003_[John_S_Lucas,_Paul_C_Southgate,_Craig_S_Tucker]_A(z-lib.org)

Water Quality 73

the day, but low DO at night can stress or kill the farmed 4.3 ­Effects of Water Quality
species (Figure 4.6). on Aquatic Animals

When daily feeding rates exceed 30–40 kg/ha, DO The effects of these water‐quality variables tend to fol-
concentration will often fall to less than 3 or 4 mg/L at low three patterns (Figure 4.8). In Figure 4.8 ‘Performance’
night. Low DO stresses cultured species, so mechanical includes factors such as level of feeding, growth, behav-
aeration is necessary at higher daily feeding rates iour, survival, fecundity and immunity.
(s­ection  4.4.3). Dissolved oxygen levels must be moni-
tored, even in ponds with mechanical aeration, to avoid 4.3.1  Water Temperature
excessive feed input for the amount of aeration applied. Temperature is perhaps the most important water‐qual-
Water exchange is sometimes used to flush plankton and ity variable because it directly or indirectly affects other
nutrients from ponds and improve water quality. water‐quality variables, natural productivity and culture
However, this practice can lead to water‐quality species. Managing water temperature is economically
d­ eterioration in receiving water bodies. unfeasible in outdoor aquaculture, and farmed species
that grow well at the water temperature range of a par-
Ponds have a large capacity to assimilate wastes ticular site must be selected.
resulting from addition of feeds. As mentioned above,
bacteria mineralise organic matter to carbon dioxide, Performance(a)
ammonia and phosphate. Ammonia is lost to the atmos-
phere by volatilisation (diffusion). It is also oxidised to temperature
non‐toxic nitrate by nitrifying bacteria. Nitrate can be pH
denitrified to nitrogen gas, which diffuses into the salinity
atmosphere. Carbon dioxide is converted to organic
carbon by photosynthesis or diffuses from pond waters Performance(b)
to the atmosphere. Bacteria can transform carbon diox- ammonia
ide in sediment to methane, which also diffuses to the Performance nitrite
atmosphere. Sediment usually has a large capacity to fix toxic pollutants
phosphorus in insoluble iron, aluminium and calcium
phosphates. Some of the organic matter in ponds is (c)
relatively resistant to microbial decay and accumulates
in sediment as stable organic matter. Although the oxygen
waste‐treatment capacity of ponds is large, many ponds food availability
receive greater inputs of nutrients in feed than can be
assimilated quickly, and water quality deteriorates. Increasing value of component
When this happens, the cultured species will be Figure 4.8  General responses of aquatic animals to increasing
adversely affected and feed conversion efficiency will levels of environmental variables. (a) A distinct optimum level
decline. with decreasing performance at high and low levels. (b)
Decreasing performance as levels increase. (c) Increasing
When ponds are drained for harvest, organic matter performance as levels increase (although performance may
and nutrients in the water are discharged. The decom- decrease at very high level). Source: Reproduced with permission
position of new organic matter in pond bottoms can be from Craig Tucker, 2017.
hastened by allowing pond bottoms to dry and crack to
facilitate aeration (Figure 22.9). By doing this each time
a pond is drained, relatively low organic matter con-
centrations can be maintained indefinitely in pond
bottoms. Of course, pond bottoms may eventually
become saturated with phosphorus, which can lead to
greater dissolved phosphorus concentrations and
denser blooms of algae.

Feed inputs to ponds must not exceed the capacity of
ponds to assimilate the resulting wastes. The most relia-
ble way of determining how well a pond is assimilating
wastes from feeding is to monitor early morning DO
concentrations. Feeding and mechanical aeration rates
must be adjusted to maintain DO concentrations above
3–4 mg/L in the early morning when the lowest concen-
trations occur.

74 Aquaculture
Table 4.4  Critical water temperatures for six representative fish species.

Species Lower critical1 range (°C) Optimum2,3 Upper critical range (°C)
range (°C)

Oreochromis mossambicus (Java tilapia) 10–14 18‐(28‐32)‐35 36–42
Sciaenops ocellatus (red drum)  8–15 18‐(22‐28)‐30 34–40
Ictalurus punctatus (channel catfish)  0–10 15‐(25‐30)‐34 35–40
Micropterus salmoides (largemouth bass)  0–10 12‐(25‐30)‐32 32–38
Morone saxatilis (striped bass)  0–6 10‐(14‐24)‐28 30–34
Oncorhynchus mykiss (rainbow trout)  0–4 5‐(10‐16)‐20 22–26

1 Upper and lower critical temperatures describe the ranges over which significant disturbances in metabolism may occur even if the fish are
slowly acclimated to those temperatures. The more extreme value in the range is an approximation of the critical thermal maximum or
minimum—the temperature at which only brief survival is expected.
2 The optimum range is the temperature range over which feeding occurs.
3 Values in parentheses are the ranges for fastest growth.

Increasing temperature increases the rate of physical restriction of ion loss. Freshwater fish tend to accumu-
processes, chemical reactions and metabolism and late water because it is hypertonic to the environment, so
growth of organisms. Chemical reaction rates double it must excrete water and retain ions. On the other hand,
with a 10 °C increase in temperature. This relationship osmoregulation for marine fish requires constant intake
applies to aquatic animal growth within their range of of water and excretion of ions. Because marine fish are
temperature tolerance. The factor of increase is called hypotonic to the environment, they lose water. To replace
the Q10, which is usually about 2 for aquatic animals. Of this water, the fish takes in salt water; but to prevent the
course, if temperature exceeds optimum, the growth of accumulation of excess salt, it must excrete salt.
aquatic animals will decline.
Invertebrates such as decapod crustaceans and bivalves
Water temperature ranges for some common aquacul- tend to conform to their environmental salinity more
ture species are provided in Table 4.4. This table gives the than fish. They tend to be isotonic in seawater, although
lower critical range, the range over which feeding occurs, the ion concentrations of their body fluids differ from
the optimum range for growth and the upper critical range. seawater. Thus, they must regulate the uptake and loss of
To illustrate interpretation of data in the table, tilapia may particular ions across their body surface. Crustaceans
die at water temperatures near 10 °C and 40 °C. They will and especially bivalves living in freshwater or dilute
not eat at temperatures below about 18 °C or above 35 °C. brackishwater have lower body fluid concentrations than
The best growth is achieved at temperatures of 28–32 °C. those in seawater. However, they are hypertonic to their
environment and must regulate water and ions in the
The relationship between temperature and disease is same manner as freshwater fish.
complex because temperature affects host and pathogen.
The immune system of aquatic animals functions best Each species has an optimum salinity range. Outside of
within the optimum temperature range for growth this range the animal must expend considerable energy
(Table 4.4). Sudden changes in temperature also impair for osmoregulation at the expense of growth and other
the effectiveness of the immune system. processes. If salinity deviates too much from the opti-
mum, the animal will die because it cannot maintain
4.3.2 Salinity internal salt balance.
Aquaculture species have a range of tolerance for salin-
ity, for they must maintain a suitable salinity of internal Many freshwater fish can live in waters with salinities
fluids through osmoregulation or ion regulation. up to 5–10‰, but they may not reproduce or grow well
Freshwater fish and crustaceans have body fluids more at salinities above 3–4‰. Some species such as tilapia
concentrated in ions than the surrounding water; they and rainbow trout can tolerate high salinity. Marine
are hypersaline or hypertonic to their environment. s­ pecies are adapted to growth in ocean water and do not
Saltwater species have body fluids more dilute than the grow well at low salinity. Estuarine species are adapted to
surrounding water; they are hyposaline or hypotonic to a wide range of salinity. Most species of farmed marine
their environment. Osmoregulation in freshwater fish shrimp survive and grow well at salinities of 1–2‰ up to
involves the uptake of ions from the environment and 35–40‰. Suboptimum salinity is especially stressful on
aquaculture species when temperature also is outside the
optimum range.

4.3.3 pH Water Quality 75
The optimum pH for growth and health of most fresh-
water aquatic animals is in the range of 6.5 to 9.0. Acid behavioral and physiological changes. Ventilation volume
and alkaline death points are approximately pH 4 and increases because less oxygen is available in a given
pH 11. Marine fish evolved in highly buffered seawater amount of water. Fish minimise extraneous activity to
that is not subject to wide variation in pH. Consequently, reduce metabolic oxygen demand. Certain physiological
most marine animals cannot tolerate as wide a range of responses to low DO concentrations increase the capa-
environmental pH as freshwater animals, and the opti- bility for gas exchange at the gills. These adaptions and
mum pH is usually between pH 7.5 and 8.5. Some species responses allow warmwater pond fish to survive for days
evolved in estuaries where there is typically large varia- even when DO concentrations are as low as 1–2 mg/L.
tion in pH in response to variations in river discharge At some point, compensatory responses are no longer
and tidal flow. Brackishwater inhabitants are rather sufficient and oxygen demand of tissues exceeds the
tolerant of extremes of pH. amount that can be supplied. At about that point, fish
swim to the surface in an attempt to exploit oxygen in the
Gills are the primary target of elevated, environmental surface film. Eventually, the energy requirement for
H+ concentration. For freshwater fish, changes in gill metabolism in the brain is not met and fish die. Adult
structure and function cause reduced ion uptake, warmwater fish can live for several hours at DO concen-
increased ion loss, and reduced respiratory efficiency trations as low as 0.3–0.5 mg/L and fingerlings may sur-
(Boyd and Tucker, 1998). Respiration is further compro- vive short exposure to lower concentrations. Small fish
mised by blood acidosis, which decreases the affinity of consume more oxygen per unit weight than large fish
haemoglobin for oxygen. Under mild acid stress, animals because of their higher metabolic rate per unit weight.
expend extra metabolic energy for maintenance of gill Small fish are also more effective at using oxygen in surface
function at the expense of growth and immune function. films than are large fish.
Under extreme acid stress, the animal is not capable of
maintaining homeostasis and dies. Coldwater fish usually die at a slightly higher oxygen
concentration than required to kill warmwater fish. For
Acid stress is most common in bodies of water where example, rainbow trout may die at DO concentrations of
pH has declined because of human activities. For 2.5–3.5 mg/L, and they do not grow well at DO concentra-
example, long‐term acidification of lakes because of tion below 5 mg/L. Crustaceans are remarkably similar
acid precipitation has disastrous effects on natural fish to fish in their tolerance to low DO concentrations.
populations in certain areas of Europe and North
America. Low pH in aquaculture systems usually can Wide daily fluctuations in DO typically occur in
be controlled through liming. intensive aquaculture ponds. In aquaculture ponds, DO
concentrations often exceed 15 mg/L in the afternoon
4.3.4  Dissolved Oxygen but fall below 5 mg/L by dawn. Usually, growth of most
Most of the oxygen carried in fish blood is associated species decreases if DO concentrations before sunrise
with haemoglobin in red blood cells, although a small drop below 25% saturation. Of course, some aquacul-
fraction dissolves in the plasma. Loading and unloading ture species such as tilapias and, especially, clariid and
of haemoglobin with oxygen is governed by oxygen ten- pangasiid catfishes are particularly tolerant to low DO
sion. At the gills, oxygen tension in water is higher than concentrations. Concentrations of DO above saturation
in the blood, and oxygen is loaded onto haemoglobin. In provide little benefit because the oxygen‐transporting
the tissues, oxygen is used rapidly and tissue fluids have pigment will fully load with oxygen when water is satu-
a lower oxygen tension than blood entering the tissues rated with DO.
from the arterial system. So, haemoglobin unloads
oxygen to the tissue fluids. Invertebrates do not have 4.3.5  Carbon Dioxide
haemoglobin in their blood. Most species contain Aquatic animals excrete carbon dioxide produced in
another pigment, haemocyanin, which functions in cellular respiration across their gills. When carbon
much the same manner as haemoglobin. dioxide concentration in water increases, the rate of
excretion decreases leading to accumulation of blood
In general, coldwater fish are more sensitive to hypoxia carbon dioxide and a decrease in blood pH. These con-
than are warmwater fish. Critical DO concentrations ditions decrease the affinity of respiratory pigments for
that negatively affect growth are lower than 5–6 mg/L for oxygen, which reduces respiratory efficiency and
trout and salmon and 3–4 mg/L for warmwater fish. decreases the tolerance of animals to low dissolved
oxygen concentrations.
As DO concentrations fall below critical levels, fish
compensate for decreased availability of oxygen through Tolerance of fish and crustaceans to environmental
dissolved carbon dioxide concentrations varies greatly.
Marine fish evolved in environments with stable, low

76 Aquaculture bubble disease may develop. Symptoms are bubble
f­ormation in gut and mouth, hyperinflation of the swim
dissolved carbon dioxide concentrations and are bladder and low‐level mortality. Death is often caused by
intolerant of dissolved carbon dioxide. Long‐term expo- secondary, stress‐related infections.
sure to more than 5 mg/L may reduce growth. An upper
limit of 20 mg/L dissolved carbon dioxide has been sug- Although animals may potentially develop gas bubble
gested for the health of salmonids. Warmwater fish disease any time there is a positive ΔP, occurrence is
evolved in environments where fluctuations in dissolved strongly influenced by the depth at which animals spend
carbon are more common and, therefore, can tolerate most of their time. Higher hydrostatic pressure at depth
higher concentrations. Concentrations of dissolved car- means that a larger gas concentration is necessary for
bon dioxide greater than 60 to 80 mg/L have a narcotic supersaturation. Water depth can also affect the poten-
effect on aquatic animals and even higher concentra- tial for developing gas bubble trauma because rates of
tions may cause death. photosynthesis and solar heating decrease with depth.

Most studies have concentrated on the effects of long‐ Despite the frequent occurrence of supersaturated
term exposure to specific carbon dioxide concentrations. conditions in surface waters, gas bubble trauma rarely
Such information has application to culture systems occurs in aquaculture ponds. Apparently, if supersatura-
where carbon dioxide concentration is relatively stable tion of surface water reaches harmful levels in ponds,
over time. Carbon dioxide concentrations in ponds may animals will move into deeper waters where ΔP is lower.
change by an order of magnitude between night and day. In most ponds, supersaturation will only threaten eggs or
No studies have been made of the effects of short‐term fry restricted to the surface by lack of mobility. In shal-
exposure of fish or crustaceans to fluctuating carbon low ponds with clear water and abundant submersed
dioxide concentrations so it is difficult to assess the over- vegetation, the entire water volume can become strongly
all importance of carbon dioxide to pond aquaculture. supersaturated and there is no refuge for animals.

4.3.6  Gas Supersaturation 4.3.7  Ammonia and Nitrite
Water can become supersaturated with gases by several Ammonia exists in two forms, un‐ionised ammonia
processes: rapid increase in temperature; mixing of (NH3) and ammonium ion (NH4+), in a pH‐ and temper-
air‐saturated water masses of different temperatures; ature‐dependent equilibrium:
expulsion of air into surrounding water during ice for-
mation; air entrainment by falling water; leaks on suction NH4 NH3 H , K 10 9.3.
side of pumps; submerged aerators; photosynthesis As pH rises, NH3 increases relative to NH4+. Water
(oxygen only); or sudden changes in barometric pres- temperature also causes an increase in the proportion of
sure. Gas supersaturation is expressed as the ΔP value un‐ionised ammonia, but the effect of temperature is less
which is calculated by subtracting local barometric than that of pH. The common analytical procedures for
pressure from total gas pressure in the water or by
measuring it directly with a saturometer. Table 4.5  Decimal fractions of total ammonia‐nitrogen existing
as un‐ionised ammonia‐nitrogen at various pH values and water
Supersaturation is an unstable condition and as gases temperatures. Values were calculated using the ‘ammonia
come out of solution they form bubbles. If the dissolved calculator’ at http://fisheries.org/hatchery for a typical, dilute fresh
gases diffuse across the gill before coming out of water with a TDS concentration of 250 mg/L.
­solution, emboli will form in the vascular system and
other tissues: a condition called gas bubble disease. Temperature (°C)
Acute gas bubble disease occurs at high levels of super-
saturation, usually at ΔP values of about 15 kPa or pH 5 10 15 20 25 30 35
greater. Eggs will float to the surface, and larvae and fry
may exhibit hyperinflation of the swim bladder, exoph- 7.0 0.001 0.002 0.003 0.004 0.005 0.007 0.010
thalmos (‘pop-eye’), cranial swelling, swollen gills, blood 7.5 0.004 0.005 0.008 0.011 0.016 0.023 0.032
in the abdominal cavity, gas bubbles in yolk sac and dis- 8.0 0.011 0.017 0.024 0.035 0.050 0.069 0.093
tention and rupture of yolk sac membrane. In juvenile 8.5 0.035 0.051 0.073 0.103 0.141 0.189 0.246
and adult fish, the most common symptoms of acute gas 9.0 0.103 0.146 0.200 0.267 0.342 0.424 0.507
bubble disease are gas bubbles in the blood and bubble 9.5 0.266 0.350 0.442 0.535 0.622 0.699 0.765
­formation in gill filaments, on the head, in the mouth 10.0 0.534 0.630 0.715 0.784 0.839 0.880 0.911
and in fin rays. The eyes will also protrude. Mortality
rates can be high and death is caused by bubbles that Source: Adapted from Boyd and Tucker (2014).
restrict blood flow. When aquatic animals are exposed to
ΔP values of 5–15 kPa on a continuous basis, chronic gas