of the phytoplankton Boyd and Tucker (1998, 2014) provide additional infor-
community often causes worse environmental condi- mation on toxin‐producing algae, off‐flavors caused by
tions than existed before use. algae and phytoplankton bloom management.
Copper‐containing products are the most commonly 4.4.9 Mineral Amendments
used algicides in aquaculture. The traditional copper for- Several uses of mineral amendments have already been
mulation used as an algicide is copper sulphate pentahy- discussed. In addition, in fish hatcheries and nursery
drate, CuSO4 · 5H2O, containing 26% copper. Although ponds, it may be necessary to increase salinity for certain
copper sulphate readily dissolves in water, it quickly dis- species. This can be done by applying industrial‐grade,
appears from solution through precipitation and reaction mine‐run salt which is about 96–98% sodium chloride.
with soils. In some countries, brine solution (100–250‰ salinity)
from coastal seawater evaporation ponds may be applied
Copper toxicity to plants and aquatic animals is as an alternative to salt.
strongly affected by other water‐quality variables, most
notably pH, alkalinity and hardness. This makes it diffi- Saline surface and groundwaters found in some inland
cult to formulate safe, effective dosages. In most waters, regions are used to culture marine and brackishwater
those three variables are highly correlated (higher alka- species. These waters often have different proportions of
linity waters have higher pH and hardness than lower major ions than found in seawater. In particular, low con-
alkalinity waters) and general guidelines have been centrations of potassium and magnesium have been
developed based on total alkalinity. One such guideline is associated with poor survival and growth of some shrimp
to multiply total alkalinity (expressed as CaCO3) by 0.01 and fish species. The specific requirements of marine
to determine the algicidal dosage for CuSO4 · 5H2O. For and brackishwater species for major ions are not known,
example, if the total alkalinity of 135 mg/L as CaCO3, the but several studies have shown that increasing potas-
treatment rate would be 1.35 mg/L CuSO4 · 5H2O (or sium concentration and sometimes magnesium concen-
0.35 mg/L as Cu). Using copper sulphate in waters with tration allowed normal growth of these species. Minerals
low pH (<7.5) and low alkalinity (<50 mg/L) may be dan- such as fertiliser grade potassium chloride (muriate of
gerous even at the rate calculated using this formula. potash), potassium magnesium sulphate (Kmag®), and
magnesium sulphate (Epsom salt) have been applied to
Copper sulphate can be applied by several methods. The ponds to correct ionic imbalances.
most common are broadcasting crystals over pond sur-
faces, dissolving the chemical in water and spraying the 4.4.10 Bacterial and Enzyme Products
solution over the pond, and suspending copper sulphate in Aquaculture production is ultimately limited by the
porous bags where aerator currents can disperse the chemi- capacity of the pond microbial community to treat
cal as it dissolves. Copper sulphate can also be used to treat wastes produced during culture. To intensify aquacul-
mat‐forming algae around pond edges. The usual method is ture production beyond that achievable in static‐water
to broadcast copper sulphate crystals over algal mats. ponds, the production system must be designed or man-
aged to increase waste‐treatment capacity. One approach
Copper ion can be chelated with organic compounds to aquaculture intensification is to export wastes pro-
such as triethanolamine or ethylene diaminetetraacetic duced during culture and treat them in separate units of
acid. Chelated copper is toxic to phytoplankton, but it is the overall culture system (as in recirculating aquacul-
not nearly as toxic as cupric ion to aquatic animals. Thus, ture systems) or by discharging wastes to public waters
chelated copper products for phytoplankton control can (as in raceways and net‐pen culture).
be used in acidic, low‐alkalinity waters with less fear of
harming the culture species. Moreover, in high alkalinity
88 Aquaculture to control. Water‐quality management in cages, raceways
and recirculating systems is much simpler, although the
An alternative approach to intensifying aquaculture engineering technology used to manage the system may
production is to increase the capacity of the pond’s be sophisticated. Design and management of these sys-
microbial community to remove or transform metabolic tems are presented in Tidwell (2012).
wastes. This is routinely accomplished, for example, by
using aeration to provide more oxygen, which stimulates Cages are enclosures used to hold and grow fish
microbial activity (this general approach to environmen- (Figures 4.13 and 3.6). They may be located along a shore
tal remediation is ‘biostimulation’). or pier, or anchored and floating offshore, either in fresh
or salt water. Environmental conditions inside cages (and
Another approach is to supplement naturally occur- therefore potential fish yield) depend on the quality of
ring microorganisms by adding microorganisms grown the surrounding water and the rate of water flow through
in mass laboratory cultures or by adding enzyme prepa- the cage. Water flow is induced by tides, surface cur-
rations derived from laboratory cultures. This general rents, and other natural water movements that provide a
approach is called bioaugmentation. continual supply of high‐quality water and sweep away
wastes produced during culture. The only control the
Many microbial products are aggressively marketed culturist has over water quality is selecting good sites
for aquaculture use. Specific claims for bacterial supple- based on long‐term records of water quality, climate,
ments include improved water quality, reduced algal current velocities and other conditions.
growth, enhanced sediment conditions and improved
aquaculture production. Enzymes are organic catalysts Flow‐through systems consist of raceways, tanks, or
that can accelerate chemical reactions but are not small ponds with water passing once through the cul-
expended in the process. Enzyme preparations used as ture unit (Figure 4.14). Water entering the culture unit
water‐quality enhancers are usually extracts from provides DO, and water leaving carries away waste
homogenates of yeast cells, and they function extracel- products. Water may be reconditioned and reused as it
lularly. Proponents of these products argue that adding passes through a series of culture units, but unlike recir-
enzymes to ponds increases the activity of extracellular culating aquaculture systems, water is not reconditioned
enzymes and favors greater rates of waste assimilation by and passed through the same unit more than once. Fish
microbes. production is a function of flow rate because water flow
determines the amount of DO available to meet fish
Wastes in aquaculture ponds consist of uneaten feed, metabolic demands and the rate at which wastes are
faeces, and remains of plankton—ordinary organic mat- diluted. For trout and salmon aquaculture, which is the
ter that readily decomposes. Decomposer organisms most common type of flow‐through aquaculture, rela-
respond to substrate (nutrient source) and if environ- tionships between production and water flow are refined
mental conditions are conducive, their abundance to the point where facility design and production deci-
increases rapidly until substrate is depleted. Because the sions are easily calculated.
abundance and variety of microorganisms respond to
substrate, and microorganisms capable of assimilating Production in flow‐through systems can be increased
aquacultural wastes are always present, it seems unlikely by providing supplemental oxygen, either by aeration
that lack of bacteria impairs sediment and water quality. (exposing water to air) or oxygenation (exposing water to
This disappointing conclusion has been verified by pure oxygen). Oxygen can be supplied directly to the cul-
numerous replicated pond studies that have failed to ture unit or to water flowing between culture units when
show significant improvements in sediment and water units are arranged in a series. Aeration in serial raceways
quality in ponds or laboratory systems treated with these
products. is usually accomplished by gravity fall over weirs between
individual raceways. Production cannot be increased
A few studies have shown that microbial or enzyme
amendments can increase fish and shrimp survival. The infinitely by providing supplemental oxygen because
mechanism by which these products enhance survival water‐quality variables other than DO, such as accumu-
is puzzling, but it is not related to water or sediment lation of ammonia or dissolved carbon dioxide, eventu-
quality improvements. ally affect fish performance.
4.4.11 Water‐Quality Management in Cages, Recirculating aquaculture systems (RAS) confine fish
Raceways and Recirculating Systems
in tanks at very high densities. Oxygenated water flows
Pond water quality and its management are complex into the tank and water containing fish wastes flows out.
because all biological, chemical and physical processes A large portion of the water flowing from the culture
are affected by external factors such as sunlight, water tank is recycled back to the tank after DO is added and
temperature, wind, rainfall and others. These factors vary
hourly, daily and seasonally, and are difficult or impossible waste products are removed.
Rapid fish growth is obtained by providing high‐
quality feeds and maintaining optimum environmental
Water Quality 89
Figure 4.13 Cage culture of European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) near Marseille, France. Source:
Reproduced with permission from John Hargreaves, 2017.
conditions for growth and health. Water temperatures pollution to receiving waters. Fertilisers and feeds are
are controlled throughout the year (RAS are usually applied in semi‐intensive and intensive culture, and
indoors), and rates of oxygen use and waste production effluents from these systems have a greater potential for
can be estimated with good accuracy. These rates can be causing water pollution.
used to precisely design systems that incorporate a series
of unit processes, each engineered to perform a certain Feed is the main source of nutrients in aquaculture
task (Fig. 4.15). Particulate solids are removed by filters, discharges. The fate of the nutrients and other wastes
toxic dissolved nitrogen compounds are removed by depends upon the type of culture system. In static ponds,
bacterial nitrification in biological filters, carbon diox- water remains in ponds for long periods and natural pro-
ide is removed by gas strippers, and oxygen is added cesses degrade organic matter, nutrients are sequestered
by aerators or oxygenators. Processes to control pH in sediment, ammonia diffuses into the air, denitrifica-
and alkalinity, remove nitrate and dissolved organic tion returns nitrogen to the air, and suspended solids
matter, and disinfect the water may also be required settle out. A portion of the wastes enter natural waters
depending on production goals and the species farmed. when overflow occurs following rainfall or when ponds
In most facilities, monitoring and control devices over- are drained for harvest. Nevertheless, static ponds tend
see important processes so that environmental condi- to serve as sediment traps, and a relative small percent-
tions never deviate from acceptable ranges. age of the carbon, nitrogen and phosphorus applied in
feed is discharged. For example, the discharge from
4.5 Effluents channel catfish ponds contained about 3.1% carbon,
28.5% nitrogen and 7.0% phosphorus applied in feed
Discharge from extensive aquaculture facilities are low in (Boyd and Tucker, 1998).
concentrations of nutrients and organic matter. Thus,
they are usually not considered a significant source of In ponds with water exchange, the hydraulic retention
time is less, and natural processes have less time to remove
suspended solids, organic matter, and nutrients resulting
from aquaculture. For example, a typical watershed pond
90 Aquaculture
Figure 4.14 A large flow‐through aquaculture facility used to grow rainbow trout (Oncorhynchus mykiss) in the Snake River Canyon, Idaho, USA.
The facility is supplied with artesian springwater with a year‐round temperature of 15 °C flowing freely from several springs in the canyon wall.
Water is diverted through the raceways and is discharged into the Snake River in the foreground. Source: Reproduced with permission from
University of Idaho Aquaculture Research Institute, 2017.
filtration to remove solids in the southeastern United States will usually discharge
two or three times its volume during the winter and early
fish tank nitrification to remove ammonia spring, and discharge little or no water during the rest of
degassing to remove carbon dioxide the year. Most of the feed is applied during warm months
when discharge is minimal. However, in a coastal shrimp
aeration to add oxygen pond in Central America, water may be exchanged at a
rate of 5–10% of pond volume per day flushing ponds
sterilisation to destroy pathogens completely within 10–20 days.
Figure 4.15 Processes used to recondition water in recirculating
aquaculture systems. Reproduced with permission from Craig In flow‐through culture systems, culture units are
Tucker, 2017. flushed many times per day. In trout raceways, it is not
uncommon for raceway units to be flushed three times
per hour. It is possible to remove uneaten feed and faeces
that settle in ‘quiescent zones’ at the ends of raceways,
but suspended solids and nutrients pass through the
system into natural waters.
In cage culture, all of the feed ingredients not con- Water Quality 91
verted to fish biomass and removed at harvest enter
the water body into which cages are installed. Some of water‐quality deterioration resulting from the use of
the solid waste is heavy enough to settle to the bottom manufactured feeds and to improve water quality
beneath the cages while some of the lighter solids before culture water is discharged to the outside
remain suspended in the water. Fish excrete ammonia, environment.
carbon dioxide and other wastes that enter the water as ●● Source water may be of impaired quality because of
soluble nutrients. low pH, high turbidity or pollution. Turbidity can be
controlled by eliminating its source on the watershed
There are two types of water re‐use systems. or by the use of a settling basin to clarify water before
Outdoor systems recycle water from grow‐out units use. Sources of pollution usually cannot be controlled
into treatment ponds and the water is eventually by aquaculturists, and highly polluted sites must be
returned to the culture units. These systems will dis- avoided.
charge when heavy rainfall occurs. Indoor systems ●● Low pH can be corrected by liming in ponds or other
rely on waste‐treatment devices such as settling culture units. Fertilisers are used to increase natural
basins, mechanical filters, foam fractionators, biologi- productivity in pond aquaculture.
cal filters, etc. However, they will discharge to the out- ●● Manufactured feeds are used to promote high pro-
side when basins and f ilters must be cleaned and when duction of fish and shrimp. Feeding wastes impair
freshwater must be added to dilute TDS (salinity) con- water quality, but mechanical aeration can be
centration that increases over time. applied to increase the availability of DO and
allow greater production. There is no method of
In recent years, there has been increasing concern reducing inputs of ammonia‐nitrogen to ponds
about negative impacts of aquaculture effluents on aside from lowering fertilisation rates or stocking
natural water bodies into which they are discharged. and feeding rates. However, the natural processes
Many governments have developed regulations to of nitrification, uptake by microorganisms and vol-
limit the volume and improve the quality of aquacul- atilisation usually prevent excessive concentrations
ture effluents. Limits for concentrations of selected of ammonia.
water‐quality variables and discharge volume have ●● Nitrite toxicity may be counteracted in freshwater
been established for aquaculture effluents in some ponds by application of chloride ion. Turbidity from
nations. Producers comply with these limits in order suspended soil particles in culture ponds can best be
to discharge effluents. In other nations, best manage- removed through treatment with liming materials,
ment practices (BMPs) are mandated. These practices calcium sulphate or aluminium sulphate.
usually include methods for reducing discharge and ●● Aquatic plants are a critical component of pond eco-
for assuring better conversion of feed nutrients to systems but some plants interfere with pond manage-
biomass of culture species (Tucker and Hargreaves, ment, endanger farmed animals or impair product
2008). Many aquaculture associations have developed quality. The goal of aquatic plant management is to
BMPs for voluntary adoption by their members and encourage growth of certain phytoplankton at the
growing numbers of consumers in developed coun- expense of noxious plants or harmful algal blooms.
tries are seeking aquaculture products resulting ●● Water quality in cages and raceways is controlled by
from environmentally responsible culture methods. water flow through the system to provide oxygen-
Several organisations such as the Global Aquaculture ated water and flush away wastes. Water quality in
Alliance, the Aquaculture Stewardship Council, cages is managed primarily by selecting good sites.
GLOBAL G.A.P., Friends of the Sea, and some gov- Production in raceways is directly related to water
ernment agencies have developed eco‐label certifica- flow but can be increased to a point by using aera-
tion programs for several aquaculture species (Boyd tion to provide more oxygen. RAS have high produc-
and McNevin, 2015). tion potential because water quality is maintained at
optimum levels using a series of discrete engineering
4.6 Summary processes to recondition water and return it to the
culture unit.
●● Water quality is a critical consideration in aquacul- ●● Effluents from aquaculture facilities can cause pollu-
ture. Water quality in aquaculture is managed to cor- tion of water bodies into which they are discharged.
rect problems with the facility’s water supply, to Thus, practices must be implemented at aquaculture
enhance the productivity of pond systems, to mitigate facilities to reduce the volume and improve the quality
of effluents.
92 Aquaculture aquaculture and fisheries. North American Journal of
Aquaculture, 73, 403–408.
References Colt, J. (2012). Dissolved Gas Concentration in Water.
Elsevier, Amsterdam.
Boyd, C. E. (2015). Water Quality, an Introduction, 2nd Colt, J. and Bouck, G. (1984). Design of packed
edition. Kluwer Academic Publishers, Boston. columns for degassing. Aquacultural Engineering, 3,
251–273.
Boyd, C. E. and McNevin, A. A. (2014). Aquaculture, Resource Tidwell, J. H., ed. (2012). Aquaculture Production Systems.
Use, and the Environment. Wiley Blackwell, Hoboken. Wiley Blackwell, Ames, IA.
Tomasso, J. R. (1996). Environmental requirements of
Boyd, C. E. and Tucker, C. S. (1998). Pond Aquaculture aquaculture animals—a conceptual summary.
Water Quality Management. Kluwer Academic World Aquaculture, 27(2), 27–31.
Publishers, Boston. Tucker, C.S. and Hargreaves, J. A. (Eds.) (2008).
Environmental Best Management Practices for
Boyd, C. E. and Tucker, C. S. (2014). Handbook for Aquaculture Aquaculture. Blackwell, Ames, IA.
Water Quality. Craftmaster Printers, Auburn, AL.
Boyd, C. E., Tucker, C. S. and Somridhivej, B. (2016).
Alkalinity and hardness: critical but elusive concepts in
aquaculture. Journal of the World Aquaculture
Association, 47, 6–40.
Boyd, C. E., Tucker, C. S. and Viriyatum, R. (2011).
Interpretation of pH, acidity and alkalinity in
93
5
Resource Use and the Environment
Claude E. Boyd, Aaron A. McNevin and Craig S. Tucker
CHAPTER MENU 5.7 Chemicals, 105
5.1 Introduction, 93 5.8 Water Pollution, 107
5.2 An Overview of Resource Use and Environmental Issues, 94 5.9 Best Management Practices, 109
5.3 Land Use, 97 5.10 Environmental Advocacy in Aquaculture, 111
5.4 Water Use, 99 5.11 Summary, 112
5.5 Energy Use, 101 References, 112
5.6 Feed‐Fish Use, 102
5.1 Introduction planet’s surface necessary to supply the combined demand
of all goods and services consumed by humanity and to
The human population has rapidly increased since the assimilate the resulting wastes. According to the World
beginning of the Industrial Revolution (ca. 1750), and Wildlife Fund Living Planet Report 2012, the ecological
currently stands at about 7.4 billion. The United Nations footprint of humanity has become so great that 1.5 planets
Department of Economic and Social Affairs estimated a would be necessary to sustainably supply the current
global population of 9.7 billion by 2050 and 11.2 billion by demand for resources and ecological services. Obviously,
2100 — increases above the 2015 population of 31% and this state of the environment cannot continue without dis-
51%, respectively. The current population requires huge astrous effects on both ecosystems and humans.
amounts of natural resources, and resource acquisition
alters land use and degrades ecosystems. The demand for The world food system contributes significantly to
resources and resulting negative environmental impacts depletion of natural resources and is the greatest threat
will increase in the future if humans continue to use to the degradation of our ecosystems. Modern food pro-
resources anywhere near the current rate. Aggravating duction has increased food supply at a slightly greater
the effects of population growth, societies of many coun- rate than the increase in human population since the
tries are becoming more affluent and this is expected to 1950s. But, the ability to increase the food supply has a
increase resource use and associated impacts per capita. cost: much more water, nutrients and agrochemicals, but
only slightly more land, are required than in the 1950s.
The world’s ecosystems must supply the goods and ser- The tremendous use of resources for food production is
vices demanded by the population, and assimilate the well illustrated by the estimate by the Food and
resulting waste and other negative impacts. This depletes Agriculture Organization (FAO) of the United Nations
non‐renewable resources, results in some renewable that 32% of global end‐use energy consumption is by the
resources being used faster than they can be regenerated, world food‐production system.
and causes changes in land‐ and water‐use patterns that
lessen biodiversity. Waste loads often exceed the assimi- Fisheries products provide about 6.5% of total protein
lation capacity of ecosystems causing serious changes in and 16.5% of animal protein for human consumption.
ecosystem structure and function (also lessening biodi- Aquaculture is responsible for roughly half of global pro-
versity). The ability of the world’s ecosystems to supply duction of fisheries products for human consumption
services to mankind obviously has limits. The ecological (Figure 1.5). Annual production from capture fisheries is
footprint of humanity is a measure of the area of the not expected to increase in the future, and aquaculture
must supply the future increased demand for fisheries
Aquaculture: Farming Aquatic Animals and Plants, Third Edition. Edited by John S. Lucas, Paul C. Southgate and Craig S. Tucker.
© 2019 John Wiley & Sons Ltd. Published 2019 by John Wiley & Sons Ltd.
94 Aquaculture 5.2 An Overview of Resource Use
and Environmental Issues
products (section 1.3). Aquaculture is a small but signifi-
cant and growing part of the world food system, but it is a World aquaculture production comprises four major
major component of global fisheries production. Because components:
aquaculture is a small sector of the world food system, it ●● inland aquaculture of fish in ponds;
does not use a large share of most resources — the excep- ●● inland and marine culture of fish in cages;
tion being fishmeal and fish oil for inclusion in aquacul- ●● crustacean culture in coastal ponds; and
ture feeds (Boyd & McNevin, 2015). ●● marine culture of bivalve molluscs and seaweed.
The nature and significance of environmental impacts
Simply producing enough food for the future is a daunt- vary widely among the different types of aquaculture.
ing task, but to do so without depleting non‐renewable
resources, reducing the regeneration rate of renewable Bivalve mollusc and seaweed culture do not require
resources and causing ecological disaster is an even land, freshwater, nutrient inputs in fertilizers and feeds,
greater challenge (Figure 5.1). But the challenge must be or large amounts of other resources. Seaweeds and
met; otherwise, the future of the human race is in jeop- bivalves are suspended in the water or laid on the bottom
ardy. Heroic effort should be devoted to improving to obtain their nutrients from the surrounding environ-
resource use efficiency and lessening the environmental ment (Figures 5.2 and 5.3; also see Chapters 15, 24 and
impact of global food production. Small sectors such as 25). Seaweeds remove nutrients from the water, and
aquaculture must be included in the effort, because lessen the possibility of dense phytoplankton blooms in
excessive resource use and associated negative effects on eutrophic coastal habitats. Bivalves filter plankton and
global ecosystems result from the cumulative influence
of all human endeavours — large or small — on natural
resources and the environment.
Figure 5.1 Europe at night as seen from the International Space Station. This image shows the amazing extent of human development
(and energy use) in the region. Providing adequate food for the world’s growing population without depleting resources is humankind’s
greatest challenge. Source: Photograph courtesy of the National Aeronautics and Space Administration (NASA).
Figure 5.2 Intertidal oyster farm with oysters grown in mesh baskets at Medoc, southern France. Source: Photograph by Mike Peel
(www.mikepeel.net). Reproduced under the terms of the Creative Commons Attribution License, CC‐BY‐SA 4.0.
Figure 5.3 Intertidal culture of red seaweed (Eucheuma sp.) at Jambiana, Zanzibar, Tanzania. Source: © FAO Aquaculture photo library/
S. Venturini.
96 Aquaculture Table 5.1 List of negative impacts on biodiversity resulting
from aquaculture.
other particulate matter from the water, also reducing the
risk of excessive plankton blooms in coastal waters. ●● Introduction of non‐native aquaculture species
Although bivalve and seaweed culture may have localised that may become endemic and damaging to aquatic
negative impacts (such as deposition of faecal material ecosystems,
near shellfish farms), these cultures are considered— at
worst — environmentally benign and usually provide val- ●● Eutrophication and other water pollution effects and
uable ecosystem and economic services (Boyd et al., 2005; associated changes in flora and fauna in water receiving
Shumway, 2011). This is an important point because effluents from aquaculture facilities,
nearly half of global aquaculture production consists of
farmed seaweeds and shellfish (Figure 1.17) and these ●● Conversion of mangroves, wetlands, and other areas of high
activities have, on balance, positive impacts. biodiversity to ponds,
Freshwater pond aquaculture requires land and water, ●● Excessive freshwater use,
and fertilisers and feeds must be applied to increase pro- ●● Exploitation of natural fish stocks for fishmeal and oil to use
duction above natural productivity. Aquaculture in
coastal ponds typically does not use freshwater, but it in feeds or live food for culture species,
requires land, fertilisers and feeds. Cages and raceways ●● Transfer of disease and parasites from farmed animals to wild
require little or no land at the culture site, but feeds are
applied. Fertilisers and feeds are only partially converted stock,
to harvestable biomass, and nutrients and organic wastes ●● Genetic alteration of wild stock resulting from escaped farm
are discharged into natural water bodies. Freshwater
used in raceways can be from diverted streams or rivers stock,
or from groundwater aquifers whereas the water neces- ●● Destruction of birds and other predators that feed on farm stock,
sary for cage culture is the natural water bodies in which ●● Effects of antibiotics and hormones used in aquaculture
cages are placed.
facilities on the flora and fauna of water bodies receiving farm
The upshot is that aquaculture — like other types of effluents,
food production — uses land, water, nutrients, energy ●● Carbon emissions resulting from energy use and contributing
and other resources. The production and use of these to climate change,
resources result in conflicts over land and water use; pol- ●● Disruption of the continuity of natural habitat by farms,
lution of natural water bodies with nutrients, organic e.g., loss of buffer areas and corridors for animal
matter, suspended solids, agrochemicals, and antibiotics; movements.
introduction of exotic species; and effects of predator
control on wildlife. Excessive harvest of pelagic, oceanic Source: Modified from Diana (2009).
fishes to make fishmeal and oil can negatively affect
marine food chains. Before moving on to discussions of impacts, it is
important to point out that the negative effects of
An overview of the possible negative environmental aquaculture must be assessed in proper context.
impacts of aquaculture on biodiversity was provided Environmental impacts of aquaculture, or of any activ-
by Diana (2009). Some additional impacts associated ity, are difficult to objectively evaluate. There is little
with resource use are provided along with Diana’s list agreement on the relative importance of various envi-
of aquaculture‐biodiversity issues (Table 5.1). Boyd and ronmental effects, largely because they vary so greatly
McNevin (2015) followed the impacts mentioned in among different types of aquaculture and from one
Table 5.1 in assessing resource use and negative environ- facility location to another. Impacts are usually assessed
mental impacts of aquaculture. Their conclusion was in the context of current societal standards, and are
that aquaculture (exclusive of shellfish and seaweed often subjective and involve trade‐offs among values.
aquaculture) was the major consumer globally of fish- As a broad example of trade‐offs that must be consid-
meal and fish oil, and that it often contributed to nega- ered; impacts viewed as negative must be weighed
tive impacts on biodiversity at the local level. The next against the benefits of aquaculture. First and foremost,
five sections of this chapter focus on the effects of marine aquaculture is necessary if the world’s population wants
shrimp and fish aquaculture in five important areas: land to eat seafood. But further, aquaculture can provide a
use, water use, energy use, feed‐fish use, chemical use dependable source of safe, high‐quality protein that can
and water pollution. The focus on fish and marine shrimp be produced efficiently, often with less overall negative
is intentional because, as mentioned previously, impacts impact than protein obtained from corresponding cap-
of seaweed and bivalve aquaculture are generally posi- ture fisheries or from terrestrial animal agriculture.
tive. The impacts of bivalve aquaculture are thoroughly Aquaculture and ancillary activities — such as process-
summarised by Shumway (2011). ing, distribution, facility construction and equipment
manufacturing — provide employment for more than
100 million people worldwide and international trade
of aquaculture products is a crucial part of the econo-
mies of many countries.
5.3 Land Use Resource Use and the Environment 97
The world’s continental area of 130 million km2 consists aquaculture feeds were used in 2014. An additional area
of inland water (3.5%), forest and woodland (31%), agri- of about 112 000 km2 of agricultural land was required to
culture (37.8%), and urban area (3%). The remainder is in produce plant meals for aquaculture feeds in 2014. The
miscellaneous uses, or it is unsuitable for human use. total area devoted to aquaculture currently should be
Aquaculture facilities occupy a small proportion of the around 322 000 km2. Of course, a small additional
land surface, and the largest use of land in aquaculture is amount of land is used for support area for servicing
for ponds. Land use at the farm level for ponds includes cage culture and for installing raceways. We believe that
the water surface area plus the surrounding support area this amount of land is less than 1000 km2 and negligible
(Figure 5.4). A factor of 1.5 times the production water considering the large potential for error in the calcula-
surface area — upon which production is typically tions made above. The land area for aquaculture is
reported — is about average for aquaculture land use at around 0.66% of the total agricultural area of 49 000 000
the farm level (Jescovitch et al., 2014). Based on extrapo- km2 and 0.25% of the world land area.
lation of the estimated global water surface area of
110 000 km2 in 2005 (Verdegem & Bosma, 2009), the Aquaculture is thus a minor contributor to overall land
total amount of land devoted to pond aquaculture at use, but effects may be significant at the local level.
farm level is at present around 210 000 km2 (140 000 km2 Aquaculture projects are often sited on flood plains of
of ponds and 70 000 km2 of additional land). Average rivers, lower ends of catchments, and in wetland areas
land use for plant meals, e.g., soybean, cereal grains, in (Figure 5.5) that tend to have greater biodiversity than
feed averages about 0.274 ha/t of feed, and 41 million t of many other types of land cover. There has been particular
concern about siting aquaculture farms in coastal man-
grove areas. Although shrimp and milkfish ponds have
been placed in mangrove areas, the overall destruction of
Figure 5.4 Aquaculture farm showing water surface area (some ponds in foreground are empty) plus support area. Source: Reproduced
with permission from Aaron McNevin, 2017.
98 Aquaculture
Figure 5.5 Aquaculture farm in a low‐lying coastal area. Source: Reproduced with permission from Aaron McNevin, 2017.
mangrove habitat by aquaculture farms was less than the production area is expected to be the source of
10% of the global mangrove area in the 1980s (Boyd & increased aquaculture production needed to supply
McNevin, 2015). Nevertheless, in some countries, aqua- future demand. Of course, more land will be needed for
culture may have caused up to 30 to 40% of mangrove ingredients in aquaculture feeds, but the amount of
loss and, in some local areas, it has resulted in greater land for feed ingredients per tonne of aquaculture pro-
percentage losses. duction would be the same regardless of whether the
increased future demand results from intensification or
Mangrove areas (Fig. 5.6) are inferior sites for aquacul- expansion of the production area. Intensification will
ture and most governments have developed regulations decrease the amount of land needed per tonne of pro-
to avoid mangrove clearing. Although mangrove removal duction at farm level. Additional aquaculture produc-
for aquaculture is not as common as in the past, it still tion area is less likely to intrude on areas of higher
occurs in some countries. Small‐scale farmers with no terrestrial biodiversity than would expansion of the ter-
other land available to them and little capital to invest in restrial agricultural area that would be required for
pumps to lift water out of the intertidal region are prob- aquaculture feed ingredients.
ably responsible for the greatest share of mangrove con-
version to ponds at present. From a land use perspective alone, it would be more
desirable to increase global aquaculture production
Aquaculture is intensifying. To illustrate, channel cat- through intensification than to accomplish the increase
fish production in the USA was less than 2000 kg/ha of through expansion of land‐based production. Of course,
pond water surface in the early 1970s; but, today, the any change in production methodology involves trade‐
average annual production is over 5000 kg/ha, and many offs of resource use and environmental impacts. For
farmers produce over 10 000 kg/ha. Similar increases land‐based food production, the use of land — especially
have been seen with marine shrimp, tilapia and other the clearing of land — is the major factor affecting ter-
common aquaculture species. Thus, as occurred in restrial biodiversity. Nevertheless, there should be a
terrestrial agriculture over the past 50 years, aquacul- thorough assessment of all the pros and cons of aquacul-
ture production intensity will be likely to increase ture intensification.
in the future. Intensification rather than expansion of
Resource Use and the Environment 99
Figure 5.6 Mangroves on L’île de Moucha,
in the Gulf of Tadjoura off the coast of
Djibouti. Mangroves are important
components of tropical and subtropical
coastal ecosystems. They have intrinsic
aesthetic value and provide valuable
ecosystem services such as protecting
coastlines from erosion and providing
habitat and food for many aquatic,
terrestrial and avian animals. Although
mangrove systems are convenient
locations for brackish water aquaculture,
they are not well‐suited for that purpose
and should not be used as facility sites.
Source: Photograph by Benlilkon93,
Reproduced under the terms of the
Creative Commons Attribution License,
CC‐BY‐SA 4.0.
5.4 Water Use 1) Green water is rainwater that evaporates during use.
This water is used almost entirely for agriculture, and
In traditional assessments of water available for human it would have evaporated anyway. Moreover, green
use, renewable available freshwater is considered to be water is not included in estimates of renewable, avail-
non‐flood stage stream flow and reservoir storage able freshwater (Table 5.2).
(Table 5.2). Estimated use does not include rainwater
that falls on the earth only to be transpired by crop 2) Blue water is surface water or groundwater that evap-
plants and groundwater from wells. This procedure is orates during use. Blue water use is basically the same
followed here, because rainwater used by crops would as traditional consumptive water use, and it can be
have been evaporated or transpired had not crops been calculated with reasonable accuracy.
involved, and because removal of groundwater by wells
lowers the water table and lessens the amount of ground- 3) Grey water is the amount of water needed to treat the
water (base flow) contributing to non‐flood stage flow waste resulting from water use.
of streams.
There is no objective way of estimating grey water.
Consumptive water use by humans has traditionally Therefore, we do not support adoption of total water use
been calculated as surface water plus groundwater from (summation of green, blue, and grey water) as a means of
wells that evaporates as a result of its use. There is, how- assessing water use for agriculture and aquaculture.
ever, a recent tendency to divide water use into three
categories: Estimates of major consumptive uses of water show that
agriculture is the major consumer, accounting for more
than 80% of total global water use (Table 5.3). Consumptive
Table 5.2 Global supply of accessible, renewable freshwater. Table 5.3 Global water use estimates.
Component of freshwater Volume (km3/yr) Usage
Total runoff 39,700 Purpose (km3/yr) (%)
Flood stage flow (75% of total) 29,250 Water for irrigation 3,200 80.6
Livestock water 46 1.2
Non‐flood stage flow (assume 100% available) 10,450 Industrial water
Domestic purposes 400 10.1
Accessible (70% of available) 7,315 Total 324 8.2
3,970
Reservoir storage (assume 100% available) 6,197 –
Accessible, renewable supply 13,512
100 Aquaculture
Figure 5.7 Overhead irrigation of wheat in Colorado, USA. Water used to grow terrestrial crops for use in aquaculture feeds is a major
contributor to overall water use in some types of aquaculture. Source: Photograph by Gene Alexander, USDA Natural Resources
Conservation Service (photogallery.nrcs.usda.gov No. NRCSCO87001), via Wikimedia Commons.
water use by agriculture amounts to roughly 30% of avail- feeds (Figure 5.7) is ca. 275 m3/t, and about 41 million t
able, renewable freshwater. Nevertheless, water supply is of aquaculture feed were used in 2014. Thus, water use
not distributed in accordance with population density and for feeds is ca.1 km3/yr, and is the primary water‐
arid regions with high population densities often have a consuming activity in cage and raceway aquaculture.
natural scarcity of freshwater. Even in some areas where Consumptive water use by aquaculture is likely to total
water is normally plentiful, water shortages may develop about 122 km3/yr or only 3.7% as much as used in agri-
during periods of drought. Some countries do not have culture. If green water is included in the calculation,
adequate water distribution infrastructure or suffer from water use by aquaculture would increase to about 429
political unrest or armed conflicts that interfere with km3/yr (Verdegem & Bosma, 2009).
water distribution. Water pollution may also make water
unsuitable for some uses. There are several ways of reducing the direct use of
freshwater in pond aquaculture to include production
Consumptive water use in freshwater aquaculture ponds intensification, water reuse, construction techniques to
averages about 12 700 m3/ha (Boyd & McNevin, 2015). minimise seepage loss, and water level control to capture
Using an average global yield from pond aquaculture of as much rainfall as possible. Increasing production from
2496 kg/ha (Verdegem & Bosma, 2009), water use would 3000 kg/ha to 6000 kg/ha in a pond with annual water
be 5088 m3/t, and direct water use in freshwater pond use of 12 000 m3/ha would lessen water use from 4000
aquaculture would be ca. 111 km3/yr. Freshwater is usu- m3/t to 2000 m3/t. Of course, assuming no change in feed
ally not added to ponds supplied with brackish or ocean conversion ratio (FCR), the water embodied in feed
water. Cage culture and raceways have little direct con- would not increase per tonne of production. Water reuse
sumptive water use — possibly around 0.25 km3/yr that can be accomplished in ponds if they are not drained for
is mainly contained in harvested biomass. Consumptive harvest or if the water is drained from ponds and stored
water used for making feed ingredients for aquaculture for reuse. For example, many channel catfish ponds in
Resource Use and the Environment 101
Figure 5.8 Shrimp farming with a reservoir for water re‐circulation. The reservoir consists of the two wide canals and the two large ponds
in the upper right‐hand corner of the picture. Source: Reproduced with permission from Aaron McNevin, 2017.
the USA are operated for 10 years or longer, without splash water into the air — may increase water loss by
draining. Harvest‐sized fish are removed with a large‐ exposing more water surface area to air, but it does not
mesh seine and are replaced with fingerlings. The cycle increase water use as much as does water exchange.
of partial harvest and restocking may continue indefi-
nitely, although ponds are drained periodically to reno- 5.5 Energy Use
vate eroded levees. In some systems, fish may be cultured
intensively in one section and water circulated to another The world uses a tremendous amount of energy for
section for purification before returning to the culture industry, agriculture, transportation, domestic, and
section (Figure 5.8). This procedure does not usually municipal activities and other purposes. The total global
conserve water, because the combined area for produc- consumption of primary fuels was about 541 × 1018 joules
tion and waste treatment equals that of a non‐partitioned or 541 exajoules (EJ) in 2014. Only about 60% of this
pond with the same amount of production. Seepage loss energy is end‐use energy — the remainder is consumed
can be as much as 50 cm/mo or more in ponds, but seep- in transforming the energy in the primary fuels to energy
age rates usually can be reduced to 10–20 cm/mo, or used for a specific purpose.
less, by proper siting and construction practices. Rainfall
can be captured by maintaining water levels in ponds In 2008, end‐use consumption of energy for all pur-
10–15 cm below overflow levels. Captured rainfall par- poses was estimated to be 294 EJ, of which 95 EJ (32%)
tially offsets the future need for pumped water from was used in the world food system. Most of this energy
other sources. Water exchange is a particularly bad prac- was used for processing, distributing, retailing, prepara-
tice that seldom improves water quality in large aquacul- tion and cooking (Table 5.4). The share for production of
ture ponds, but it increases water use and pollution the food at the farm‐gate (or dock, in case of capture
potential of aquaculture facilities. fisheries) was only 21.4% of total energy use in the food
system.
Mechanical aeration is more effective than water
exchange in maintaining adequate DO concentrations in An FAO report (FAO 2011), using data from 2008,
ponds. Mechanical aerators — especially those that estimated energy use for capture fishery to be 2 EJ/yr,
102 Aquaculture used in producing feedstuffs, manufacturing feed, and
Table 5.4 End‐use of energy in the world food system in 2008. delivering it to farms (Figure 5.9). Such indirect energy
use is often called embodied energy and the embodied
End‐use Energy (EJ) energy content of aquaculture feed ranges from around 4
to 10 GJ/t. Aeration is also a major energy input to aqua-
Plant crops 12.8 culture. For example, at an aeration rate of 1.5 kW/t of
Livestock 5.1 production and 1000 hr of aeration during a crop, con-
Fisheries and aquaculture 2.4 sumption of electricity would be ca. 5.4 GJ/t of produc-
Processing and distribution 40.9 tion. Facility construction can be amortised over several
Retail, preparation and cooking 33.8 years and will seldom be greater than 2.0 GJ/t of produc-
Total 95 tion. Energy to pump water for filling ponds and replac-
ing evaporation and seepage loss usually will not exceed
Source: http://www.eia.gov/cfapps/ipdbproject/iedindex3. 0.5 GJ/t of production.
cfm?tid=90&pid=44&aid=8
Energy conservation is important, because most
which is rather large considering the energy use for all energy used in aquaculture is from non‐renewable
terrestrial animal production accounted for only 5.1 EJ/yr. sources. The combustion of fossil fuels is also the major
In the same report, aquaculture was estimated to use source of greenhouse gases that contribute to global
only 0.4 EJ/yr — much less than did capture fisheries. climate change.
However, few studies of energy use in aquaculture have
been made, and the FAO estimate appears too low. 5.6 Feed‐Fish Use
Several life cycle analyses (LCAs) of aquaculture produc-
tion of tilapia, channel catfish, salmon, shrimp and Fish and other aquatic organisms are capable of convert-
mussels give an average total energy use of 26 GJ/t. ing protein in their feed to edible animal protein with
Aquaculture production in 2008 (the year of the FAO good efficiency. The percentage of feed protein recov-
report) was 53 t, giving an overall energy use for aquacul- ered in harvest biomass and in processed fillets for five
ture of 1.4 EJ/yr. However, almost no data are available common species averaged 30.9% and 19.4%, respectively
for production of species raised with lower energy inputs (Table 5.5). Compared to terrestrial meat animals,
(such as certain types of carp aquaculture), so there is aquatic species generally need somewhat higher crude
considerable uncertainty regarding overall energy use. protein concentrations in their feed (although differ-
About the best that can be said is that energy use in ences are not great; Naylor et al., 2009), but they tend to
aquaculture was probably somewhere between 0.4 and have better feed conversion efficiency than terrestrial
1.4 EJ/yr in 2008. animals. Aquaculture species vary greatly with respect to
feed protein conversion and some species are superior
There are several direct uses of energy in aquaculture while others are inferior to broiler chickens in this regard.
which include facility construction, pumping water, aer-
ation and farm operations. Considerable energy is also
Figure 5.9 Harvesting rice in Mississippi,
USA. Manufactured feeds embody most of
the energy used in many types of
aquaculture. Energy is used to produce
fertilizers and manufacture farm
equipment; grow, harvest and transport
crops; process and transport the
feedstuffs, and manufacture and transport
finished feeds. Source: Reproduced with
permission from Kenner Patton, 2017.
Resource Use and the Environment 103
Table 5.5 Ranges in FCR, crude protein concentration in feed, and typical feed protein recovery in finished animals and edible meat
for several sources of animal protein.
Feed Crude Typical feed protein recovery (%)
conversion protein
Meat animal Ratio in feed (%) Whole body Edible meat
Channel catfish 1.6–2.2 28–32 25.9 12.1
Tilapia 1.5–2.0 28–34 25.7 11.9
Shrimp 1.2–1.8 35–40 31.2 13.0
Trout 1.2–1.4 42–45 28.9 26.7
Salmon 1.0–1.2 40–45 43.0 33.3
Broiler chickens 1.6–2.0 18–24 34.0 20.0
Swine 2.5–3.0 14–20 20.0 10.0
Beef cattle 5.0–7.0 10–20 10.0
4.0
However, aquaculture species usually convert protein 1994 at 7.4 million t. The production is currently around
more efficiently than do swine or beef cattle. 4.5 to 5 million t/yr and is not expected to increase in
the future. Fish oil is a by‐product of fishmeal and the
Production of aquaculture animals appears to be at annual production from pelagic fish is ca.1 million t/yr.
least as efficient as traditional meat animals in converting Thus, the yield of fish oil from making fishmeal is about
feed ingredients, and feed protein in particular, to meat 1:4.5. It requires about 4.5 kg live fish to make 1 kg fish-
and animal protein for human consumption. Of course, meal. Thus, the current production of fishmeal requires
the production of aquaculture species is much less than a feed‐fish input of 20–23 million t annually. An addi-
that of traditional meat animals. The Alltech 2015 Global tional 1–1.5 million t/yr of fishmeal is made from fish
Feed Survey1 revealed that the total global production of processing scraps. At present, about 60% of the global
animal feed in 2014 was 980 million t. Animal feed con- supply of fishmeal and 80% of fish oil are included in
sumption by species category was as follows: poultry, 439 aquaculture feeds. Feeds for salmon, trout, shrimp and
million t; swine, 256 million t; cattle and other ruminants, miscellaneous marine fish feeds consume about 80% of
196 million t; aquaculture, 41 million t; pets, 22 million t; the fishmeal and 92% of the fish oil used in
equines, 11 million t. aquaculture.
The major difference in aquaculture feeds and other Another source of fishmeal and fish oil for aquacul-
animal feeds is that a large amount of fishmeal and ture feeds is ‘trash’ fish or by‐catch, consisting of
fish oil is included in feed for some aquaculture aquatic organisms captured in trawl nets that do not
s pecies — especially in feeds for marine shrimp and car- have a market value for direct human consumption.
nivorous fish such as trout and salmon. Fishmeal is Trawl fishing has been acknowledged as a destructive
included in aquaculture feed because it is of high protein practice, but it is a traditional means of fishing in many
content with a balanced amount of amino acids. It also parts of the world. By‐catch was once considered rela-
contains phosphorus, calcium and essential omega‐3 tively useless but in certain areas it has gained a market
fatty acids. Fish oil has a high ratio of n‐3 to n‐6 fatty as a feed component for aquaculture. As an unfortunate
acids and this high ratio is considered to be beneficial consequence, use of by‐catch for aquaculture feed f osters
to human cardiac health. Vegetable oils have a lower the indiscriminate catch of any species because — large
ratio of n‐3 to n‐6 fatty acids and it is widely believed or small — there are now markets for this previously
that inclusion of fish oil in aquaculture diets enhances valueless product.
the n‐3 to n‐6 fatty acid ratio in edible meat of aqua-
culture species. The capture of pelagic fish and by‐catch to produce
fishmeal and fish oil can affect biodiversity in the ocean.
Fishmeal production from the capture of the pelagic Overfishing of these fisheries could be passed through
fish (menhaden, anchovies, herring, sardines and o thers — the food chain to cause a reduction in the production of
collectively called feed‐fish here; Figure 5.10) peaked in some omnivorous or carnivorous species important in
the fishery for human consumption. There is a limited
1 http://www.alltech.com/sites/default/files/global-feed-survey- supply of fishmeal and oil that could restrict the future
2015.pdf production of aquaculture — especially those species
104 Aquaculture
Figure 5.10 Fishing for Atlantic menhaden (Brevoortia tyrannus) off the coast of the USA. Menhaden are one of several fish commonly
caught for reduction to fishmeal and fish oil. Coastal menhaden fishing uses spotter airplanes to locate large menhaden schools and
direct net‐setting activities. Here, two ‘purse boats,’ launched from the carrier boat in the distance, are completing a purse‐seine set by
hardening up the net to concentrate fish. After fish are concentrated in the net, the carrier boat will come alongside and fish will be
pumped into the hold. Catch per seine set ranges from a few to more than 50 t. Source: Reproduced with permission from Omega Protein
Corporation, 2017.
for which feeds contain a high percentage of fishmeal 1 kg of farmed fish (FIFO > > 1). For fish receiving diets
and oil. with low levels of fishmeal and oil, far less than 1 kg of
feed‐fish are needed to grow 1 kg of farmed fish (FIFO < < 1).
Using wild‐caught fish in aquaculture feeds is perhaps
the most significant environmental impact of aquacul- One point of concern is that it appears to be ecologi-
ture (or at least the most discussed). Most other environ- cally inefficient when fish are fed diets with high levels of
mental impacts of aquaculture are localised (so‐called fishmeal and oil. On the face of it, this is true. But this
near‐field effects), readily understood and often reversi- superficial conclusion neglects other considerations.
ble with simple changes in management or technology. Feed‐fish and high‐valued farmed fish and shrimp do not
The concern with feed‐fish use in aquaculture is global, substitute equally either socially or economically, so
with far‐reaching implications (far‐field effect). The FIFO accounting is too simple to assess the deeper sig-
worrisome concept is that farming fish and crustaceans nificance of feed‐fish use in aquaculture. Most fish
on feeds containing high levels of fishmeal and fish oil caught for reduction to fishmeal and oil are not com-
does not increase global fish production. In other words, monly used as human food and using the meal and oil in
each kilogram of farmed fish consumes more than one animal feeds adds value. The apparent ecological ineffi-
kilogram of feed‐fish in the form of fishmeal or fish oil. ciency of the process is also misleading because ineffi-
ciencies are inevitable in nature when organisms at one
The amount of feed‐fish needed to grow a kilogram of trophic level consume biomass from a lower trophic
fish or crustacean in aquaculture is called the fish in–fish level. In fact, the efficiency of food use in aquaculture is
out (FIFO) ratio and is calculated from the FCR, the per- considerably better than in nature, where ecological inef-
centages of fishmeal and fish oil in the feed, and the ratio ficiencies are on the order of 10‐fold for each change in
of the quantity of feed‐fish required to produce a unit of trophic level.
fishmeal or fish oil (Boyd and McNevin, 2015). For fish
receiving diets with high inclusion rates of fishmeal and Increased demand for fishmeal and fish oil has caused
oil, several kilograms of feed‐fish may be required to grow dramatic increases in prices since 2000. Increased costs,
together with criticism of the use of fishery products in Resource Use and the Environment 105
aquaculture feeds, has induced efforts to refine diets and
develop dietary substitutes for fish products. There is magnesium oxide. Hydrated lime is made by adding
scope for reduction of fishmeal and fish oil in aquacul- water to burnt lime.
ture feeds and considerable progress has been made in
replacing fishmeal with oil‐seed meals, although much Liming materials are used primarily to neutralise acid-
less progress has been made in replacing fish oil (Naylor ity in pond bottom soil and water. Liming also increases
et al., 2009). the total alkalinity and total hardness concentration in
water. There is no particular environmental risk associ-
Large reductions in use of fishery products in aquacul- ated with agricultural limestone. Burnt and hydrated
ture feeds could be achieved by growing aquatic animals lime are caustic, and workers should wear gloves and eye
with low dietary dependence on fish products. This will protection.
require shifts in social and economic values that may
eventually become necessary but at present are difficult Boyd and McNevin (2015) placed the use of liming
to envision. Farming carnivorous species using feeds materials in aquaculture at about 5.5 t/yr: and about
with high inclusion rates of fishmeal and oil is based on 2.75% of the amount used in agriculture. There are abun-
consumer demand for those products. Society’s role in dant supplies of limestone in many countries. However,
shaping global aquaculture should not be understated. quarrying limestone can have adverse environmental
Provided that consumers are willing to pay for products effects and lime production, and the reaction of lime-
with high rates of resource use and that they can be stone and lime with acidity contributes to global carbon
grown profitably, farmers will continue to grow them. emissions.
Overall use of fish products in aquaculture feeds could
fall dramatically if social values (or production econom- The main chemical fertilisers used in aquaculture are
ics) change to favour farming ‘low trophic-level fish,’ urea and triple superphosphate, but a variety of other
such as carps and catfishes that can be grown on feeds fertiliser compounds are used to a lesser extent. There is
with little, if any, fishmeal or fish oil. little worker safety and no human health risk in fertiliser
use. Nitrogen and phosphorus from fertiliser added to
5.7 Chemicals ponds to increase primary productivity and foster greater
aquaculture production can enter natural water bodies
The chemicals most widely used in aquaculture are and contribute to eutrophication.
fertilisers and liming materials. But, a wide array of
other chemicals are used to include oxidants, coagulants, The use of fertilisers in aquaculture was estimated at
osmoregulators, algicides, herbicides, fish toxicants, 207 000 t of nitrogen and 103 000 t of phosphorus (Boyd
antifoulants, disinfectants, anaesthetics, antibiotics, and McNevin, 2015). This represents only 0.20% and
agricultural pesticides and hormones. Many of these 0.27% of global use of N and P in fertiliser, respectively.
chemicals may be toxic or have other negative impacts There are limits on the availability of apatite, the ore for
on the biota when discharged into natural water bodies. phosphate fertiliser production and on natural gas nec-
Some chemicals pose a danger to farm workers through essary to convert atmospheric nitrogen to ammonia for
toxicity or as fire or explosion hazards. A few chemicals nitrogen fertiliser manufacturing. Moreover, fertiliser
may contaminate aquaculture products and be a health manufacturing is energy intensive — 57.46 GJ/t of nitro-
hazard to consumers. Some chemicals are necessary for gen and 7.03 GJ/t for phosphorus in fertiliser. Thus, fer-
efficient aquacultural production, and most of them are tiliser conservation should be practiced in all types of
safe provided they are stored, used, and properly dis- food production including aquaculture.
posed of. Of course, chemicals should be used only
when necessary and certain ones are not effective for Organic matter, mainly animal manures, grass and
their intended purpose or are banned because of known certain plant processing by‐products, also are used as
environmental, worker safety and food‐safety risks. fertilisers in aquaculture. The recycling of nutrients
from agricultural wastes by using them as pond fertilis-
Liming materials consist primarily of agricultural ers is a wise use of resources that conserves chemical
limestone and lime made by burning limestone. fertilisers and extracts value from wastes. Of course,
Agricultural limestone is calcium carbonate or a mixture effluents from ponds fertilised with organic matter will
of calcium and magnesium carbonate (limestone) that be enriched with nitrogen and phosphorus to about the
has been finely pulverised. Burnt lime is made by burn- same extent as effluents from ponds treated with chemi-
ing limestone at high temperature in a kiln to drive off cal fertilisers.
carbon dioxide. The product consists of calcium and
A variety of antibiotics are used to treat various dis-
eases of aquaculture species (Table 5.6). Most of these
antibiotics are also used in human or veterinary medi-
cine, and few were developed specifically for aquacul-
ture. The greatest use of antibiotics is with high‐value
species such as rainbow trout, salmon and shrimp, but
they are also used in the culture of Pangasius species in
106 Aquaculture antibiotics may be toxic to non‐target bacteria, algae and
Table 5.6 Antibiotics used in global aquaculture. Those micro‐crustaceans. However, the greater concern is that
with an asterisk also are commonly used in human medicine. Note repeated exposure of microorganisms to antibiotics
that antibiotic use is highly regulated in most countries can lead to selection of spontaneous mutants that are
and the number of antibiotics used in a particular country will resistant to antibiotics. Resistance can render an antibi-
be far less than this list indicates. otic worthless for its intended purpose of aquaculture
disease control. A broader concern is the horizontal
Class Compound transmission of antibiotic resistance from bacteria at
Aminoglycosides aquaculture sites to bacteria responsible for human
Neomycin* diseases. Moreover, antibiotic residues in aquaculture
β‐Lactams; penicillins Gentamycin* products can be potentially harmful to consumers.
Fenicoles Streptomycin (obsolete)
Fluoroquinolones Amoxicillin* Although aquaculture is not a major user of antibiot-
Ampicillin* ics, aquaculture production tends to be concentrated in
Macrolides Chloramphenicol small areas. Disease control could lead to locally large
Non‐fluorinated guinolones Florfenicol and frequent inputs of antibiotics. These areas could
Sulfonamides Ciprofloxacin* possibly become ‘hot spots’ for the development of
Enrofloxacin* microbial resistance to antibiotics that by horizontal
Trimethoprim Flumequin* transmission could lead to human health concerns.
Tetracyclines Erythromycin* Most antibiotics that are a food‐safety concern have
Oxolinic acid been banned by governments for use in aquaculture.
Sarafloxin Nevertheless, these compounds are still used by some
Sulfamethazine producers, and importing countries analyse aquaculture
Sulphamerazine products for residues of certain antibiotics:
Sulphadimethoxine
Trimethoprim* ●● antibiotic use should be a last resort;
Chlortetracycline ●● banned antibiotics should not be applied; and
Oxytetracycline* ●● antibiotic treatments should follow recommendations
Tetracycline*
on product labels.
Vietnam, channel catfish in the US and for a few fish
s pecies in other countries. Alternative approaches to aquatic animal health man-
agement that are often more effective than antibiotic use
There is little reliable information on world antibiotic include development and use of vaccines (Chapter 12)
use. Countries in the EU used 13 288 t in 1999 and 16 200 and improved biosecurity. Biosecurity measures include:
t were used in the US in 2006, and the greatest use was
for human medicine in the EU and for livestock produc- ●● providing disease‐free animals at stocking
tion in the US. Thus, global use is likely to be at least ●● preventing — to the extent possible — disease organ-
twice the combined use in the EU and USA. It is also dif-
ficult to find data on the quantities of antibiotics used in isms from entering culture systems from outside
aquaculture. However, the total use in USA aquaculture sources
in the late 1990s was estimated to be 135 000 kg/yr, and ●● maintaining good water quality to avoid impairing the
about 385 600 kg were used in salmon culture in Chile animals immune function and making them more sus-
in 2007. In Norway, where salmon production was over ceptible to disease.
twice that of Chile in 2007, only 649 kg of antibiotics
were used. Parasiticides, fungicides and disinfectants include a list
of chemicals too long to present here. Most of these
The usual means of administering antibiotics in chemicals can be used safely, but nearly all of them can
a quaculture is through medicated feed. From 70 to 80% be toxic to aquatic life if used in excessive concentration
of the antibiotics in medicated feed enter the water via or accidentally spilled into water bodies. Of these, a few
urinary excretion and defaecation of uneaten feed. The compounds deserve mention because of high risks of
antibiotic remains in the water until degraded by natural adverse efforts or wide usage.
processes and adsorbed by sediment. In natural waters,
Malachite green (a triarylmethane dye) is sometimes
used to control protozoal and fungal infections. This
compound is extremely toxic to aquatic animals, and it
accumulates and persists in the tissues of animals. Thus,
malachite green poses a food‐safety hazard to consumers
because it can affect kidneys, liver, spleen, heart, eyes,
lungs and bones of vertebrates. In addition, malachite
green can negatively affect immune and reproductive sys- Resource Use and the Environment 107
tems, and it has genotoxic and carcinogenic potentials. Table 5.7 Aquatic herbicides used in aquaculture. Herbicide use is
regulated in most countries and the types of herbicides used
A wide variety of chemicals are used to disinfect ponds in a particular country may be less than this list indicates.
before stocking or water added to replace seepage and
evaporation loss. Of course, insecticides — especially Common Chemical formula
those with a residual life — should not be applied to kill name
invertebrate hosts of certain disease vectors. Moreover,
there is inadequate information on many of the other Copper CuSO4⋅5H2O
compounds used as disinfectants to assure that they are sulphate
safe for workers, the environment and consumers. Chelated 2‐aminoethanol copper and others
Chlorine compounds have been widely used for sanita- copper
tion in other endeavours, e.g., tank culture, and they can Diquat 6,7‐dihydrodipyrido[1,2‐a:2′,1′‐c]pyrazinediium
be safely used as disinfectants in aquaculture with proper dibromide
precautions. Endothall (7‐oxabicyclo[2.2.1]heptane‐2,3‐dicarboxylic
acid)
Hormones are widely used for spawning fish, and the Fluridone 1‐methyl‐3‐phenyl‐5‐3‐(trifluoromethyl)
use of 17‐methyltestosterone (MT) for sex‐reversal in phenyl∣‐41H∣‐pyridinone
tilapia and other fish species has drawn considerable Glyphosate N‐(phosphonomethyl) glycine
attention. There is concern over the effects on fish and Simazine 6‐chloro‐N,N′‐diethyl‐1,3,5‐triazine‐2,4‐diamine
other organisms in water bodies receiving effluents con- Paraquat 1, 1′‐dimethyl‐4, 4′‐bipyridinium dichloride
taining MT. Moreover, there is potential for MT expo- 2, 4‐D 2,4‐dichlorophenoxyacetic acid
sure by workers at aquaculture facilities and the question
of whether residues could occur in aquaculture prod- heeded, these products can be used safely. Boyd and
ucts. Most research on MT indicates that it can be used McNevin (2015) gave a similar opinion on the use of
safely if effluents containing MT are held for two weeks fish anaesthetics, fish toxicants, coagulants, antifoulants,
before discharge. There appears to be little risk of resid- osmoregulators, feed additives, bacterial cultures and
ual MT in tissues of marketable size fish that were treated their derivatives, and fuels and lubricants used in
at the fingerling stage with MT. Nonetheless, there is still aquaculture.
considerable concern by many about the safety of MT
use in aquaculture. 5.8 Water Pollution
Oxidants used in aquaculture include potassium per- In feed‐based aquaculture, only 20 to 40% of nitrogen
manganate, peroxide compounds, and nitrate com- and phosphorus applied in feed is recovered in harvest
pounds. The largest risk associated with these products biomass. A similar recovery of feed nitrogen is realised in
is likely to be an accidental spill resulting in a high local shrimp culture. But, no more than 10% of phosphorus in
concentration that could kill aquatic organisms. Nitrate feed is contained in the shrimp at harvest, because
fertilisers have a great explosion potential and must be shrimp do not have as much phosphorus in their bodies
stored separately from petroleum products and not as fish — in which the bone is made of calcium phos-
exposed to sparks or open flames. Sodium sulphite is a phate. The recovery of feed carbon is between 10 and
reducing agent used as a post‐harvest dip for shrimp to 20% in both shrimp and fish. In fertilised ponds, produc-
prevent enzymes from forming discolorations in the tion depends upon natural food organisms, and the
shrimp’s shell. Spent sodium sulphite solutions must be recovery of nitrogen and phosphorus applied to ponds is
held until the sulphide has completely oxidised and then usually less than in feed‐based aquaculture. The organic
carbon in fertilised ponds is primarily from photosyn-
treated with liming material to increase their pH before thesis, and there is inadequate information on primary
discharge into natural waters. productivity rates in aquaculture ponds to allow a relia-
ble assessment of typical carbon transfer efficiency from
The most common algicide used in aquaculture ponds photosynthesis to harvest biomass of aquaculture spe-
cies. However, a few studies have placed the efficiency at
is copper sulphate. This compound has a history of safe less than 5%.
use in municipal water‐supply lakes and reservoirs for
algal control. It is safe for use in ponds at a concentration The portion of the nutrient input to aquaculture that is
equal to 0.01 times the total alkalinity. Despite over- not recovered in harvest biomass represents nutrients that
whelming evidence of the safety of copper sulphate when
applied according to instructions on the product label,
some still prefer that it is not used in aquaculture.
Several aquatic herbicides are used in aquaculture
(Table 5.7). If treatment rates, and if worker and environ-
mental safety precautions given on herbicide labels are
108 Aquaculture The system loads for elements are variable among spe-
Table 5.8 Recovery of carbon, nitrogen and phosphorus applied cies. For example, the system load for tilapia was 56 kg
to ponds in tilapia at harvest. N/t at a typical FCR. For channel catfish at a typical FCR
of 2.0, the N loading would be around 72 kg N/t. The
Chemical fertilizera Manureb Feedc FCR has the greatest effect on system loads. In the tilapia
example, at an FCR of 2.0 the system load of nitrogen
Nitrogen would increase to 72.5 kg/t. The concentration of nitro-
gen in the feed also affects the system load. In the tilapia
Input (kg/ha) 108.0 198.7 500 example, a feed with 5% N had a system load of 56 kg/t,
45.3 165 but had the feed contained 5.5% N, the N load would
Removal in fish (kg/ha) 26.0 22.8 have been 64 kg/t at the same FCR. The concentration of
33.0 elements in harvest biomass of a given species usually is
Recovery (%) 24.1 fairly constant and does not affect the system load appre-
ciably. The system loads for major species usually are
Phosphorus around 60 to 90 kg N/t, 12 to 22 kg P/t and 400 to 900 kg
C/t at typical FCRs for common species.
Input (kg/ha) 47.2 141.9 130
13.2 48 The fate of feed nutrients in aquaculture systems is
Removal in fish (kg/ha) 7.6 9.3 36.9 discussed in Chapter 4. In summary, a large proportion
of the wastes from feeding is assimilated in ponds —
Recovery (%) 16.1 e specially in those without appreciable water exchange to
flush nutrients out. In other words, only a small propor-
Carbon tion of the total nutrient loading from feeds is eventually
discharged from ponds. In raceways and tanks, much of
Input (kg/ha) – – 4270 the solid wastes can be removed from the culture system,
– 726 but in cage culture the entire system load is discharged
Removal in fish (kg/ha) – – 17.0 into the body of water containing the cages. Thus, aside
from cage culture, the load of an element discharged into
Recovery (%) – natural water is usually less than the input loading — and
can be far less in properly managed ponds and raceways.
a Twelve applications each of 45 kg/ha of 20‐20‐5 fertilizer;
947 kg/ha tilapia (12.1% C, 2.75% N, 0.8% P). As shown in Chapter 4, feed use in aquaculture is a
b Total of 28 380 kg/ha fresh cow dung (0.7% N and 0.5% P); source of acidity because each mole of ammonia N oxi-
1646 kg/ha tilapia. dised to nitrate N produces two moles of H+. The acidifi-
c 10 000 kg/ha feed (42.7% C, 5.0% N, 1.3% P); 6000 kg/ha tilapia. cation potential resulting from feed may be calculated by
the following equation:
enter the water of the culture system and can potentially
become pollutants in natural waters receiving the dis- Acidification potential (kg CaCO3 /t) production
charge from aquaculture production units. Only a portion
of the nutrients applied to ponds in fertilisers is recovered Nf FCR NBB 7.14 1000
in harvest biomass, but because of the relatively low pro-
duction in fertilised ponds, the pollution potential is not where Nf and NB = decimal fractions of N in feed and har-
great. Much higher production is achieved with feeding, vest biomass, respectively; 7.14 = CaCO3 equivalent of
and most of the concern about water pollution by aqua- acidity from nitrification of 1 kg ammonia N; 1000 = kg/t.
culture is directed at feed‐based aquaculture.
In the tilapia example used earlier, the acidification
Data from pond culture of tilapia were used to esti- potential would be 400 kg CaCO3/t tilapia. A portion of
mate recovery of added nitrogen and phosphorus in the potential acidity will be expressed in the culture sys-
ponds receiving fertiliser or feed and of feed carbon tem, but the remainder will be expressed in the receiving
recovery in ponds with feeding (Table 5.8). The differ- water body.
ence between the input of a nutrient in fertiliser or feed
and the output of this nutrient in harvest biomass enters The carbon loads for aquaculture systems were not
the culture system. This difference is the system load of calculated above, because the more important variable is
an element. For illustration, the system load of nitrogen the BOD (biochemical oxygen demand)2 associated with
will be calculated for tilapia production with feeding. In
the example (Table 5.8), FCR = 1.67, the feed contained
5.9% N, and live tilapia contained 2.75% N. Thus, 1670 kg
feed containing 83.5 kg N was used to produce 1000 kg
tilapia containing 27.5 kg N. The system load for N was
56 kg N/t tilapia. These calculations can be combined
into the following equation for estimating system load:
System loadi kg/t Fi FCR Bi 1000
where i = a particular element; F = decimal fraction of 2 The amount of oxygen required by aerobic microorganisms to
elementi in feed; Bi = decimal fraction of element I in the decompose the organic matter in that water: high levels of organic
harvested biomass; FCR = feed conversion ratio. matter lead to high BOD and potentially low DO levels from the
aerobic decomposition.
the carbon input in feed. Moreover, nutrients from feed Resource Use and the Environment 109
support organic matter production by photosynthesis. In
principle, the organic matter produced in photosynthesis global anthropogenic discharges of N and P, respectively,
does not create an oxygen demand, because the amount into natural waters. Aquaculture is highly concentrated
of oxygen produced by photosynthesis offsets the oxygen in small areas, and although it produces only a small
consumed in decomposing the organic matter produced share of global water pollution, it can be a major con-
in the process. The feed, however, produces an oxygen tributor to water pollution locally. The major impact of
demand as a result of oxidation of uneaten feed and fae- aquaculture effluents is to contribute to eutrophication,
ces, and fish respiration. This demand can be described but they also can be a source of suspended solids.
simply as:
Aquaculture effluents are rather dilute making them
C O2 CO2 . difficult to treat. Coarse solids can be removed from the
final draining effluent of ponds by sedimentation, and
It can be seen from the equation that 1 kg organic carbon solids can be removed from raceways and other flow‐
requires 2.67 kg O2 for oxidation. This stoichiometry through systems. The three most effective ways of reduc-
applies to the overall respiration in any ecological sys- ing the pollution potential of aquaculture are:
tem. In addition, oxidation of 1 kg of ammonia N by 1) Good feed management to include high‐quality feed
nitrifying bacteria consumes 4.57 kg of oxygen. The
equation for estimating total BOD imposed by feed for containing no more nitrogen and phosphorus than
the production of 1 kg of harvest biomass is: necessary to satisfy nutrition requirements.
2) Feeding conservatively to avoid overfeeding and a sig-
Feed BOD kg O2 /t production Nf FCR NB 4.57 nificant quantity of uneaten feed.
1000 Cf FCR CB 2.67 3) The use of plenty of mechanical aeration to avoid
stressing fish (stress reduces appetite and encourages
where Cf and CB = decimal fractions of carbon in feed disease) and to provide plenty of oxygen for waste
and harvest biomass, respectively; Nf and NB = decimal oxidation.
fractions of nitrogen in feed and harvest biomass, These procedures usually allow a good FCR minimis-
respectively ing both system and environmental waste loads.
Although feed management is a central theme in reduc-
Tilapia feed contains about 43% C and tilapia are ing the risk of pollution from all aquaculture facilities,
around 12% carbon. For the tilapia example used above, specific management practices to mitigate pollution vary
feed BOD would be about 1854 kg O2/t. Of course, the widely among different types of aquaculture systems.
greatest part of the BOD is from fish respiration. In These practices are summarised by Tucker and
ponds and raceways, most of the feed BOD will be Hargreaves (2008).
expressed in the culture system. The BOD concentration In some countries limits have been placed on dis-
in effluent from aquaculture facilities seldom exceeds 25 charges from ponds. The limits may be concentration
to 50 mg/L — much lower than in domestic sewage or limits, load limits or both for selected variables. Feed
other common effluents. But, in cage culture the entire input or production amount limits have also been
feed BOD is imposed on the receiving water body. imposed — especially in cage culture. In the USA, no
specific effluent limitation guidelines have been devel-
The concentrations of nitrogen, phosphorus and BOD oped by the USA Environmental Protection Agency
in effluents from aquaculture ponds usually range from (EPA). The EPA does suggest that producers use best
0.3 to 5 mg/L ammonia N, 2–10 mg/L total N, 0.1 to 0.5 management practices to minimise negative environ-
mg/L phosphorus, and 5–20 mg/L BOD. With respect to mental impacts.
other variables, the usual values are: pH, 7–9; dissolved
oxygen, >3 mg/L; total suspended solids, 30–50 mg/L. 5.9 Best Management Practices
The initial and final effluent 5 to 10% discharge from
aquaculture ponds may be more concentrated in these Practices that can conserve resources and lessen pollu-
variables. Nevertheless, the pollutional strength of pond tion and other negative impacts have been used widely in
effluent is rather modest in comparison with average agriculture, forestry, mining and other endeavours where
domestic wastewater: pH, 6.5–8; dissolved oxygen, <1 non‐point source pollution is common. These ‘so-called’
mg/L; total suspended solids, 210 mg/L; ammonia N, best management practices (BMPs) are based on a com-
25 mg/L; total nitrogen, 4 mg/L; total phosphorus, bination of research findings, practical experience, adop-
7 mg/L; BOD, 190 mg/L (Tchobanoglous et al., 2003). tion from other industries and logic. Thus, it was possible
to recommend BMPs for aquaculture in response to
Boyd and McNevin (2015) estimated that aquaculture environmental criticism. Since the late 1990s, many BMP
effluents might contribute as much as 2% and 2.9% of
110 Aquaculture Criticisms of BMPs are that their adoption is usually
voluntary and it is difficult to verify their use. Farmers
Table 5.9 Best management practices (BMPs) for use of water can selectively implement BMPs, and those that are less
quality enhancers in channel catfish culture. costly and easier to implement are likely to be chosen
over others. To our knowledge, no studies have deter-
●● Store amendments under a roof where rainfall will not wash mined the extent of BMP use or ascertained the com-
them into surface waters. pleteness when a particular suite of BMPs is implemented.
This uncertainty quickly led those attempting to improve
●● Apply copper sulphate at a rate not exceeding on‐one environmental performance of aquaculture to the con-
hundredth (0.01) of total alkalinity (in mg/L). Do not release clusion that eco‐label certification requiring compliance
pond water for 72 hr after application of copper sulphate. with specific standards was necessary to ensure real
improvement in environmental performance. Of course,
●● Apply sodium chloride at rates not exceeding 200 mg/L per BMPs can be used as a means of attaining compliance
application. with eco‐label certification standards or governmentally–
imposed regulations.
●● Apply lime (calcium oxide or hydroxide) at rates not
exceeding 100 kg/ha per application. The logic for eco‐labelling in aquaculture was as
follows:
●● Apply agricultural limestone and gypsum (calcium sulphate) ●● many consumers prefer fisheries products that are
at rates not exceeding 5000 kg/ha per application and 2000
kg/ha per application, respectively. produced responsibly.
●● nations want to protect exports, so they cannot be
programs for individual aquaculture species or for differ-
ent kinds of aquaculture production systems have been depended on to assure responsible aquaculture.
developed. An example of a set of BMPs recommended ●● aquaculture — despite its negative impacts — can
in channel catfish production in Alabama is provided in
Table 5.9. The example in Table 5.9 concerns mitigating be more environmentally‐responsible than capture
the effects of using water quality enhancers; sets of BMPs fisheries.
can be developed for any environmental issue (water ●● buyers seek products from responsible aquaculture.
conservation, antibiotic use, pollution control, erosion ●● voluntary adoption of BMPs by producers is unveri-
prevention and so forth). fiable and unreliable evidence of environmental
stewardship.
There are no reliable estimates of the extent of adop- ●● sourcing responsible aquaculture products if facili-
tion of BMP programs or of the effectiveness of the pro- tated by eco‐labelling.
grams in aquaculture. The BMPs presented by most The development of eco‐labelling programs has been
organisations appear to cover most negative impacts of undertaken by the aquaculture industry, governments,
aquaculture. However, many of the programs do not pro- private companies, and environmental non‐governmental
vide sufficient details for installation of BMPs at the farm organisations (eNGOs). The legitimacy and effectiveness
level. The only example of an evaluation of the benefits of these programs vary, but in principle, the standards
of BMPs known to us is summarised in Table 5.10. The for an eco‐label should be established through stake-
BMPs used in this study were limiting daily feeding rate holder involvement according to guidelines of the
to 110 kg/ha versus feeding rates up to ≈ 50 kg/ha; 28% International Social and Environmental Labelling
versus 32% crude protein in feed; and capturing rainfall Alliance (ISEAL). The standard should be transferred to
to partially offset the need for using pumped groundwa- an independent certification body that will oversee par-
ter by maintaining at least 7.6 cm of water storage capac- ticipants (producers) who agree to comply with the
ity in the pond. The BMPs lessened effluent, phosphorus standards. A third‐party audit of participating producers
discharge and groundwater use. Production did not dif- for compliance with standards is done by an external,
fer between BMP and non‐BMP ponds. The example independent, accredited body. Product traceability from
clearly shows benefits of BMPs, and there is a general farm to consumer must be in place. Regular re‐auditing
consensus that BMPs can be effective — without reduc- and performance review is necessary for continuation of
ing production — if implemented properly. certification.
To illustrate, the World Wildlife Fund (WWF) held
Table 5.10 Effects of best management practices (BMPs) on three aquaculture dialogues with stakeholders to develop
variables in Mississippi catfish ponds. standards for several species. The Aquaculture
Stewardship Council (ASC) holds the standards, and
Variable BMP ponds Non‐BMP ponds producers register with the ASC in order to embark
on the effort of obtaining certified status. The ASC has
Overflow (cm) 52 96
Phosphorus discharged (kg/ha) 1.6 4.3
Groundwater added (cm)
18 47
Source: Modified from Tucker and Hargreaves (2006).
several third‐party auditing bodies accredited by Resource Use and the Environment 111
Accreditation Services International. These auditing
bodies go to participating production facilities and make and monitor aquaculture’s impacts. This issue is further
audits for compliance with the standards and traceability complicated by a reduction in academic research into
requirements. The products from certified farms can be aquaculture production and associated impacts. From
sold under the eco‐label of the ASC provided that chain about 1980 to the late 1990s, much of the academic
of custody certification is obtained. research in aquaculture concerned production‐related
issues and ways to enhance output. This work was greatly
There are several other aquaculture eco‐labelling informed by the interactions with the private sector of
efforts in addition to the ASC. The most important ones the aquaculture industry. In the early 2000s, research
are the Global Aquaculture Alliance (GAA) Best was turning to more specified analyses in narrow disci-
Aquaculture Practices certification, Friends of the Sea plines — such as genomics — that tended to have more
and GLOBAL G.A.P. academic benefit than enhanced aquaculture productiv-
ity. Thus, the type of research data that had once pro-
5.10 Environmental Advocacy vided accurate depictions of the global aquaculture
in Aquaculture sector is no longer available for present‐day production.
The aquaculture industry grew at an approximate annual The new generation of eNGOs in the aquaculture
rate of 10% since the 1990s. This is faster than any other sector are focused primarily on marine or coastal
form of animal protein production. This growth rate aquaculture — a result of the major donors also being
resulted in an incredibly steep learning curve concerning focused or segregated institutionally into key areas of
which practices had adverse environmental impacts. In work. Most eNGOs working on aquaculture are funded
particular, the conversion of coastal land for pond con- through marine conservation elements of private donors
struction — notably for shrimp aquaculture — and the with substantially less attention to freshwater aquacul-
rapid expansion of marine cage production of salmonids ture. The other forms of funding of these eNGOs is via
alarmed many of the eNGOs that sought to protect high‐ private‐sector partnerships, primarily involving major
biodiversity areas from degradation. The result was a retailers. With retail‐ and private‐sector funding eNGOs
series of advocacy and engagement campaigns that iden- attempt to help identify ‘better’ sources of aquaculture
tified the negative impacts of aquaculture. The aquacul- product or engage directly with aquaculture producers
ture industry responded with the development of an to improve production practices. It should be noted that
industry advocacy group, the GAA, to refute many of the very few of the eNGOs engaged in promoting sustaina-
claims of the environmentalists. As BMPs were popular- ble seafood can maintain consistent engagement with
ised and certification programs grew, much of the nega- the aquaculture production sector because their opera-
tive advocacy diminished; however, their engagement tions are typically located in developed countries whereas
with the aquaculture sector did not. over 90% of aquaculture production takes place in the
developing world. The result is much higher levels of
While eNGOs were successful at identifying negative engagement with buyers of aquaculture product rather
impacts of aquaculture and encouraging public responses than producers of the product.
to these issues, these groups were generally ill‐equipped
to intervene and provide practical solutions that lessen All of the issues raised in this chapter are concerns
such impacts. The more common approach of eNGOs is of the eNGO community. However, because of the need
to consolidate information from other sources and use for distinction to donors, eNGOs choose different strat-
that information as a means of evaluating the environ- egies to differentiate themselves. Coupled with specific
mental impact of the production of specific species in eNGO‐company partnerships, the environmental advo-
specific regions. Much of this type of analysis grew out of cacy community is left divided on a clear vision for
the evaluation of wild‐capture fisheries where a specific aquaculture into the future. While attempts have been
fishery can be evaluated based on its status and manage- made to provide unity, often these efforts result in more
ment. Aquaculture operations, however, vary greatly, ambiguous and generalised ambitions for aquaculture.
even when farms are growing the same species using Further, there is no agreement on what constitutes the
similar systems, and there is wide variation in the envi- greatest environmental threats of aquaculture and, as
ronmental impacts of farms in the same geographic such, effective work in a specific impact area seldom
region. Thus, eNGOs often are making generalisations occurs. Boyd and McNevin (2015) asserted that there
about aquaculture’s impact using only information that should be little disagreement among eNGOs on con-
can be gathered in the literature, because they do not serving the natural resources used for aquaculture
have access to farming operations where they can assess production — land, water, energy, fishmeal and oil, and
the receiving waters that process waste. Collective
approaches to lessen impacts associated with these
issues would no doubt foster greater clarity on what has
112 Aquaculture ●● Energy is used in aquaculture to construct facilities,
pump or aerate water and for other farm operations.
been described as the ‘sustainable seafood movement.’ Considerable energy also is used in producing feed-
Further, the advantage of focusing on natural resources stuffs, manufacturing feed, and delivering it to farms.
utilised in aquaculture production has the added benefit
of conveying a value proposition to producers to become ●● Fish are relatively efficient at converting food to bio-
more efficient. mass, and this affords aquaculture the potential to be
one of the most resource‐efficient forms of animal
5.11 Summary agriculture. Relatively large amounts of fishmeal and
fish oil are used in feeds for some aquaculture spe-
●● Aquaculture is a small but significant and growing part cies — especially marine shrimp and carnivorous fish
of the world food system. Aquaculture — like all such as trout and salmon. The global supply of fish-
human activities — uses resources and impacts the meal and oil is limited and could restrict the produc-
environment. tion of certain species.
●● Environmental impacts vary widely among different ●● Most chemicals used in aquaculture, such as liming
types of aquaculture. Seaweed and shellfish farming agents, fertilisers, oxidants, and herbicides) have
have generally positive impacts on the environment. negligible environmental impacts when used properly.
Aquaculture based on manufactured feeds may have Misuse of antibiotics for disease management can be a
significant impacts because considerable land, energy significant issue but problems can be reduced (or
and water are used in feed production. Also, feed avoided) through proper husbandry and development
nutrients not converted to harvestable biomass repre- and use of vaccines.
sent a possible source of pollution.
●● Only 20 to 40% of the nitrogen, phosphorus and car-
●● Aquaculture is a minor contributor to overall land use, bon in feed is recovered in harvested biomass. The
but farms often are sited in sensitive, important remainder represents nutrients that enter the water of the
ecosystems. Increased aquaculture production needed culture system and can become pollutants in effluent‐
to supply future demand will be likely to come from receiving water bodies. Using good feed management
production intensification rather than expansion of is a key to reducing pollution from aquaculture.
production area.
●● Environmental advocacy over the past two decades
●● Water use varies widely among different types of has resulted in substantial improvement of aquacul-
aquaculture, with freshwater ponds being the most ture production practices. Improvement often relies
water‐intensive system. Water availability is an important on adoption of BMPs developed through research and
site‐selection criterion for some aquaculture facilities. practical experience.
References Hua, K., and Nichols, P. D. (2010). Feeding aquaculture
in an era of finite resources. Proceedings of the National
Boyd, C. E. and A. A. McNevin. (2015). Aquaculture, Academy of Science, 106(36), 15103–15110.
Resource Use, and the Environment. Wiley Blackwell, Shumway, S. E. (Ed.) (2011). Shellfish Aquaculture and the
Hoboken. Environment. John Wiley and Sons, West Sussex.
Tchobanoglous, G., Burton, F. L. and Stensel, H. D. (2003).
Boyd, C. E, McNevin, A. A., Clay, J. and Johnson, H. M. Wastewater Engineering. McGraw Hill, New York.
(2005). Certification issues for some common Tucker, C. S. and Hargreaves, J. A. (2006). Water‐level
aquaculture species. Reviews in Fisheries Science, 13, management and BMPs cut water use and pond
231–279. effluents. Global Aquaculture Advocate, 9(3), 50–51.
Tucker, C. S. and Hargreaves, J. A. (Eds.) (2008).
Diana, J. S. (2009). Aquaculture production and Environmental Best Management Practices for
biodiversity conservation. BioScience, 59(1), 27–38. Aquaculture. John Wiley and Sons, West Sussex.
Verdegem, M. C. J. and Bosma, R. H. (2009). Water
FAO (Food and Agricultural Organization) (2011). withdrawal for brackish and inland aquaculture, and
“Energy‐Smart” Food for People and Climate. FAO, options to produce more fish in ponds with present
Rome. water use. Water Policy, 11(1), 52–68.
Jescovitch, L. N., Chaney, P. and Boyd, C. E. (2014). Land to
water surface area ratio in pond aquaculture. World
Aquaculture, 45(4), 44–46.
Naylor, R. L., Hardy, R. W., Bureau, D. P., Chiu, A., Elliott,
M., Farrell, A. P., Forster, I., Gatlin, D. M., Goldburg, R. J.
113
6
Reproduction, Life Cycles and Growth
John S. Lucas and Paul C. Southgate
CHAPTER MENU 6.4 Growth, 120
6.1 Introduction, 113 6.5 Summary, 124
6.2 Reproductive Physiology, 113 References, 125
6.3 Life Cycles, 116
6.1 Introduction These cues trigger hormonal changes within the animal
brought about by stimulation of the pituitary gland.
Only the three major groups of aquaculture animals will
be considered in this chapter: fishes, bivalve molluscs Hormonal systems are often complex, and the pitui-
and decapod crustaceans (the last two being the major tary gland contains three major hormones concerned
‘shellfish’ groups). The fundamental morphologies of the with reproduction:
species in these three groups are extremely different (see
invertebrate and vertebrate textbooks), as are their 1) gonadotrophin‐release hormone (GnRH), which con-
reproductive systems, life cycles and patterns of growth. trols the release of gonadotrophin from the pituitary;
However, although the details and requirements of the
various life stages of aquaculture species vary enor- 2) gonadotrophin‐release‐inhibitory factors (GnRIF,
mously, there are some common patterns. More specific primarily dopamine), which inhibits the release of
details are included in the appropriate chapters for the gonadotrophin from the pituitary; and
various groups and species.
3) gonadotrophins (GtHs), which regulate the release of
Reproduction, life cycles and growth of seaweeds have gonadal steroid from the gonad. Gonadotrophins are
some distinctly different features to animals and they are composed of GtH I (follicle‐stimulating hormone,
treated in Chapter 15. FSH) and GtH II (luteinising hormone, LH).
6.2 Reproductive Physiology The major male and female gonadal steroids are 11α‐
ketotestosterone and 17β‐oestradiol, respectively. They
6.2.1 Fishes control the major aspects of reproduction such as repro-
The sexes in most cultured fishes are separate and their ductive behaviour, oocyte maturation, spermatogenesis
paired gonads are located dorsolaterally in the body and ovulation. These steroids also have a negative‐feed-
c avity. Reproductive activity is confined to a particular back influence on GtH production from the pituitary.
season of the year. Reproduction is usually triggered by This hormonal system is shown in Figure 6.1. Although
environmental cues, such as increase in day length or low levels of GtH are found in the blood throughout the
water temperature (in temperate and tropical species), year, final maturation of gametes and ovulation is
or changes in salinity or turbidity (in tropical species). brought about by a surge in GtH levels in response to
final environmental cues.
Knowledge of this system allows farmers to control
reproduction in captivity and obtain spawnings of high‐
quality eggs when required. Control of reproduction also
allows hatchery managers to plan for maximum food
Aquaculture: Farming Aquatic Animals and Plants, Third Edition. Edited by John S. Lucas, Paul C. Southgate and Craig S. Tucker.
© 2019 John Wiley & Sons Ltd. Published 2019 by John Wiley & Sons Ltd.
114 Aquaculture Stimulus
production for larvae and juveniles when needed. In cap- Sensory mechanisms
tivity, however, the final environmental cue for repro-
duction is often lacking. In this case eggs do not undergo Brain
final maturation and fishes do not ovulate or spawn
because of the lack of a surge in GtH levels. This problem Hypothalamus
is usually overcome in one of two ways:
1) Environmental manipulation. The environmental cues GnRH Dopamine
necessary for gamete maturation and spawning are LHRHa action Pituitary
provided. This method requires precise knowledge of HCG action +–
the factors governing reproduction in a particular spe-
cies. Important cues include water temperature, salin- GtH I GtH II Steroids
ity, photoperiod and food availability. (FSH) (LH) negative
2) Hormonal manipulation. The final GtH surge is arti- feedback
ficially attained by injection of appropriate hormones
into the fish. Although crude pituitary extract can be Gonad
used, human chorionic gonadotrophin (hCG) and
luteinising hormone‐releasing hormone analogue Gonad Final gonad
(LHRHa) are more commonly used. growth maturation
The actions of these hormones are detailed in Table 6.1
and their points of action within the maturation sequence Figure 6.1 The reproductive endocrine physiology of fishes
of fishes are shown in Figure 6.1. The development of showing the sites of action of LHRHa and HCG used to artificially
current methods used for endocrine manipulation of induce maturation. Source: Reproduced with permission from Paul
reproduction in cultured fishes is described by Zohar Southgate.
and Mylonas (2001).
Successful spawning induction depends on a number Table 6.1 The actions of human chorionic gonadotrophin (HCG)
of factors, which are outlined in the following sections. and luteinising hormone‐releasing hormone analogue (LHRHa)
in influencing reproduction in fishes.
6.2.1.1 Stage of Maturity of Brood Fish
Artificial surges in GtH will be an ineffective spawning Hormone Action
inducer unless female oocytes have previously reached a HCG
certain stage of development. To determine this, a small LHRHa Acts directly on the gonad to induce the release of
sample of developing eggs is removed from the female gonadal steroids (sex hormones)
and observed microscopically. Cannulation is used, in Acts on the pituitary gland to stimulate the
which a fine plastic tube is passed up through the oviduct release of gonadotrophins (GtH)
to the ovary. Gentle suction by the mouth at the other
end of the tube provides a small sample of eggs for ●● hCG is usually used at doses of 250–2000 IU (interna-
inspection. Eggs must possess yolk globules and be of a tional units of activity) per kg of fish.
certain size. The required size can only be determined
experimentally for each species, but, as a rule of thumb, Initially, a mid‐range dose is used, and the optimum dose
they should be at least half the diameter of eggs at spawn- is determined by trial and error. If the dose is too low, it
ing. Ripe males should exude milt (sperm suspension) will fail to induce a spawning. Too high a dose will cause
from the genital opening when pressure is applied to the final oocyte maturation to occur too rapidly and will
abdomen. result in poor egg quality. A very low dose will be effec-
tive if the oocytes are very mature (determined by
6.2.1.2 The Correct Hormone to Use cannulation).
and Hormone Dose
hCG and LHRHa are the preferred hormones for spawn- 6.2.1.3 Method of Hormone Administration
ing induction in fishes; both are available commercially Hormones are administered either intramuscularly (IM)
and available in purified form: (Figure 6.2) or intraperitoneally (IP) (into the body cav-
●● LHRHa is more effective in bringing about oocyte ity). IM injections have the disadvantage of hormone loss
when the needle is withdrawn; however, minimising the
maturation. It is usually used at a dose of 10–50 µg/kg injection volume can reduce this. IP injections avoid the
fish; and problem of hormone loss but can result in damage to
internal organs or injection into the intestine, where the
hormone will be ineffective.
Figure 6.2 Intramuscular injection of hormones into Eurasian Reproduction, Life Cycles and Growth 115
Perch, Perca fluviatilis. Source: Reproduced with permission from
Tomáš Policar. 6.2.2 Decapod Crustaceans
The sexes in cultured decapod crustaceans are separate,
Hormones can also be administered as a pellet con- and the paired gonads are located dorsally and laterally
taining the hormone. The hormone is mixed with cho- to the gut. Their reproduction and gonad maturation are
lesterol and compressed to form a pellet, which is hormonally regulated. Synthesis and release of crusta-
injected into the muscle. The advantage of using pellets cean reproductive hormones occur in response to both
is that the release of the hormone into the blood system exogenous and endogenous factors and, in the wild,
occurs more evenly and does not result in a sudden reproduction is closely related to seasonal cues. These
increase in hormone levels as does liquid injection. seasonal cues are similar to those for fishes, e.g., photo-
Cellulose can also be incorporated into the pellet to help period, water temperature and food availability.
regulate the rate of hormone release: the more cellulose,
the slower the rate of release. Reproduction in decapod crustaceans is controlled by
6.2.1.4 Timing of Hormone Administration hormones released from the sinus gland and associated
Hormonal induction of spawning is more successful if centres (X‐organ and Y‐organ). These glands which
the hormone is administered so that spawning occurs at secrete fundamental hormones for the body’s function-
a time of day when it would occur naturally. To achieve ing are surprisingly located within the eyestalks. The
this, knowledge of the time of natural spawnings and the hormones are gonad‐inhibiting hormone (GIH), moult‐
length of time taken between hormone administration inhibiting hormone (MIH) and several other hormones.
and ovulation (‘latent period’) is required. For example, A commonly used technique to induce reproductive
barramundi spawn naturally after dark and have a latent maturation in shrimps is eyestalk ablation. This removes
period of ca. 36 hr. Therefore, the hormone is adminis- the eye, together with the eyestalk and its source of
tered at around 7am to induce a spawning after dark the hormones. Eyestalk ablation is usually unilateral (applied
following night. to one eye only) and is achieved by:
As detailed above, GnRIF inhibits the release of GtH ●● cutting off the eyestalk; or
within the pituitary (Figure 6.1). GnRIF is actually dopa- ●● cauterising, crushing or ligaturing the eyestalk.
mine, and dopamine antagonists such as pimozide and In females, ablation results in an increase in total ovarian
domperidone can block its inhibitory action. Thus, mass, owing to the acceleration of primary vitellogenesis
spawning may be facilitated by administering a dopa- and the onset of secondary vitellogenesis. In males, abla-
mine antagonist to brood fish. The use of a combination tion induces spermatogenesis, enlargement of the vas
of GnRIF analogue (e.g., LHRHa) and a dopamine antag- deferens and hypersecretion in the androgenic gland.
onist (e.g., domperidone) to induce ovulation and spawn- Eyestalk ablation also removes the source of MIH and
ing of cultured fishes is known as the ‘Linpe method’. A other compounds that control moulting (ecdysis).
mix of LHRHa (stimulating GtH) and pimozide (inhibit- Reproduction in decapod crustaceans is often character-
ing GnRIF) is now available commercially and spawning ised by a pre‐copulation or pre‐spawning moult.
kits are available for the major groups of cultured fishes
such as carps and salmonids. Gonad maturation after ablation can be very rapid in
shrimps, and females can develop full ovaries within 3–4
days. As in other arthropods, the spermatozoa of deca-
pod crustaceans have no flagella and are non‐motile. At
mating, the male inserts a spermatophore (a large bundle
of the non‐motile spermatozoa) into a ventral receptacle
for the spermatophore (thelycum) or onto the ventral
surface of a female shrimp near her gonadophore.
Fertilisation occurs externally upon ovulation and pas-
sage of the oocytes through the gonadophore, and the
eggs are shed into the water column. Spermatophores
may remain implanted through several ovarian matura-
tion cycles and may fertilise as many as six spawnings
within one moult cycle. The spermatophores are lost
with the moulted exoskeleton.
Copulation in decapod crustaceans typically involves a
soft‐shelled female that has just moulted and a hard‐
shelled male. As decapod crustaceans moult regularly
(section 6.4.2), it means that fertilisation can be reliably
116 Aquaculture
Spawning Figure 6.3 Generalised life cycle of a pearl
oyster (Pinctada species) showing the
Egg and sperm phases of culture. Source: Reproduced with
Fertilised egg permission from Paul Southgate.
Veliger larva
Adults Larval rearing and
early nursery culture
Nursery culture and (land based)
grow-out
(ocean-based) Umbone veliger
Metamorphosis
Spat/juvenile
obtained by keeping female and male broodstock together factors and levels of endogenous energy reserves. Gonad
in appropriate conditions. This is used for hatchery pro- maturation in bivalves relies on both direct food intake
duction in freshwater prawns, freshwater crayfish, crabs, and utilisation of endogenous energy reserves. Water
lobsters and some shrimp species. In the groups other temperature is perhaps the major influence on gonad
than shrimps, the fertilised eggs are not shed into the maturation in bivalves. However, the relationship
water column but are retained on the female’s abdominal between increased water temperature and increased
appendages (pleopods) until the larvae or juveniles hatch natural phytoplankton production (food availability) is
(Figure 23.11). Females brooding eggs on their pleopods also very important.
are known as ‘berried’ or ovigerous females. More
detailed information on the reproductive biology of deca- Major spawning cues for bivalves include a change in
pod crustaceans is provided by Mente (2008). water temperature, a change in salinity, lunar periodicity
and chemicals (pheromones) associated with water‐
6.2.3 Bivalve Molluscs borne gametes from other individuals. With the excep-
Most species of cultured bivalves have separate sexes, tion of lunar periodicity, these are commonly used to
but the total situation within the group is more complex induce spawnings in bivalve hatcheries (section 6.3.2).
than in cultured fishes and decapod crustaceans. Some Gametes are generally liberated into the surrounding
cultured bivalves, e.g., some scallops, are hermaphro- water where they are fertilised. In some species (e.g.,
dites with eggs and sperm produced simultaneously. Ostrea edulis), fertilised eggs are retained in the mantle
Other bivalves change sex during development. Usually cavity, where they develop into swimming larvae before
these are protandrous, with younger individuals being being released. More detailed information on the repro-
male and older individuals being female. Some bivalves ductive biology of bivalve molluscs can be found in
may undergo more than one change (often annually) Chapter 24 and in Gosling (2002).
in functional sexuality and are said to show rhythmic
consecutive hermaphroditism (e.g., Ostrea species). 6.3 Life Cycles
Although the causes of sex change in bivalves are poorly
understood, gene‐activated components that respond to 6.3.1 Sequence of Stages
environmental factors, sex‐determining genes and food At breeding, mature adults shed their eggs and sperm
supply have all been suggested. freely into the water or the male impregnates the female.
Fertilisation results in a zygote and subsequently an
As in fishes and decapod crustaceans, gamete matura- embryo, which develops within the egg. Embryonic
tion in bivalves is influenced by a number of exogenous development occurs over a period of hours or days,
factors, including water temperature, food availability, depending on the species and temperature. In a typical
light intensity and lunar periodicity. It is also influenced life cycle for a species with planktonic larvae, the embryo
by endogenous factors including hormones, genetic emerges from the egg as a swimming larva that develops
Generalised life-cycle in aquaculture Reproduction, Life Cycles and Growth 117
Broodstock Hatchery Nursery particular genetic traits (Chapter 7). In many cases the
from wild Larvae Juveniles broodstock are simply obtained from the field (Figure 6.4)
and there is no opportunity for selective breeding to
Selected improve the aquaculture‐related attributes of the spe-
Broodstock cies. As described, most aquatic animals become sexu-
ally mature and reproduce during the warmer months of
Maturity Grow-out the year in response to increasing photoperiod, food
availability and rising water temperature. Knowing the
Commercial major cues for gonad maturation, conditions in an aqua-
size culture hatchery can be manipulated to bring on sexual
maturity outside natural reproductive seasons. This is
MARKET known as broodstock conditioning, and it allows hatch-
ery production on a year‐round basis.
Figure 6.4 How typical stages in aquaculture relate to the 6.3.3 Spawning
generalised life‐cycle stages of animals in aquaculture.
Source: Reproduced with permission from John Lucas. Methods of inducing spawning in individual species and
groups are described in detail in other chapters. However,
into a juvenile over a period of days, weeks or even, in it is appropriate to summarise these methods in five
some species, months. This may be a progressive pro- categories:
cess, as in shrimps and most fish; or the larva may
abruptly settle out of the plankton onto an appropriate 1) Mild stress, such as abrupt temperature increases,
substrate and metamorphose into a juvenile, as in e.g., with bivalves.
bivalves and other benthic invertebrates. The juvenile
then grows progressively over a period of months or 2) Manipulation of the body’s reproductive hormones
years until it reaches sexual maturity and the life cycle is by injection or implantation of reproductive hor-
completed (Figure 6.3). mones into the body in fishes or by eyestalk ablation
in shrimps.
From the viewpoint of aquaculture, these life cycles
can be divided into the following sequence of stages: 3) Transfer of gametes from a container where animals
●● broodstock conditioning to produce ripe adults for have spawned to one where they have not and thus
allowing the pheromones associated with gametes to
spawning; trigger responses from the recipients, e.g., bivalves.
●● spawning, either natural or induced (the latter being
4) Gonad stripping, where the broodstock is known to
more common in aquaculture); have ripe gametes. The body is appropriately mas-
●● egg fertilisation; saged to expel gametes, e.g., fishes (Figure 6.5,
●● larval rearing; Figure 17.2), or gonads are removed from a sacrificed
●● postlarval and juvenile rearing; animal and lacerated to release gametes, e.g., bivalves
●● grow‐out rearing to commercial size; or (section 24.4.2.1).
●● grow‐out rearing to sexual maturity for use in
breeding.
In the intensive aquaculture of a species, many of these
phases in the life cycle require different culture techniques
(Figure 6.4). For example, different culture methods are
used for larvae, juveniles and grow‐out stock in the inten-
sive culture of bivalves, shrimps and fishes. In extensive
aquaculture, however, there may be very little change in
culture techniques throughout all phases of the life cycle,
e.g., extensive culture of tilapia and freshwater crayfish.
6.3.2 Broodstock Selection and Figure 6.5 Stripping eggs from pikeperch, Sander lucioperca.
Conditioning Source: Reproduced with permission from Tomáš Policar.
This involves selecting appropriate animals to serve as
sources of gametes. It may involve selecting stock with
118 Aquaculture
Figure 6.6 The life cycle of the mud crab, Scylla serrata, showing successive developmental stages. Source: Holme et al. 2009. Reproduced
with permission from Elsevier.
5) Injecting a neurotransmitter substance directly into developing embryos to be cleaned of any debris and
the gonad of a ripe animal, presumably to cause gonad chemicals from spawning induction and fertilisation.
contractions, e.g., some bivalves. The embryos need to be maintained in finely filtered
(e.g., 0.2–2 µm) water to reduce levels of bacteria,
It is evident that the greatest range of induced spawning which may potentially invade the surfaces of the eggs.
techniques is used with bivalves (section 21.4.2). To an Developing embryos must also be provided with
extent this reflects the fact that they are more readily a dequate levels of DO.
induced to spawn than fishes or shrimps.
The larval stages of most fishes, crustaceans
6.3.4 Egg Fertilisation (Figure 6.6) and bivalves (Figure 26.13) are plankto-
Where the broodstock sexes are segregated, fertilisation trophic, i.e., dependent on exogenous food supplies for
involves mixing an appropriate amount of sperm sus- much of their development. When larvae first hatch,
pension with a suspension of eggs. The objective is to however, they usually have sufficient energy reserves in
run a course between low fertilisation rates, as a result of the yolk to support development for a day or more before
insufficient sperm, and polyspermy, where eggs are pen- they need to feed (Figures 19.7). These reserves were
etrated by a number of sperm. When an egg is penetrated present in the egg and were passed on from the mother.
by a sperm, its surface often changes to prevent further Fish larvae, for example, do not develop a functional gut
sperm penetration. If, however, sperm are very abun- until some time after hatching: they depend on endoge-
dant, other penetrations may occur before the barrier nous reserves until they can support themselves by
is established and the egg then contains two or more feeding on appropriate foods in their environment
haploid nuclei from sperm. These may result in a poly- (Figure 6.7). The embryos and early larval stages of most
ploid (2+n) zygote and the subsequent embryo develops aquatic animals utilise lipids and proteins from yolk
abnormally. reserves to fuel development to this point. It has been
estimated, for example, that embryonic development in
6.3.5 Larval Rearing some bivalves utilises about 70% of the lipid reserves of
Embryonic development is usually a brief process, from the egg. Bivalve larvae must again rely on endogenous
a few hours to several days duration. It requires the energy reserves at the end of larval development, during
metamorphosis, when they temporarily lose the ability
to feed. Unlike fishes and crustacean larvae, which retain
the ability to feed during the steady transition from larva
Feeding Reproduction, Life Cycles and Growth 119
Endogenous
behaviour also occurs in some freshwater and marine
Fe fishes that have normal larval stages.)
E
Direct development occurs in freshwater crayfish, but
H giant freshwater prawns, Macrobrachium species, and
the mitten crab, Eriocheir sinensis, have larval stages.
Mixed They must migrate downstream to saline water to release
the larvae, which cannot survive in freshwater.
S
Planktotrophic larval development is often the most
Exogenous demanding phase of the life cycle for aquaculture, and it
can last from several days to many weeks (or even months
Re in the case of rock lobsters, section 2.8.1). The larvae are
physically and physiologically fragile. They require well‐
Figure 6.7 Diagram showing early development in salmonids. controlled environmental parameters, such as DO, pH,
E, eye development of embryo inside the egg; Fe, fertilised egg; N waste levels, bacterial populations, levels of organic
H, hatching with large yolk sac; S, larva beginning to seek and inorganic particles, and temperature. They are often
external food; Re, completion of yolk sac resorption and total reared in large tanks to buffer these environmental
reliance on exogenous nutrition. Source: Kamler 1992. parameters; but tank culture also has hazards, such as
Reproduced with permission from Springer. Adapted from contamination and pathogenic bacterial blooms.
Raciborski 1987.
As the larvae switch from relying on their endogenous
to juvenile, bivalve larvae must accumulate substantial yolk reserves to feeding, providing appropriate food
energy reserves during larval development. In some becomes a major aspect of larval culture. There are two
gastropod species, e.g., abalone and trochus, the eggs main forms of food:
contain sufficient nutrient reserves to support larval ●● microalgae for the filter‐feeding larvae of molluscs and
development through to the juvenile phase without the
larva having to feed. This lecithotrophic development is shrimps; and
usually relatively brief and associated with relatively ●● rotifers and brine shrimp nauplii for the larvae of fishes
large and yolky eggs.
and older larvae of shrimps (Chapter 9).
The life cycles of freshwater animals often differ from Larval density is important in hatchery culture. The
this general pattern in that planktonic larval stages are advantage of high larval density is that more postlarvae
omitted or much abbreviated. This is known as direct can be produced per unit volume of culture tank.
development. Freshwater species with direct develop- However, if larval density is too high, it may compromise
ment typically produce larger and yolkier eggs than water quality and affect larval growth and survival. The
related marine animals (Figure 23.11). These large eggs culture tank water is usually changed at regular intervals
support a comparatively longer embryonic period to to maintain water quality. The density of larvae is
result in the hatching of juveniles or near‐juveniles. The reduced as they grow, despite inevitable attrition that
eggs are usually protected during this long embryonic reduces numbers. If 1 larva/mL of culture water com-
development and the adult may carry them, e.g., on the pletes development, this corresponds to one million lar-
female crayfish’s pleopods or in the mouths of some vae from 1000 L of culture water. Aquaculture hatcheries
fishes, or there may be ‘nests’ and patterns of reproduc- use larval rearing tanks up to 20 000 L and produce tens
tive behaviour to protect broods of eggs. (Egg‐protecting of millions of postlarvae per rearing (Figure 3.9).
Ponds may be used to provide very larger volumes for
rearing larvae. Even with low densities of larvae this can
result in very large numbers of postlarvae, e.g., tonnes of
megalopae of Eriocheir sinensis are produced in this way
in China (section 23.3.3.3).
6.3.6 Postlarval and Juvenile Rearing
It is necessary to provide appropriate substrates in the
culture system to induce bivalve larvae to settle (sec-
tion 24.4.2.4). The larvae are selective because, at settle-
ment and metamorphosis, they attach and make a habitat
commitment for the remainder of their lives. For fishes
and shrimp larvae there is no abrupt metamorphosis.
120 Aquaculture culture process involves a hatchery or hatchery/nursery
selling juveniles to farmers, because the hatchery aspect
There is, however, a progressive change in behaviour of is most technically demanding and because one hatch-
shrimp postlarvae as they become more substrate ori- ery can supply many farms. In some Asian countries,
ented. The same pattern occurs in benthic fishes. such as Taiwan, this process is taken further. There is a
series of specialised facilities with expert technical staff
Marine and brackish water crabs, such as portunid who culture specific early‐development stages of fishes.
swimming crabs, are another example of an abrupt met- During development, the fishes may be sold on to other
amorphosis from planktonic larval stages to the first facilities a number of times. Each facility may culture
benthic juvenile stage. In this case it is via a transitional equivalent stages of a number of species.
megalopa stage.
Many aquaculture industries rely on natural recruit-
The early part of this period is often a particularly dif- ment of juveniles, thereby omitting the more technically
ficult one for rearing fishes, as the post‐metamorphic demanding phases of aquaculture outlined in sections
juveniles must be progressively weaned off live feeds onto 6.3.2 to 6.3.6. Relying on natural recruitment is possible
artificial diets. Standard procedures have been developed by knowing the times and localities of this recruitment,
for the major culture species (section 9.5.1), but this is and other biological information about the recruits, such
often one of the particular points of difficulty in develop- as settlement substrate. It may be possible to stock
ing the culture process for a new fish species. coastal ponds with recruits, for example, juvenile fishes,
shrimps or crabs, by filling the ponds at high tides when
Post‐metamorphic juveniles of fishes, decapod crusta- juveniles are abundant in the adjacent inshore waters.
ceans and bivalves are very small, physiologically fragile Alternatively, the recruits may be netted from coastal
and vulnerable to predation. Over a period of days to water when they are abundant and then stocked into
months or more than a year, they are grown in protected ponds. In the case of bivalve larvae, it is a matter of pro-
conditions, before they are put out into the grow‐out viding appropriate substrates to attract late‐stage larvae
environment. This may involve a period of culture in to settle from the water column (Figure 24.8). In freshwa-
tanks at the hatchery and an onshore nursery, followed ter ponds, the adults may be allowed to mate and care for
by a period of protected culture in ponds or in the field. their eggs, and then the juveniles are removed at an
Care is required to minimise mortality when transferring appropriate stage. There may still be several commercial
juveniles from the relative comfort of the hatchery (e.g., stages in this process, whereby juveniles from the field,
adequate feed and filtered water, which may be heated obtained from privately‐owned settlement substrates or
above ambient temperature to pond or field nursery con- by fishing, are sold to farmers for grow‐out.
ditions with cooler water, fluctuating food supply, preda-
tors, disease and fouling. Natural recruitment is often the start of an extensive or
semi‐intensive culture process, i.e., where the stocking
6.3.7 Grow‐out Rearing rate is low to moderate, and capital is limited. It is the
basis of many major aquaculture industries, but there are
Grow‐out rearing is the final phase, during which the inherent problems. As in fisheries, that natural recruit-
juveniles are put out into the adult environment and ment levels vary from year to year and are not completely
reared until being harvested (Figure 6.4). They are predictable in time. Fishing pressure on the wild adult
not treated in the same manner throughout this phase: stock may reduce its reproductive output and hence nat-
factors such as food pellet size, pond size and mesh ural recruitment. Another disadvantage of relying on
sizes of protective or enclosing nets, may be varied as natural recruitment is that there is no opportunity for
they grow. stock improvement. It is impossible to select for desira-
ble traits for aquaculture.
6.3.8 Other Considerations
6.4 Growth
In some cases, the whole culture process from spawn-
ing broodstock to final marketing takes place on one 6.4.1 Size vs. Age
farm. In other cases, the cultured organisms may be
sold several times during the culture process. There are two patterns of growth occurring simultane-
Broodstock may be captured by fishing or reared by a ously in many developing organisms.
specialist farm and sold to the hatchery. Late larval
stages or early juveniles are often sold from a specialist 1) Absolute growth rate, e.g., rate of increase in length
hatchery to nursery growers. Juveniles ready to be or mass per unit time, which is quite insignificant
grown out may be sold from a specialist nursery to initially, when the animal is very small (Figure 6.8).
grow‐out farms. This is because the different phases of
culture require particular expertise and facilities, and
there is efficiency of specialisation. Most typically, the
Reproduction, Life Cycles and Growth 121
the relationship between size and age. One form of the
VBGF is:
Size Lt L 1 be Kt 1
Absolute growth rate
Length (mm) Relative growth rate
mm per year where Lt is size at time t; L∞ is maximum size; b and K are
growth coefficients; b is related to the ratio of maximum
size to initial size; and K is the growth coefficient, often
used in comparisons of growth rates between popula-
tions and species.
However, over short periods of measurement (months),
when the animals are in the exponential (rapid) phase of
growth, a relative growth coefficient can be obtained
from the following equation:
k ln L2 ln L1
t2 t1
Per cent increase per day where L1 is size at initial time t1 and L2 is size at final
time t2
This growth coefficient, k, is readily calculated and
is widely used in comparative studies.
Age 6.4.2 Growth in Decapod Crustaceans
Figure 6.8 Generalised curves of size, absolute growth rate and Growth in decapod crustaceans basically follows the pat-
relative growth rate vs. age. Source: Reprinted from Malouf & tern outlined in section 6.4.1. The pattern of growth is,
Bricelj (1989) with permission from Elsevier Science. however, complicated by moulting, whereby the animal
sheds its old exoskeleton (Figure 6.9) and expands rapidly
Then, as the animal grows, its capacity to feed and during a short period before a new exoskeleton hardens.
assimilate food progressively increases. The absolute Thus, growth appears to occur in a series of abrupt steps
growth rate increases and the size vs. age curve rather than as a continuous process (Figure 6.10). The
becomes steeper. This is known as the exponential details of the moult cycle are outlined in Table 6.2.
phase of growth.
2) Relative growth rate, e.g., growth increment / unit Figure 6.9 A pile of moulted shells (exuviae) from mitten crabs,
body mass / unit time, is very rapid during the early Eriocheir sinensis. Source: Reproduced with permission from John
phases of an animal’s life cycle (Figure 6.8). Relative Lucas.
growth may be 200% and more during the first week
or so of development and remain at a high level dur-
ing the early months.
The size vs. age relationship reflects the changing pat-
tern of absolute growth, producing a sinusoidal curve
(Figure 6.8). There is an upper inflection in the size vs.
age curve, and the upper curve typically slopes gently
towards a final theoretical size, L∞, which it would reach
if the animal lived indefinitely.
Some complex logistic equations, such as the von
Bertalanffy growth function (VBGF), are used to describe
122 Aquaculture
Moult soft-shelled body intermoult period. This is the critical period of growth,
body inflation although there is no change in external dimensions: body
size is constrained by the exoskeleton. The animal does
Size Feeding and not feed during the periods immediately before, during
tissue growth and after moulting. Its shell is too soft for feeding and it
(instar) is vulnerable. It is ironic that it is the period of non‐
growth, although it is the period when the soft‐shell
Rapid moulting phase body expands rapidly by absorbing water.
(larvae juveniles) 6.4.3 Energetics of Growth
The growth rate depends on the extent to which net
Time energy intake exceeds metabolic rate and to what extent
Figure 6.10 Staircase‐like pattern of growth of decapod energy is diverted from growth to reproduction in mature
crustaceans with moults to expand in size by ‘inflation’ animals (section 8.5). Energy committed to gametogene-
alternating with hard‐shelled periods (instars) during which sis is a major component of the body’s resources in many
the animal is fixed in size externally, but it feeds and grows tissue. aquaculture animals. Hence gametogenesis is at least part
Source: Reproduced with permission from John Lucas. of the cause for growth tapering off with age.
The decapod crustacean’s life is divided into a series of The energy for growth may be expressed as a summa-
ntermoult periods or instars, which may be numbered tion of input and outputs.
consecutively for convenience. These are punctuated by
moults (or ecdyses) and the animal continues to moult, G I M E F R
unless it goes into a final terminal instar when it ceases to where G is the growth rate; I is the food ingestion rate;
moult (terminal anecdysis). The moults are more fre- M is the metabolic rate; E is the excretion rate of waste
quent earlier in life and the size increments at moults are molecules from metabolism; F is faeces production; R
relatively larger, but absolutely smaller. The result is a is gametogenesis; and [I – (E + F)] is the net rate of
size vs. age relationship that fundamentally looks like the energy intake, i.e., food intake – (excreted wastes and
relationship in Figure 6.8, but with a stepwise pattern faeces). The equation can be expressed as net rate of
instead of a smooth curve (Figure 6.10). energy intake less energy spent of metabolism and
reproduction:
The processes of:
●● moulting to expand body dimensions; and G I E F M R
●● growth, in terms of adding organic tissue and energy content. or
are completely independent (Table 6.2). The decapod
crustacean feeds and builds up organic tissue during the G I E F M during immaturity.
Table 6.2 Events in the moult cycles of decapod crustaceans, from one inter‐moult period (instar X) to the next (instar X + 1).
Phase Duration1 External body Organic tissue mass Behaviour Shell
size
Intermoult Days–weeks Fixed Increasing Feeding; locomotion Hard
(instar X)
Pre‐moult Hours–several Little change Decreasing = > increased Ceasing feeding; may seek Shell decalcifying
days Rapid organics in body fluids cover
Moult Minutes–hours increase Low Rapidly shedding old shell; Old shell discarded;
no feeding; may be new soft shell
Post‐moult Hours–days Little change Begins increasing concealed
Intermoult Days–weeks Fixed Increasing Begins feeding Hardening
(instar X+ 1) (months2) Feeding, locomotion Hard
1 The durations of the stages are correlated with the animal’s size: small size = short durations: large size = long durations.
2 Last instars of some large species, e.g., marine lobsters, mud crabs, which may cease moulting (terminal anecdysis)
6.4.4 Measuring Growth Reproduction, Life Cycles and Growth 123
6.4.4.1 General Figure 6.11 A laboratory muffle furnace of the kind that is used to
Growth rate often needs to be measured accurately. It is deternmine ash‐free dry weight. Metal or porcellaine crucibles are
a means of predicting when the stock will be ready for used to contain samples in the furnace and tongs are used to
harvest. Suboptimum growth rates draw attention to fac- handle the crucibles. Source: Cjp24 2009. Reproduced under the
tors such as poor health, inadequacy of feeds and nutri- terms of the Creative Commons Attribution Share‐Alike Licence,
tion, poor environmental quality, poor source of stock, CC BY‐SA 3.0.
etc., that may be adversely influencing performance.
the prerogative of laboratory studies that require very
The change in size of a substantial representative sam- accurate data. Linear measurements and wet weights are
ple of the cultured organisms must be measured to used to assess size or growth on farms.
determine the growth rate. For example, an appropriate
sample from each pond on a shrimp farm may be at least It is often sufficient for the farmer to observe meas-
100 individuals. The size of aquaculture organisms may ured growth, but in some circumstances, there may be
be measured in various ways, but different methods are a need to calculate growth rates, e.g., for comparisons
more appropriate for seaweeds, fishes, bivalves and crus- of growth rate through the seasons or between
taceans. The four main measurements of size used in batches. The growth rate may be calculated on the
aquaculture are: assumption of linear growth, i.e., change in size over
period of measurement. However, as illustrated in
1) A linear dimension measured with callipers or a ruler, Figure 6.8, growth is not linear. The growth coefficient
according to size. The advantages of this method are provided in section 6.4.1 can be used as a more accu-
that it is quick and easy, and the animal is not unduly rate value for comparisons of relative growth rate,
harmed. The disadvantages are that it cannot be used provided the animals are in the appropriate phase of
for flexible organisms (e.g., seaweeds) and it does not growth.
give an indication of the condition (fat/thin) of the 6.4.4.2 Fishes
animal. Fish length has long been used as an assessment of size
and growth. Two measurements are typically used:
2) Wet weight of the whole living organism, weighed 1) Standard length (SL). SL is measured from the tip of
on scales. The advantage of this method is that it
is quick and easy, and the animal is not unduly the snout to the base of the caudal fin (tail). In fish
harmed. larvae, however, SL is from the tip of the snout to the
tip of the notochord.
3) Dry weight. The dead organism, or a quantified piece
of it, is dried in an oven at ~50 °C until all moisture
has evaporated. The advantages of this method are
that it gives a tissue weight value that is not compli-
cated by water content and the value is relatively
quickly and easily obtained.
4) Ash‐free dry weight (AFDW). Once the dry weight of
an organism (or piece of it) is determined, it is heated
in a furnace at ca. 500 °C for ca. 24 hr. All organic mol-
ecules are burnt off in the furnace, leaving only inor-
ganic ash. The weight of ash is then subtracted from
the dry weight. This gives the AFDW, which is the
dry weight of organic matter in the animal. Animal or
tissue sample dry weight – AFDW = dry weight of
organic matter. Knowing the amount of organic mat-
ter in an animal is a very accurate measure of size. A
particular advantage of AFDW determinations is that
it is accurate for animals with large inorganic compo-
nents, such as shells (e.g., bivalves), for which it is dif-
ficult to estimate organic tissue content because it is
relatively small. A less accurate alternative to AFDW
is to use Condition Index (section 6.4.4.3).
Determining dry weight and AFDW requires specialised
equipment such as a drying oven, 500 °C furnace
(Figure 6.11) and a sensitive balance. As such, these are
124 Aquaculture lengths (CL) measured. These latter groups of crustaceans
have well‐developed abdomens. These tend to flex vigor-
2) Total length (TL). TL is measured from the tip of the ously when they are held, hindering total length measure-
snout to the tip of the caudal fin. However, measuring ments, which are therefore not usually used.
TL may be inaccurate if there is damage to the caudal
fin, and it is difficult to measure accurately in larvae Because of the moult cycle, the water content of deca-
and juveniles. pod crustacean tissues is very variable and wet weight
measurements of a few individuals are dubious. Wet
As noted previously, length takes no account of fatness weight measurements of large groups of individuals are
or condition of a fish and may therefore be a poor guide more reliable as they accommodate the range of moult
to tissue growth. Consequently, wet weight is also fre- conditions in the group. The mean wet weight is deter-
quently used for size and growth assessment, usually in mined, and this has the advantage of being more rapid
combination with length. than taking individual carapace measurements.
6.4.4.3 Bivalve Molluscs Frequency of moulting is a fair indication of relative
Bivalves are usually assessed in terms of shell dimen- growth rate in a cultured population of decapod crusta-
sions, e.g., shell length (SL), shell height (SH) and shell ceans, but it is difficult to assess. It is no substitute for
width (SW). If only one dimension is used, as is often the size measurements.
case, it is the largest dimension of the shell. This is not
the same in all bivalves, e.g., the largest shell dimension 6.5 Summary
is SH in oysters, SL in mussels and often SW in clams.
●● The sexes in most cultured fishes are separate.
The major problem with shell dimensions as a measure Reproductive activity is naturally confined to a par-
of growth is that they take no account of tissue growth. ticular season of the year and usually responds to envi-
The bivalve shell tends to grow, regardless of the condi- ronmental cues, such as increase in day length or water
tion of the animal. The shell may increase at a slow rate temperature. These trigger hormonal changes con-
while tissue mass declines. This can occur during peri- trolled by the pituitary gland.
ods of low phytoplankton abundance or adverse environ-
mental conditions or disease. Shell increments in these ●● Knowledge of these cues and of hormonal control of
circumstances only show apparent growth and this is reproduction allows control of fish reproduction in
where AFDW determinations would show the true state hatcheries through environmental manipulation and
of the tissue and bivalve health. by hormonal injection.
There is also a simpler method for determining the ●● Reproduction in decapod crustaceans is controlled by
relative amount of tissue in relation to the shell. This is hormones released from the sinus gland and associ-
called the condition index (CI) and is a quantitative ated centres (X‐organ and Y‐organ) located within the
measure used in commercial aquaculture, primarily to eyestalks. The hormones include gonad‐inhibiting
assess reproductive condition: hormone (GIH) and moult‐inhibiting hormone (MIH)
and a commonly used technique to induce reproduc-
CI wet weight of meat(g)/volume of shell cavity(mL) tive maturation in shrimps is eyestalk ablation. Eyestalk
ablation is usually unilateral (applied to one eye only).
or
●● Most bivalves have separate sexes and reproductive
CI dry weight of meat (g)/volume of shell cavity (mL) maturation is influenced by exogenous factors such as
water temperature and food availability. Major spawn-
Volume of shell cavity can be determined by immers- ing cues for bivalves include a change in water tem-
ing an intact bivalve and then measuring the displaced perature, a change in salinity, lunar periodicity and
water. chemicals (pheromones) associated with water‐borne
gametes from other individuals. Gametes are generally
The CI is used for table oysters and is more concerned liberated into the surrounding water where they are
with gonad condition and how ‘fat’ the oysters are than fertilised.
with tissue growth per se.
●● The life cycle of aquaculture animal includes: brood-
6.4.4.4 Decapod Crustaceans stock conditioning to produce ripe adults for spawn-
Decapod crustaceans tend to be measured by a linear ing; spawning, either naturally or induced (the latter
measurement of their carapace. Crabs, with a carapace being more common in aquaculture); egg fertilisation;
that is broader than it is long, have their maximum cara- larval rearing; postlarval and juvenile rearing; grow‐
pace width (CW) measured. Other decapod crustaceans, out rearing to commercial size; or grow‐out rearing to
shrimps, freshwater prawns, freshwater crayfish, etc., with sexual maturity for use in breeding.
carapaces that are longer than broad have their carapace
●● Growth in decapod crustaceans is complicated by Reproduction, Life Cycles and Growth 125
moulting and growth appears to occur in a series of
abrupt steps rather than as a continuous process. The ●● Linear measurements are most often used to assess
decapod crustacean’s life is divided into a series of growth rates of aquaculture animals, but determin-
intermoult periods or instars. ing dry weight or ash‐free dry weight (organics con-
tent) provides a more accurate measure of tissue
growth.
R eferences Malouf, R.E. and Bricelj, V.M. (1989). Comparative biology
of clams: environmental tolerances, feeding and growth.
Raciborski, K. (1987). Energy and protein transformation In: Clam mariculture in North America, J. Manzi, M.
in sea trout (Salmo trutta L.) larvae during transition Castagna (eds.), Elsevier, New York. pp. 23–73.
from yolk to external food. Polskie Archiwum
Hydrobiologii, 34, 437–502. Mente, E. (2008). Reproductive Biology ofCcrustaceans:
Case Studies of Decapod Crustaceans. CRC Press. 565 pp.
Gosling, E. (2002). Bivalve Molluscs, Biology, Ecology and
Culture. Fishing News Books. Blackwell Publishing, UK. Zohar, Y. and Mylonas, C.C. (2001). Endocrine
443 pp. manipulations of spawning in cultured fish: from
hormones to genes. Aquaculture, 197, 99–136.
Kamler, E. (1992). Early Life History of Fish: An Energetics
Approach. Chapman & Hall, London. 267 pp.
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