Seaweed and Microalgae 327
(a)
(b)
Figure 15.10 Seaweed (Kappaphycus) drying and packing on the island of Nusa Lembongan, Indonesia. (a) Fresh (foreground) and partly
dried (background) seaweed laid out on mats on the ground. Seaweed is turned regularly by the farmer using rakes to ensure it dries over
a couple of days. (b) Seaweed is combined from multiple farmers in small operations using paid staff to provide a uniform dried product.
This dried biomass (<40% moisture) is packaged into large bags for transport to seaweed traders and processors. Source: Reproduced with
permission from Nicholas Paul.
15.3 Microalgae as sources of high‐value, fine chemicals such as carot-
enoids and fatty acids, nutraceuticals and cosmaceuti-
15.3.1 Introduction cals, and animal feed ( Table 15.4). Others are used in
Microalgae are taxonomically extremely diverse being wastewater treatment and in agriculture as soil condi-
found in many Phyla and in almost every environment tioners. Microalgae are also proving to be potential
in nature. This great taxonomic and environmental sources of bioactive compounds such as antibiotics
diversity is also reflected in the wide range of metabolites and anti‐cancer drugs and, although some of these
they produce. Several species are grown commercially compounds can be produced by chemical synthesis,
many others will probably have to be produced through
Table 15.4 Summary of the major current commercial or near commercial microalgae products, their value (based on current market prices), comparative market size and the species
used for their production and cell content of the product. All algae, with the exception of those species marked with an asterisk are produced photoautotrophically; those marked
with an asterisk are produced by heterotrophic culture.
Product Use Approx Value of Market Size Algae Approx Cell Content Status
product (USD/kg) (% of dry weight)
β‐carotene Nutraceutical, Natural medium Dunaliella salina Commercial
pigment >600 3–5%
Astaxanthin Nutraceutical, Natural medium Haematococcus pluvialis Commercial
pigment >2000 1.5–4%
Phycocyanin Food and cosmetic pigment medium? Arthrospira Commercial
Fluorescent marker ? small 10% Commercial
Phycoerythrin Food and cosmetic pigment >1000 medium? Porphyridium, Rhodella, Mainly R&D
Fluorescent marker ? small Arthrospira 10% Commercial
Fucoxanthin Nutraceutical >1000 medium? Diatoms & haptophytes R&D
Paramylon (β‐1,3‐glucan) Nutraceutical >2000? small Euglena >15% Commercial
Algal flour & oil Food >800 ? Chlorella protothecoides* >45% Commercial
? 100%
Docosahexaenoic acid Nutritional supplement large Crypthecodinium cohnii* 40% Commercial
(DHA) >30 >10%
Eicosapentaenoic acid Nutritional supplement large? Nannochloropsis, Mainly R&D
(EPA) >30 Phaeodactylum >10%
Tocopherols Vitamin medium Euglena etc R&D
Polysaccharides Cosmetics, viscosifyers 10‐50 medium Porphyridium, Rhodella <1% Mainly R&D
Pharmaceuticals >5 large Cyanobacteria, ? > 50% R&D
General (very high) dinoflagellates, others <<1%
Biofuel extremely large Green algae, diatoms etc R&D
>1 30%
Seaweed and Microalgae 329
Table 15.5 Estimated production costs for microalgae currently grown on a commercial scale.
Alga Estimated production Production system
cost (USD/kg dry wt)
Arthrospira (Spirulina) platensis Raceway systems. Raceways of up to about 0.5 ha area
Chlorella spp. 8–12 Centre pivot open ponds
15–18 Closed photobioreactors
Dunaliella salina > 50 Very extensive open ponds up to 250 ha in area (Production
5 costs in systems using raceway ponds are higher)
Haematococcus pluvialis Closed photobioreactors and open ponds
Algae for aquaculture (e.g., Isochrysis, > 40 Big bags (lowest cost is for largest aquaculture facility in
Tetraselmis and Skeletonema spp.) 60–1000 the USA)
Crypthecodinium cohnii Grown heterotrophically on glucose in fermenters
2
microalgae culture (Borowitzka, 2013a). Commercial also be used. Again, an appreciation of the physiology
culture of microalgae is still a very new industry and the and biochemistry is very important for successful genetic
full potential of microalgae remains to be developed with modification. Finally, managing a large‐scale outdoor
only a few hundred of the approximately 40 000 species culture of microalgae is not too dissimilar to that of
of microalgae having been studied. managing a monospecific algal bloom and, therefore,
knowledge of the ecology of the alga in its natural envi-
Although commercial culture of microalgae is a rela- ronment can be very useful.
tively new industry with only a small number of species
and products, global production has grown significantly Since most microalgae are photoautotrophs and require
in the last 30 years. However, the high cost of production light for growth, a basic feature of all microalgae culture
(Table 15.5) means that the product must also command systems (with the exception of heterotrophic cultures sys-
a high sale price. It is interesting to note that the most tems; section 15.3.4) is that they are shallow so that light
expensive microalgae produced are those grown for can reach all the cells. Commercial‐scale microalgae cul-
use as feed for aquaculture species (Chapter 9), with the ture systems may be extensive, semi‐intensive or intensive.
marked exception of ‘green water’ culture, which is a The cultures may also be either open to the air or fully con-
mixture of phytoplankton and used in the culture of tained (closed).
fish and shrimp. Some of the factors contributing to high
production costs are the high capital costs and high 15.3.2 Extensive Culture
labour costs and, for microalgae used in aquaculture, the Extensive culture systems are very large and achieve only
rather small scale of production (section 9.2.1, Figure 9.3). low cell densities of 0.1–0.5 g/L dry weight. The main
microalga grown commercially in extensive culture
Commercial‐scale micro‐algal culture requires a good systems is the chlorophyte, Dunaliella salina, which is
understanding of the life cycle of the alga being cultured, grown in extremely large shallow ponds in Australia for
algal physiology and algal ecology. For example, in culture the production of the carotenoid ß‐carotene (Borowitzka,
Dunaliella and Chlorella reproduce almost exclusively 2013b). It grows best at very high salinities (~25–30% w/v
vegetatively, whereas Haematococcus undergoes mor- NaCl – seawater salinity is about 3% NaCl) and high
phological life‐cycle changes during culture, with astax- temperature (30–40°C). ß‐Carotene production from
anthin accumulation occurring in the aplanospore stage. D. salina is greatest at high salinity and high light levels.
In order to achieve a high productivity and reliable cul- The high salinity prevents almost all other organisms
ture, and to maximise the production of the desired from growing in the ponds and competing with D. salina.
final product, an understanding of the alga’s physiology
allows identification of key environmental factors (e.g., In commercial systems in Australia, D. salina is grown in
light, pH, temperature) and nutrient requirements allowing shallow ponds of up to 250 ha each in area constructed with
the rational development of an optimal culture regime. earthen walls on the bed of a saltlake or saline mud flats
An important element in successful commercial micro- (Figure 15.11). The ponds are about 30–50 cm deep and
algae culture is selection of the correct algal species and the only water movement results from wind or
strain, and in the future genetically modified strains may
330 Aquaculture
Figure 15.11 The extensive culture ponds of the Dunaliella salina β‐carotene plant at Hutt Lagoon, Western Australia, operated by BASF.
The total pond area is over 750 ha. Source: Reproduced with permission from Michael Borowitzka.
convection. In such a system, the operator has little con- The first commercial large‐scale cultures of microalgae
trol over culture conditions other than salinity and nutri- were developed in Taiwan in the 1950s for culturing the
ent concentrations. The ponds are usually operated in a freshwater green alga, Chlorella, which is used as a health
semi‐continuous mode with part of the ponds harvested food. The algae are grown in circular concrete ponds
at regular intervals and with the medium being returned of up to about 500 m2 surface area. The ponds have a
to the ponds after microalgae cells have been harvested. centrally pivoted rotating arm that mixes the culture
Nutrients are added as required for microalgae growth, (Figure 15.12). This system results in uneven mixing,
and salinity is controlled by the addition of seawater. with the periphery of the pond being mixed much more
Since the algal density achieved in such systems is low, than the centre because of the higher velocity of the
harvesting and further downstream processing is expen- mixing arm at the outer perimeter. The larger the pond,
sive and the final product must have a high value for the the greater is the difference between the periphery and
overall process to be economical. Despite this, the actual the centre, and this limits the effective size of the ponds.
production costs for D. salina are among the lowest for Because of the inherent instability, the cultures need to
any commercially produced microalga. be grown in batch mode. Each growth cycle is started as
a small (~1 L) laboratory culture (section 9.2.2) and is
15.3.3 Semi‐Intensive Culture scaled up by a factor of 5–10 at each step (Figure 9.3).
Semi‐intensive culture systems require less land area
than extensive culture; however, better mixing of the In the 1960s, a better pond design, the ‘raceway’ pond,
cultures and improved control of culture conditions was developed. These ponds consist of long channels
result in cell densities of up to about 1 g/L dry weight. arranged in single or in multiple loops (Figure 15.13). Early
designs used a configuration consisting of relatively narrow
channels with many 180° bends and propeller pumps to
Seaweed and Microalgae 331
Figure 15.12 Central pivot ponds at a Chlorella production plant in Taiwan. The ponds at the front has an area of 0.5 ha. Source:
Reproduced with permission from Michael Borowitzka.
Figure 15.13 A 400 m2 test paddle‐wheel driven raceway pond at solids deposition. These were minimised by using a single
an algae biofuels pilot plant at Karratha, Western Australia. Source: loop (raceway) configuration, with flow rectifiers at the cor-
Reproduced with permission from Michael Borowitzka. ners. Simple geometric optimisation has also shown that a
large pond with a low length to width (L/W) ratio gives the
produce a channel velocity of about 30 cm/s. In the 1970s, largest pond area for the least wall length, and is therefore
paddle‐wheel mixers of various designs were introduced cheaper to construct. Ponds can be up to about 6 m wide
and found to be more effective, with reduced energy with the width being limited by paddle‐wheel design. Pond
requirements and reduced shear forces on the microalgae length is influenced by head loss relative to the mixing
cells. The numerous bends in the channels of the older velocity and pond depth. Details of the design considera-
designs also led to hydraulic losses and problems with tions for such systems are outlined by Borowitzka (2005).
Several factors need to be taken into account when
designing the optimally sized pond. These include:
●● optimal pond depth, taking into account the degree of
light penetration;
●● mixing velocity, which relates to the need to keep the
algae in suspension, avoiding any poorly mixed regions
and the effects of turbulence on the pond materials;
●● the energy requirement for mixing; and
●● materials from which the pond is constructed.
This pond design was first developed for high‐rate
oxidation ponds used in the treatment of wastewater, but
was soon also applied to the ‘clean’ culture of a range of
microalgae, especially the cynobacterium, Arthrospira
platensis (commonly referred to as Spirulina). Raceway
ponds are also used in Israel for culturing D. salina, and
in Hawaii for culture of Haematococcus pluvialis. The
raceway ponds are more efficiently mixed than the
circular central pivot ponds and pond size can be up to
1 ha in area with a depth of about 30 cm.
The ponds are either constructed of concrete, or with
concrete walls or earthen walls lined with a plastic liner.
The plastic liner is replaced with a concrete bottom in
the region of the paddle wheel. Experiments have shown
that microalgae productivity increases with increasing
flow rate and that a velocity of at least 10 cm/s is necessary
332 Aquaculture essential to maintain axenic conditions, a feature that is
not required for the tubular photobioreactors.
to avoid settlings of the cells; however, practical limita-
tions in pond design mean that velocities in the range of Tubular Photobioreactors
30 cm/s are optimal. In order to maximise productivity, Many different tubular reactor designs have been devel-
the pond depth is about 20–30 cm and cell density must oped to produce cultures of relatively high density (Zittelli
be controlled to minimise self‐shading by the cells. et al., 2013). The first large‐scale tubular photobioreactor
developed in France had a solar receptor constructed of
Although productivity of up to 30 g/m2/day dry weight five identical 20‐m2 units made of 25‐cm diameter poly-
has been reported, actual annual average productivity is ethylene tubes floating on or in a large pool of water. The
significantly lower than this with the best value of about culture circulated through these tubes and temperature
20 g/m2/day dry weight reported. Several attempts have was controlled by either floating the tubes at the water
been made to improve the productivity of these ponds by surface (to heat) or by immersing them in the water (to
increasing turbulence and improving mixing. cool). Water in the pool also provided a convenient
support for the long tubes of the solar receptor. At the
Although raceway ponds are the main culture system end of each solar receptor, a gas exchange tower removed
used for the commercial‐scale culture of microalgae, its photosynthetically produced oxygen and CO2 could be
major limitation is that the system is open to the air, added. This pilot plant was quite successful in growing
which may lead to contamination and infection by the red unicell, Porphyridium; however, it was technically
predators (mainly other algae, protozoa and fungi). These complex and expensive, and required a large land area.
systems are, therefore, best suited to microalgae that
grow in relatively extreme environments such as high A more efficient arrangement for the tubes of the solar
pH (e.g., Arthrospira species) or high salinity (e.g., receptor is to wind them helically around a tower. This is
Dunaliella species) or fast‐growing algae such as the design of the ‘Biocoil’ (Figure 15.14), a system developed
Chlorella, Phaeodactylum or Scenedesmus species,
which can outgrow most of their competitors.
15.3.4 Intensive Culture
In intensive cultures, the microalgae are grown under
highly controlled optimum conditions in closed photo-
bioreactors, which can result in cell densities of 1–10 g/L
dry weight. High cell densities have the advantage of
requiring a smaller area for the reactor and, in addition,
harvesting costs are also reduced significantly.
Closed culture systems include:
●● bag culture, which is widely used for the culture of
algae for aquaculture (Figure 9.3);
●● alveolar panels and other flat plate reactors of various
designs;
●● stirred tank reactors with internal illumination;
●● tower reactors with external illumination;
●● suspended narrow bags or tubes; and
●● tubular reactors in various configurations.
‘Big Bag’ Systems Figure 15.14 A 1000 L pilot scale Biocoil‐type helical tubular
Probably the most widely‐used closed culture systems photobioreactor in Perth, Western Australia. Source: Reproduced
for mass culture of microalgae are the ‘big bag’ systems with permission from Michael Borowitzka.
generally used in aquaculture hatcheries to feed larval
fish, crustaceans, molluscs or rotifers (Figure 9.3).
Although widely used, these systems are notorious for
the instability of the cultures. This instability probably
occurs because uniform mixing in the bags is very hard
to achieve, resulting in potential build‐up of cells in
unmixed areas, which, in turn, can lead to culture
collapse, especially if the culture is not axenic (bacteria
free). In order to achieve reasonably reliable cultures, it is
in the UK and optimised in Australia. Several pilot scale Seaweed and Microalgae 333
units of the Biocoil have operated in the UK and in
Australia, with volumes up to 2000 L, and a very wide range Currently the most commonly used tubular photo-
of microalgae including Chlorella, Spirulina, Dunaliella, bioreactors have the tubes arranged vertically in long
Tetraselmis, Phaeodactylum, Chaetoceros, Isochrysis, rows, usually connected to manifolds at their ends
Pavlova, Porphyridium, Haematococcus and Skeletonema (Figure 15.15). The tubes are arranged vertically form-
species have been grown successfully. The Biocoil system ing a fence‐like structure. This is the system used in the
uses low‐density polyethylene or Teflon tubing of commercial culture of Haematococcus in Israel and in
25–30 mm diameter. This narrow diameter results in much China, as well as for the production of Chlorella in
higher productivity and reduced fouling of the inside of the Germany.
tubes by the algae. The helical arrangement of the tubing
also means that there are no sudden changes in direction of Flat‐Panel Photobioreactors
flow, which not only result in significant head losses, but An alternative design for a closed photobioreactor are
can also lead to undesirable accumulation of algae. The the flat‐panel reactors first developed in the 1980s. In the
maximum temperature in the reactors is controlled by most common configuration the reactor consists of two
evaporative cooling, achieved by running water over the rectangular panels of glass or Perspex spaced about
reactor surface. The helical design also has the great 25 mm or more apart. A cheaper design in which plastic
advantage of good scale‐up properties. This means that the bags are placed within a rectangular metal framework
results obtained in smaller pilot experiments can be has also been recently developed (Rodolfi et al., 2009).
directly related to full‐scale production units. Systems such Some of the designs have a number of internal baffles.
as the Biocoil also allow continuous culture, which results The panels can be inclined to capture the optimum
in more consistent quality of the algae produced and is amount of solar irradiation and the algal culture is mixed
cheaper. In Western Australia, Tisochrysis lutea (Isochrysis by aeration or circulated by pumping. The aeration
T. ISO) has been grown in a 1000‐L Biocoil (Figure 15.14) also helps to remove photosynthetic O2, which at high
in continuous culture for more than 6 months. concentrations will limit productivity due to photores-
piration. CO2 can also be added to enhance microalgae
One of the key design features of these systems is the growth. The temperature is usually controlled by spray-
pumping system used to circulate the algal culture. ing water over the panel surface in order to cool the cul-
Several types of pumps, including centrifugal, diaphragm, tures. Some systems use a heat exchanger; however,
peristaltic and lobe pumps, as well as airlifts, have been this is too expensive for large‐scale systems. These
used, and the choice of pump depends on the degree of flat‐panel reactors can be very productive, but individ-
fragility of the algae being grown. ual reactors are very difficult to scale up to any appreci-
able size and are expensive to operate.
Figure 15.15 Vertical tubular
photobioreactors at the Algae
technologies Haematococcus pluvialis
astaxanthin production plant at Kibbuz
Ketura in Israel. Source: Reproduced with
permission from Dr S. Boussiba.
334 Aquaculture produces large amounts of hydrocarbons, which are
secreted from the cells. The sugars contained in the algal
Heterotrophic Culture biomass can also be fermented to produce ethanol, or
Closed fermenters have been developed for the hetero- alternatively the whole algal biomass can be converted to
trophic production of long‐chain polyunsaturated fatty a hydrocarbon feedstock by hydrothermal liquefaction.
acids from microalgae (Bumbak et al., 2011). Sugars such The potential advantages of microalgae over alternative
as glucose or acetate serve as the energy and carbon oleagenous crops for biofuels are that they have higher
source for the algae and this eliminates the need for light, productivity per unit area, do not compete with food crops
which is a major cost in phototrophic microalgae culture for limited resources, can be grown using saline water
systems. Furthermore, because cell density is not limited and can be grown on land unsuitable for agriculture.
by light availability, microalgae can be cultured in The high demand for liquid transport fuels however
relatively high densities with high biomass production. means that extremely large production plants (10–100 km2
Heterotrophically grown microalgae are being produced in area) would be required. In order to achieve the
commercially and have value as an aquaculture feed required high annual productivities, such plants would
(section 9.2.5). Heterotrophic culture is also used to need to be located in a region with high average annual
produce microalgae as a source of long‐chain polyun- solar irradiation and minimum cloud cover and rainfall,
saturated fatty acids mainly for use as nutrient supple- such as in the north of Western Australia or Arizona.
ments in infant formula. Large saline water sources are also required so the plant
must be located near the ocean or near a source of saline
However, only a limited number of species can be grown groundwater. Although a wide variety of culture systems
heterotrophically, and therefore photoautotrophic culture have been proposed for microalgae culture, for economic
in raceway ponds or tubular photobioreactors are likely to reasons, open raceway ponds are the most likely system
be the main systems to be used in the future. for the eventual commercialisation of algae‐based biofuels
(Fon Sing et al., 2013). Although trial batches of algal
15.3.5 Algae for Biofuels biofuels have been produced for testing, their commercial
Algae, both microalgae and seaweeds, have been proposed production still requires a very significant reduction in
as sources of renewable biofuels such as biodiesel, bioeth- production costs for them to be commercially viable.
anol, H2 and methane. Biomass‐based biofuels also have Options to reduce production costs and improve the
the advantage of a smaller CO2‐footprint because CO2 is economics for biofuels include the use of wastewaters as
fixed during photosynthesis to produce the biomass. a nutrient source and production of multiple products
Some microalgae, in particular, are excellent sources of from the algal biomass, not just biofuels.
fats (lipids) with cell contents of up to ~ 30% in actively
growing cells and over 50% in stationary phase cells and A schematic diagram of an algae‐to‐biofuel process
these lipids can be converted to biodiesel by transes- in shown in Figure 15.16. Here, the algae are grown in
terification. The green alga, Botryococcus braunii also
Light Nutrients CO2 Figure 15.16 Simplified schematic diagram
of an algae–to–biofuel process. Multiple
Saline forms of bioenergy can be produced
water including biodiesel, ethanol and/or
methane. Co‐production of animal feed and
Recycle water other products such as carotenoids, fatty
acids and protein has also been proposed.
Source: Reproduced with permission from
Michael Borowitzka.
Algae Ethanol
sugars
Algae oil Biodiesel
Remaining biomass Jet Fuel
Nutrients Anaerobic Animal Other
digestion feed product(s)?
Methane
open raceway ponds in semi‐continuous culture with Seaweed and Microalgae 335
regular harvesting of part of the biomass to maintain
maximum productivity. After harvesting the algae, the rates can damage algal cells, especially flagellates, so that
medium is returned to the growth ponds and supple- the number of species that can be cultured in these systems
mented with nutrients. This medium recycling is essential is limited as they are unsuitable for fragile species.
for economic and environmental reasons. After further
dewatering, the algae oil (and possibly also the sugars) Finally, compared to all other algal culture systems, pho-
are extracted and converted to biofuel. The remaining tobioreactors are more expensive to construct and have a
biomass could be used either as a high protein feed high energy requirement for operation, limiting their
supplement in aquaculture, or be fermented to produce application to high‐value products. Thus, for economic
methane, or the energy in the remaining biomass could be reasons, the majority of current commercial microalgae
recovered by co‐burning with coal in a power station. culture systems are open systems such as raceway ponds.
In conclusion, it is clear that there is no perfect culture
system. The final choice of culture systems will be
dependent on the species being cultured, the final
product, and on commercial considerations.
15.3.6 Choice of Culture System 15.4 Summary
The choice of culture system depends on many factors ●● Seaweed are the single largest crop by volume in the
and no single system is best for all microalgae. Table 15.6 aquaculture sector. However, the relative value of most
compares some of the main characteristics of the differ- seaweed products has decreased as they become
ent culture systems used for production of microalgae. mature commodities.
The reliability of the culture is of critical importance to ●● Seaweed is almost exclusively cultured in Asia in
commercial operations and, for this reason, closed cul- shallow coastal areas and is focussed on brown and
ture systems are seen as preferable because contamination red seaweeds of which there are seven main taxa
is easier to manage, and culture conditions are generally under cultivation; Kappaphycus, Eucheuma, Gracilaria,
easier to control. Photo‐bioreactors require a high surface Porphyra/Pyropia (red algae) and Saccharina/
area to volume ratio to maximise light availability. Laminaria, Undaria and Sargassum. New strains and
Although this results in higher cell density and productiv- species can, and should, be domesticated or bred to
ity, as well as lower harvesting costs, these systems do have support production in nascent coastal areas.
important disadvantages. For example, the high surface
area to volume ratio means that the culture heats up rap- ●● Culture methods differ with respect to species, rang-
idly during the day in outdoor systems, which can lead to ing from relatively simple propagation methods on
overheating. In addition, the high biomass in such systems long‐lines through to highly mechanised systems
requires turbulent flow to reduce light limitation and requiring industrial scale production of seed in land‐
ensure adequate gas (O2 and CO2) and nutrient exchange. based hatcheries prior to deployment in the open sea.
The circulation system required to achieve these flow Innovations in the mass culture of colloid‐producing
Table 15.6 Comparison of the properties of different large‐scale microalgae culture systems.
Light utilisation Temperature Hydro‐ Species Maximum achievable
Reactor type Mixing efficiency control Gas transfer dynamic stress control Scale‐up cell density (g/L)
Unstirred Very poor Poor None Poor Very low Difficult Very 0.2
shallow ponds None difficult
Possible
Paddle‐wheel Fair–good Fair–good Possible Poor Low Difficult Difficult 0.5–1.0
raceway ponds Possible
Possible
Stirred tank Largely Fair–good Possible Low → high High Easy Difficult 0.5–1.0
reactor uniform
Tower air‐lift Generally Good High Low Easy Difficult 0.5–1.0
reactor uniform
Flat‐plate reactor Uniform Excellent High Low → high Easy Difficult 2.0–5.0
Easy Easier 1.0–2.0
Tubular reactor Uniform Excellent Low → high Low → high
(Serpentine type)
Tubular reactor Uniform Excellent Low → high Low → high Easy Easy 1.0–2.0
(Biocoil type)
336 Aquaculture ●● Semi‐intensive culture systems require less land area
than extensive culture; however, better mixing of the
red seaweeds will be a game‐changer for the industry, cultures and improved control of culture conditions
as it was for brown seaweeds in China. result in higher cell densities. Raceway ponds are the
●● Food (dried and fresh) and food‐gelling ingredients are main culture system used for the commercial‐scale
the two drivers for seaweed production. New applica- culture of microalgae and productivity of up to 30 g/m2/
tions are constantly advocated, some near to market day dry weight has been reported. The major limita-
(niche food, functional food), some are emerging tion is that the system is open to the air, which may
(nutraceutical, fertiliser) and others are still far away lead to contamination.
(protein crops, biofuels). A biorefinery approach, creat-
ing multiple products from a single feedstock, may be ●● In intensive culture systems microalgae are grown
the only way to bring some applications to the table. under highly controlled optimum conditions in
●● The great taxonomic and environmental diversity of closed photobioreactors, which can result in cell
microalgae is reflected in the wide range of metabolites densities of 1–10 g/L dry weight. High cell densities
they produce. Several species are grown commercially have the advantage of requiring a smaller area for
as sources of high‐value, fine chemicals such as carot- the reactor and harvesting costs are reduced
enoids and fatty acids, nutraceuticals and cosmaceuti- significantly.
cals, bioactive compounds and animal feed. Others are
used in wastewater treatment and in agriculture as soil ●● Closed fermenters have been developed for the heter-
conditioners. otrophic production of microalgae, where sugars such
●● Extensive microalgae culture systems are very large as glucose or acetate serve as the energy and carbon
and achieve only low cell densities. The main species source for the microalgae eliminating the need for
of microalga grown commercially in extensive culture light, which is a major cost in phototrophic culture sys-
systems is the chlorophyte, Dunaliella salina, which is tems. Microalgae can be cultured in relatively high
grown in extremely large shallow ponds at very high densities with high biomass production in hetero-
salinities and high temperature where ß‐Carotene pro- trophic systems but only a limited number of species
duction from D. salina is greatest. can be grown heterotrophically.
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339
16
Carps
Sena De Silva and Qidong Wang
CHAPTER MENU 16.6 Diseases, 358
16.1 Introduction, 339 16.7 Genetic Improvement, 358
16.2 Biology of the Important Carps 16.8 Culture‐Based Fisheries, 359
16.9 Conclusions, 360
in Aquaculture, 341 16.10 Summary, 360
16.3 Artificial Propagation, 350 References, 361
16.4 Nutrition, 352
16.5 Culture Practices, 353
16.1 Introduction pharyngeal teeth in one to three rows, with not more than
eight teeth in any one row; lips that are usually thin and
The family Cyprinidae comprises a diverse and widely dis an upper jaw that is usually bordered only by premaxillae.
tributed group of mostly freshwater fish. Only two marine The Cyprinidae includes about 275 genera and more than
cyprinid species exist, and neither are of commercial 2400 species, although taxonomy is in a constant state of
interest. Cyprinids are collectively referred to as carps, flux. The greatest diversity of the group occurs in Asia.
barbels and minnows. Some use the term ‘carp’ to refer to Cyprinids include one of the largest freshwater fishes,
the entire family, although common usage usually restricts Catlocarpio siamensis, which may reach 3 m in length,
the term to describe only the larger cyprinid species. The and some of the smallest, with maximum lengths less
common name ‘barbel’ refers to cyprinids mostly in the than 5 mm.
genus Barbus. ‘Minnows’ comprise the smaller cyprinids,
although in many places the term is loosely used to refer to Carps include many species that have been widely
many small fishes—cyprinid or not. Many cyprinids, such introduced—often with negative consequences—out
as danios (Danio spp.) and barbs (mostly Puntius spp.), are side their native ranges either accidentally or for varying
popular ornamental aquarium fish (Chapter 26), and purposes. The common carp is native to Europe and
zebrafish (Danio rerio; Figure 16.1) are widely used as a Asia but has been introduced to every continent except
simple vertebrate model for research in developmental Antarctica as a food fish or as a species for angling.
biology, toxicology, oncology and gene function. Mostly Feral common carp are regarded as pests in Australia,
in the USA, cyprinid ‘minnows’ such as golden shiner most of the USA, and many other countries due to its
Notemigonus crysoleucas, fathead minnow Pimephales fecundity, feeding habits and resulting negative impacts
promelas and goldfish Carassius auratus are grown in on native fisheries. Koi are varieties of common carp
aquaculture to provide bait for angling. However, the that were bred for dramatic c oloration in Japan in the
focus of this chapter is fish grown for human food and for 1800s and have been widely distributed throughout the
simplicity’s sake we will refer to all cyprinids used in food world for display in ornamental ponds and aquaria
fish aquaculture as carps. (Figure 16.2). Similarly, selective breeding of the
Prussian carp Carassius gibelo in China more than a
Carps are native in North America (northern Canada thousand years ago produced the goldfish, which is
to southern Mexico), Africa and Eurasia, but were absent perhaps the most widely distributed aquarium fish in
from South America, Australasia and Madagascar. the world. In many places, koi and goldfish have
Distinguishing features of the group are the presence of escaped into the wild and are considered a nuisance
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.
340 Aquaculture example, are considered as invasive pests in some
countries whereas salmonids introduced outside their
Figure 16.1 Zebrafish are one of the most important cultured fish native range are almost never categorised in that way.
in the world. These small cyprinids (approximately 4 cm total The bias against carps in some countries must be
length) are widely used in biological and medical research. Source: viewed in the context of the tremendous contribution
Oregon State University 2013. Reproduced under the terms of of cyprinids to global food production.
the Creative Commons Attribution Share Alike license, CC‐BY‐SA
2.0, via Flickr. Carp aquaculture is extraordinarily important.
Carps are major sources of animal protein for hundreds
Figure 16.2 Koi in the Berlin Aquarium, Berlin Zoological Gardens. of millions of people in Asian, African and European
Source: Diether 2008. Reproduced under the terms of the Creative countries. Carp aquaculture also provides an impetus
Commons Attribution Share Alike license, CC BY‐SA 3.0, via or starting point for general aquaculture development
Wikimedia Commons. in many countries. Global carp aquaculture production
in 2014 was almost 28 million t, accounting for 65% of
species. Grass carp have been introduced as a biocon all inland finfish aquaculture, 56% of total finfish
trol agent for weed control into the USA, New Zealand aquaculture, and almost 40% of total animal aquacul
and other countries. Grass carp are controversial out ture production.1 Carp aquaculture production has
side their native range as society tries to balance the increased significantly since records have been kept,
usefulness of the fish for weed control against the and increased more than twofold from 2000 to 2014
potentially negative impacts on native ecosystems. In (Table 16.1). Interestingly, the percentage contribu
countries where introduced cyprinids are considered tion of carp aquaculture to total inland freshwater
as invasive or nuisance species, management plans for finfish production has remained essentially unchanged
their eradication have been developed. Identification since 1980.
of certain fish species as undesirable often shows con
siderable cultural bias: common carp and tilapias, for Carps are a diverse group of fish and many species
have traits that make them good candidates for aquacul
ture. The United Nations Food and Agricultural
Organization lists 49 species or species groups in its
2016 FishStat J database for global aquaculture produc
tion. However, six species (grass carp, silver carp,
c ommon carp, bighead carp, catla and rohu), each with
an annual production of more than 1.5 million t in 2014,
constitute 91% of total carp aquaculture. Production of
three additional species—Wuchang bream, black carp
and mrigal—represents an additional 6% of total carp
aquaculture. As such, 97% of carp aquaculture is repre
sented by nine species.
China and India dominate cyprinid aquaculture
p roduction. The importance of carp aquaculture in these
two countries stems initially from the fact that they
comprise the native ranges of the Chinese and Indian
carps that constitute the backbone of cyprinid aquacul
ture (Table 16.2). China alone contributes about 70% to
global cyprinid production. Importantly, it is evident
that countries such as Bangladesh, Myanmar, Indonesia,
Laos and Vietnam have adopted cyprinid aquaculture to
a significant extent, further emphasising the importance
of the group in global aquaculture. Throughout China,
Southeast Asia and the Indian Subcontinent, carp aqua
culture is a growing contribution to food fish production
and, therefore, food security.
1 All production data in this chapter were obtained from the 2016
FAO Fishery and Aquaculture Statistics FishStatJ database http://
www.fao.org/fishery/statistics/software/fishstatj/en
Carps 341
Table 16.1 Global annual aquaculture production of the nine most important (by weight produced) carp species and global annual
production of all cyprinid species.
Annual production (million t)
Common name Species 1970 1980 1990 2000 2010 2014
Grass carp Ctenopharygodon idella 0.093 0.156 1.054 2.976 4.362 5.537
Silver carp Hypophthalmichthys molotrix 0.271 0.449 1.134 3.035 4.100 4.968
Common carp Cyprinus carpio 0.242 0.365 1.134 2.410 3.421 4.159
Bighead carp Hypophthalmichthys nobilis 0.125 0.199 0.678 1.428 2.587 3.253
Catla Gibelion (Catla) catla 0.031 0.087 0.235 0.602 2.977 2.770
Rohu Labeo rohitus 0.031 0.090 0.245 0.733 1.133 1.670
Wuchang bream Megalobrama amblycephala 0.029 0.045 0.162 0.446 0.652 0.783
Black carp Mylopharyngodon piceus 0.018 0.271 0.038 0.149 0.424 0.557
Mrigal Cirrhinus cirrhosus 0.011 0.046 0.160 0.552 0.302 0.415
Total cyprinid aquaculture 0.907 1.573 5.628 13.859 23.110 27.882
Table 16.2 The global total and the top ten countries for aquaculture production of cyprinids for selected years. The percent contribution
to the global total from each country is given in parentheses.
Country/Territory 1990 Country/ Territory 2000 Country/ Territory 2013
Total cyprinid 5 621 860 China 13 793 350 China 26 410 010
China 4 093 124 (72.8) India 10 788 565 (78.2) India 18 809 593 (71.2)
India Bangladesh 1 703 357 (12.3) Bangladesh 3 737 358 (14.2)
Russian Fed. 628 157 (11.2) Indonesia Myanmar 1 076 604 (4.1)
Indonesia 252 296 (4.5) Myanmar 458 978 (3.3) Indonesia
Ukraine 131 725 (2.3) Russian Fed. 224 868 (1.6) Viet Nam 789 160 (3.0)
Romania Brazil Iran 473 355 (1.8)
Taiwan 80 801 (1.4) Thailand 93 948 (0.7) Pakistan 394 565 (1.5)
Iran 34 400 (0.6) Ukraine 63 154 (0.5) Russian Fed. 167 883 (0.6)
Poland 26 818 (0.5) Iran 54 566 (0.4) Laos 144 881 (0.5)
Uzbekistan 26 254 (0.5) 54 482 (0.4)
22 200 (0.4) 30 835 (0.2) 98 385 (0.4)
21 948 (0.4) 27 500 (0.2) 79 950 (0.3)
The remarkable development trend of inland aquacul n ormally gutted, cleaned, glazed and frozen whole—has
ture in Myanmar, considered to have been pioneered by created many employment opportunities for rural
rohu aquaculture, is depicted in Figure 16.3. This devel females in processing plants (Figure 16.4), with signifi
opment occurred in less than 20 years and rohu culture cant impacts on the socio‐economic conditions of poor
accounts for over 60% of national production. Rohu cul communities in developing countries.
ture in Myanmar originally focused exclusively on local
markets with a small quantity of fresh fish on ice exported 16.2 Biology of the Important
to Bangladesh. Over time rohu exports from Myanmar Carps in Aquaculture
increased up to 64 000 t. The development of an export
market for rohu has essentially exploited a niche market This chapter deals primarily with the culture of common
of expatriate Indian and Bangladeshi communities, at carp and the Chinese and Indian major carps. These fish
first in Middle East and later in Europe. Development are riverine, typically being found in large river systems.
of the Myanmar export market for rohu—which are
342 Aquaculture
Table 16.3 Feeding habits of important carps used in aquaculture.
1000 % Rohu 100
Total 80
Rohu Species Feeding habit
Production (103 t/yr ) % of production Common carp Omnivore; predominantly a grazing
bottom‐feeder.
60 Grass carp Herbivore; grazing on succulent emergent,
500 submersed, or floating aquatic plants.
40 Silver carp Omnivore; water‐column filter‐feeding on
phytoplankton and zooplankton.
Bighead carp Omnivore; water‐column filter‐feeding
principally on zooplankton.
20
Black carp Molluscivore/omnivore; grazing primarily on
snails, mussels and clams.
0 0
1990 1995 2000 2005 2010 Wuchang bream Herbivore, grazing on filamentous algae and
succulent aquatic plants.
Figure 16.3 Trends in the development of inland aquaculture Catla Omnivore; water‐column filter‐feeding
and the role of rohu in Myanmar aquaculture. Source: Data from principally on zooplankton.
FAO 2015.
Rohu Omnivore; water‐column filter‐feeding and
periphyton grazing.
Mrigal Omnivore; predominantly a grazing
bottom‐feeder.
The food habits of the various species differ considerably
(Table 16.3), which is the fundamental reason why these variety of foods available in aquatic habitats. Food habits
fish are often grown together in polycultures rather than of the major carp species also differ between life stages;
alone in monoculture like so many other aquaculture for example, grass carp fry and early fingerlings are zoo
species. That is, the different fish co‐cultured together planktivorous but the fish becomes herbivorous and feed
feed on particular types of food—such as snails, zoo on larger aquatic plants as they grow.
plankton, phytoplankton or larger aquatic plants—and
are complementary rather than competitors with respect Carps are very fecund—depending on species,
to food resource utilisation. Carp polyculture is a highly females usually produce 75 000 to 300 000 eggs/kg body
efficient culture strategy that makes full use of the wide
Figure 16.4 A processing facility for rohu in
Myanmar. Processing is usually limited to
descaling and removing gills and viscera.
Source: Reproduced with permission from
Sena De Silva, 2017.
weight—and most of the cultured carp species attain sex Carps 343
ual maturity in their third year of life. In the wild, they
spawn once per year, generally with the onset of mon fish culture and angling. Appearance is variable. Body
soonal floods. It is generally accepted that the interaction shape ranges from elongated to deep and stocky. The
of a large number of factors associated with flooding is body is fully scaled with relatively large scales, although
responsible for bringing about ovulation and spawning of natural mutations produce scaleless fish (leather carp)
Indian major carps under natural conditions. One cue that or fish with only a few scales (mirror carp; Figure 16.6).
seems to be common among the Chinese and Indian The scale edges are more darkly pigmented, giving the
major carps seems to be a sudden increase in water flow. fish a cross‐hatched appearance. Colours vary from sil
The spawning of Indian major carps may be synchronised very grey to reddish bronze. The subterminal mouth has
with the phase of the moon during the floods. In the case two obvious barbels on each side at the corners of the
of Chinese major carps, it is believed that temperature and mouth, with shorter barbels at the tip of the snout. The
photoperiod provide primary cues for maturation. fish may grow to more than 40 kg in nature, but usually
Chinese and Indian major carps have not been known to grow to 5–15 kg. Common carp biology and culture are
spawn naturally in lake waters, nor under captive condi summarised in the dated, but still useful, monograph by
tions without hypophysation (section 16.3). All the Billard (1999).
Chinese and Indian major carps are single spawners in the
wild, in that during any one spawning season the female Common carp tolerate a wide range of environmental
sheds all her mature oocytes within a very short period. conditions, which explains their wide distribution and
favoured status as a fish for aquaculture. They live in
All the major carps grow to about 1 m in length, and waters with temperatures ranging from near freezing to
generally the Chinese major carps grow to a larger size more than 35 °C; optimum water temperature for growth
than the Indian major carps. For example, it is not and propagation is 20–30 °C. Common carp survive at
uncommon for silver and bighead carp to grow up to salinities above 10‰ but grow best in fresh waters with
1.5 m in length and exceed 10–20 kg in weight. Under salinities below 5‰. They can survive for long periods at
culture conditions, however, all except broodstock are dissolved oxygen levels near 1 mg/L, or lower, but like
harvested in their second or third year, often at weights most warmwater fish, grow best when dissolved oxygen
of 1–2 kg. concentrations are near saturation (see Chapter 4).
16.2.1 Common Carp Wild common carp spawn in lakes, streams and rivers
Common carp (Figure 16.5) are native to Europe and from late spring through early summer in temperate
Asia but have been introduced throughout the world for regions, when water temperatures rise to 18–24 °C. Rising
water levels that flood littoral regions also stimulate
spawning. Spawning takes place in shallow littoral areas
with slow‐moving or stagnant water and abundant weeds
which serve as spawning substrate. Fertilised eggs stick to
Figure 16.5 Common carp from Lake
Enghien les Bains. Source: ASSOPECHE 2011.
Reproduced under the terms of the Creative
Commons Attribution Share Alike license,
CC BY‐SA 4.0, via Wikimedia Commons.
344 Aquaculture
Figure 16.6 Common carp (left) and the
irregularly scaled, mirror carp mutation
(right). Source: Reproduced with permission
from Karelj 2011, via Wikimedia Commons.
the spawning substrate and hatch in 3–5 days, depending 16.2.2 Chinese Major Carps
on water temperature. One female common carp may Several hundred cyprinid species are endemic to China
produce 100 000 to 300 000 eggs/kg body weight. and neighbouring areas but only a few are important in
aquaculture. These are referred to as ‘major carps’ based
Larval fish eat zooplankton and consume an ever‐ on potential for growth in culture and importance in
increasing variety of foods as they grow. Juveniles and food production. The Chinese major carps (sometimes
adults are opportunistic omnivores, feeding on zoo called Asian major carps) include grass carp, silver carp,
plankton, aquatic insects, detritus of plant origin, and a bighead carp, black carp, Wuchang bream and mud carp
variety of benthic organisms. Larger fish may eat fish (Cirrhina molitorella). Common carp and Crucian carp
eggs, small fish and crayfish. Adults are notorious for (Carassius carassius) are sometimes included on this list
disturbing and suspending sediments during foraging. by virtue of their importance in Asian aquaculture. The
three most important Chinese major carps—those with
Common carp used for aquaculture may be produced total 2014 aquaculture production exceeding 1 million
using volitional spawning in ponds or tanks provided with t—are the grass, silver and bighead carp.
a spawning substrate (submersed aquatic plants, palm
fronds, etc.). However, most cultured fish are spawned 16.2.2.1 Grass Carp
under controlled conditions using hormone injection to Grass carp are native to large rivers in eastern Asia, from
induce and synchronise spawning. Larval fish are raised northern Vietnam to northern China and southeast
in ponds fertilised with organic material (manures) or Russia. It has been widely introduced and is present on
synthetic fertilisers to promote appropriate zooplankton every continent except Antarctica. The fish was intro
communities (section 9.9). Fish may be fed a supplemen duced as a food fish in many Asian countries but was
tary diet of nut or cereal meals as they grow. Fry may be introduced as a biocontrol agent to control aquatic weeds
grown to fingerlings in the same pond or thinned and in Europe, the USA, New Zealand, Australia and else
transferred to another pond for growth to fingerlings. where. Grass carp have strong, torpedo‐shaped bodies
(Figure 16.7); large, clearly defined scales; and a terminal
Ongrowing to market size takes place in a wide variety mouth with no barbels. They are brownish olive dorsally,
of systems, ranging from extensive pond polycultures shading to bronze on the sides and white ventrally. Grass
based on natural food production (Figure 2.4) to intensive carp can grow to more than 35 kg in the wild. Grass carp
monocultures in tanks and cages using commercial biology is summarised by Cudmore and Mandrak (2004).
pelleted feeds. Common carp are often the primary
species raised in semi‐intensive pond polycultures with Grass carp are hardy fish, with environmental toler
Chinese and Indian major carps or tilapia. In these sys ances similar to common carp. They tolerate water tem
tems, common carp are fed farm‐made or commercial peratures from near freezing to about 35 °C, with an
feeds. Wastes excreted by the primary species promote optimum range for growth of 20–30 °C. Feeding activity
abundant phytoplankton and zooplankton growth that declines sharply below about 10–15 °C. Salinities up to
are then grazed upon by secondary carp or tilapia species.
Common carp may grow to more than 2 kg in one year in
feed‐based systems in tropical or subtropical regions.
Carps 345
Figure 16.7 Grass carp (approximately
15 kg) harvested from a lake in the USA.
This fish is much larger than those grown
in aquaculture, which are usually
harvested at less than 2 kg. Source:
Reproduced with permission from David
Cline, 2017.
10–12‰ are tolerated, but fish grow best and are healthi propagation, with occasional use of eggs collected from
est below about 5‰. Grass carp are relatively tolerant of wild sources. Artificial propagation relies on injecting
low dissolved oxygen concentrations and can tolerate brief mature broodfish with an inducing hormone (usually
exposure to oxygen concentrations as low as 0.2 mg/L. pituitary extract or luteinising hormone‐releasing
hormone analogue, LHRHa; see section 16.3). Eggs
Grass carp mature at 2–8 years depending on their are stripped from the female, fertilised, and incubated
environment. Longer maturation periods are typical of in tanks or jars with water circulation to keep eggs
cooler waters. Adult grass carp migrate to the upper suspended. Larvae are held in the hatchery for 3–5 days
reaches of large rivers to spawn. Spawning is stimulated until the yolk sac is absorbed and then transferred to
when water temperatures reach 15–20 °C and river small ponds fertilised with organic material (manures)
flows increase rapidly, corresponding to typical spring to promote zooplankton growth. Natural foods may be
and early summer conditions in native rivers. Other supplemented with soybean cake, grain by‐products or
spawning stimuli may also be involved. Males and finely ground aquatic plant fodders. Fry are usually
females congregate in groups and females release eggs thinned and transferred to fingerling‐rearing ponds for
into flowing water where they are immediately ferti further growth after a variable length of time (weeks to
lised. Females may release 75 000–150 000 eggs/kg body months) in the fry nursery pond. Length of time in nurs
weight. ery and fingerling ponds and the desired fingerling size
for ongrowing to market‐sized fish varies depending on
Fertilised eggs float downstream and hatch in 2–4 days. climate and custom.
Larvae continue to drift downstream as the yolk sac is
absorbed over a 2–3‐day period, after which fry begin A variety of systems are used for grow‐out to market
feeding on zooplankton. After about 3 weeks, feeding size. The most common methods include semi‐intensive
habit becomes increasingly herbivorous, with increasing polyculture in ponds and intensive polyculture or mono
proportions of filamentous algae, succulent aquatic plants culture in cages placed in reservoirs or large lakes. Grass
and plant‐based detritus. By 2 months the diet is almost carp are an esteemed food fish in China (and elsewhere)
exclusively plant‐based, although larger zooplankton, and increasingly they are the primary species in pond
aquatic insects and even small fish can contribute to the polycultures. In the past, most cultured grass carp were
diet throughout life. Adults prefer succulent aquatic fed lightly processed, chopped plant material (aquatic
plants with soft leaves but will consume almost any plant plants or terrestrial grasses) but use of high‐quality manu
when preferred food items are scarce. Under aquaculture factured feeds is becoming common. Secondary species in
conditions, fish may eat plants growing naturally in the polyculture include silver carp, bighead carp or Indian
pond or may be fed terrestrial or aquatic plants harvested major carps, depending on the country. Grass carp are
from nearby land or water bodies and then processed to usually the primary (or only) species when grown inten
some degree, usually by simply chopping into smaller sively in cages. If other fish are used, such as Wuchang
pieces. Increasingly, however, cultured grass carp are fed bream or silver carp, they are added to cages at much
artificial feeds such crop by‐products or manufactured lower numbers than grass carp. In India, grass carp often
pelleted feeds. are a relatively minor part of polyculture systems compris
ing Indian major carps and silver carp. Desired size at har
Grass carp spawn naturally only in large rivers, and vest is 1–3 kg depending on country and market. Fish are
will not spawn in ponds, tanks, or raceways without hor usually marketed locally as fresh fish.
mone injection. Nearly all grass carp used in aquaculture
are therefore produced using hatchery‐based artificial
346 Aquaculture release eggs into flowing water and eggs are immediately
16.2.2.2 Silver Carp fertilised. Each female may release 100 000–200 000
eggs/kg body weight. The semi‐buoyant eggs drift down
Silver carp are native to large rivers in eastern China. stream and hatch in 2–4 days. After larvae absorb their
The fish has been introduced to at least 90 countries in yolk sac, currents sweep them into slower‐moving back
Asia, Europe, Africa and North and South America. water areas and fry begin feeding on small zooplankton.
Introductions were most often made for aquaculture, Over 2–3 weeks feeding becomes increasingly phyto
but fish were also introduced as a biological agent for planktivorous, and juvenile and adult fish feed primary
phytoplankton control in eutrophic waters. Accidental on larger phytoplankton, with lesser amounts of zoo
introductions have also occurred. Silver carp biology is plankton and suspended detritus. Food is captured using
reviewed by Kolar et al. (2005). a highly efficient filter feeding process. Particles as small
as 10 µm, or less, can be captured by the fine gill rakers
Silver carp are deep‐bodied, laterally compressed and sponge‐like raker net.
fish with a scaleless abdominal keel running from the
pectoral region to anus. Adult coloration is dark grey Silver carp will not spawn in ponds, tanks, or raceways
dorsally shading to silver laterally and ventrally. Barbels without hormone injection. Eggs are stripped from the
are absent from the terminal mouth. Eyes are set well female, fertilised, and incubated in running‐water hatch
forward and below the angle of the jaw (Figure 16.8). ing jars, vats or tanks with enough water circulation to
They may grow to more than 1.2 m and 40 kg in natural keep eggs suspended. Larvae are held in the hatchery for
habitats. The gill rakers are highly modified into a 3–5 days until the yolk sac is absorbed and then trans
sponge‐like filtering apparatus. Mucus produced by ferred to small ponds fertilised with organic material
the epibranchial organ helps consolidate filtered mate (manures) to promote zooplankton growth. After 14–21
rial on the gill rakers and the overall result is a highly days, fry may be thinned and moved to fingerling‐rearing
effective food filtering and concentrating process. ponds for 3–6 months of further growth.
Within their native range silver carp live in rivers and The anatomy and feeding habits of silver carp are such
backwater lakes that are connected to large rivers. When that they do not feed to a significant degree on supple
introduced outside their native range, they adapt well to mental feeds. Their unique filter‐feeding habit makes
a variety of habitats including rivers, streams, reservoirs, them ideal for polyculture and silver carp are almost
lakes and ponds, but require flowing water for reproduc always used in systems with other fish. Silver carp poly
tion. Temperature tolerance ranges from near freezing to culture systems consist of extensive and semi‐intensive
more than 35 °C, with optimum growth temperatures of ponds as well as intensive cages and net pens. They may
24–30 °C. Feeding activity slows below 15 °C and ceases be grown in extensive ponds fertilised with organic
at about 10 °C. Salinity tolerance appears to be less than material or synthetic fertilisers, in combination with
grass or common carp, with tolerance to 7.5–10‰ and other filter feeders such as bighead carp, Wuchang
best growth below 4‰. The fish also does not appear to bream, Indian major carps or tilapia. In more intensive
be as tolerant of low dissolved oxygen as common carp. systems, silver carp are a secondary species, with grass
carp or common carp being the primary species. The
Silver carp mature at 2 years in warm climates and 5 primary culture species are fed some type of farm‐made
years, or more, in cooler waters. Spawning activity is ini or commercial prepared feed, and wastes produced by
tiated when water temperatures reach 18–25 °C and river the primary species stimulate phytoplankton blooms
levels rise, corresponding to spring and early summer that are eaten by filter‐feeding silver carp. The fish is
conditions in rivers within their native range. Adult fish mainly consumed locally and marketed alive or fresh.
migrate upstream and congregate in groups. Females
16.2.2.3 Bighead Carp
Figure 16.8 Silver carp (approximately 1 kg) harvested from a The native range of bighead carp is similar to silver carp,
‘composite’ polyculture pond in India. Source: Reproduced with although the range of bighead carp does not extend as far
permission from Les Torrans, 2017. north. Bighead carp have been widely introduced
throughout the world for aquaculture or as a biocontrol
agent for remediation of eutrophic waters. Naturally
reproducing populations are known to be established in
many countries in Asia, Europe and North and South
America. The fish is similar in appearance to silver carp;
it is deep‐bodied, laterally compressed, with a smooth,
scaleless abdominal keel running between the pectoral
fins to anus. Adult coloration is grey dorsally shading to
creamy white ventrally, sometimes with darker blotches Carps 347
on the sides (Figure 16.9). The head appears dispropor
tionately large for the body. Eyes are well forward, some than 0.5 million t to global aquaculture. Black carp are
what below the angle of the jaw and are oriented more native to large rivers in eastern China and outwardly
ventrally than those of the silver carp. Bighead carp can resemble grass carp in appearance, but are dusky‐grey
reach 40 kg in natural habitats but are usually harvested rather than bronze‐coloured. The fish has been widely
at 1–2 kg in culture. introduced throughout the world for aquaculture or as
a biocontrol agent for snails or other molluscs. Black
Reproductive behaviour, environmental tolerances, carp spawn naturally only in large rivers and will not
and general mode of feeding are similar to the closely spawn in captivity without hormone injection. Larvae
related species, silver carp (Kolar et al., 2005). Bighead and small juveniles feed on zooplankton, but as they
carp adapt well to a variety of freshwater habitats but grow the diet switches to snails, mussels and clams.
require flowing water for reproduction; they will not The molluscan diet provides a unique feeding niche
spawn in ponds, tanks, or raceways without hormone that can be utilised in polyculture. Black carp are a
injection. Spawning methods are similar to those highly revered food fish in China and are grown mainly
described for silver carp. For aquaculture purposes, the as part of pond polyculture systems where it is a minor
important distinctions between the two species are species with grass or Crucian carp. The relative high
slightly larger prey selection by bighead carp during fil prices paid for black carp has stimulated interest in
ter‐feeding and consumer preference for bighead carp emphasising black carp production. The fish will
over silver carp in China and most other Asian countries accept pelleted artificial feeds and in some areas it is
(Dey et al., 2005). now farmed as the principal fed species in polyculture,
with silver or bighead carp as the minor species utilis
Unlike in silver carp, the gill rakers of bighead carp are ing plankton communities stimulated by waste nutri
not fused at the base into a sponge‐like filtering appara ents excreted by black carp. Black carp are harvested
tus. As such, bighead carp are not as effective as silver and transported live to wholesale markets locally or
carp at retaining small particles during filter feeding. regionally, often in major cities.
Bighead carp are therefore considered primarily zoo
planktivorous whereas silver carp are considered phyto Wuchang bream are also a valuable and esteemed fish
planktivorous, although both species are highly in China. They are native to the Yangtze River basin in
opportunistic and considerable dietary overlap exists. China. Wuchang bream inhabit lakes and slow‐moving
waters and migrate into rivers to spawn. They are her
Bighead carp are grown primarily in pond polycul bivorous in nature but in culture they are usually fed
tures, filling a slightly different feeding niche than silver farm‐made or commercial feeds as the primary species
carp. They are almost always a secondary species, feed in pond or pen polyculture systems together with other
ing on zooplankton and larger phytoplankton that are carps as secondary culture species.
produced as a natural by‐product of fertilisation or feed
ing practices for the primary culture species, which may 16.2.3 Indian Major Carps
be grass carp, common carp, or others. Bighead carp are Indian major carps comprise a handful of cyprinid spe
mostly sold as fresh fish and consumed locally. cies that are endemic to the Indian subcontinent and
are important aquaculture species because of their rela
16.2.2.4 Black Carp and Wuchang Bream tively fast growth to a desirable market size. The major
Black carp and Wuchang bream (also called blunt carps are in the genera Labeo, Gibelion (Catla) and
snout bream and Wuchang fish) both contribute more
Figure 16.9 Bighead carp (approximately
5 kg) harvested from a naturally
reproducing population in the Illinois
River, USA. Source: J. Amberg, U.S.
Geological Survey.
348 Aquaculture Figure 16.11 Catla (approximately 1.5 kg) collected from
Ukai reservoir in Gujarat, India. Source: Reproduced with
Cirrhinus, and, more specifically, the term usually refers permission from Dibakar Bhakta, 2017.
to the species catla, rohu and mrigal. Major carps are
important aquaculture species in India but are also pro Environmental tolerances are not well known. The fish
duced throughout the subcontinent and Southeast survives at temperatures of about 17 to 40 °C, with an
Asia. Although Indian and Chinese major carps are optimum temperature for growth near 30–32 °C. Catla
often grown together in countries of the subcontinent, grow well at salinities up to at least 5 ‰. Catla appear to
temperature tolerance is an important distinction be somewhat less tolerant of low dissolved oxygen con
between the two groups of fish. The Chinese carps centrations than common carp.
evolved in a temperate climate and tolerate an extremely
wide range of water temperature—from near freezing Catla mature in 2 years and are stimulated to migrate
to more than 35 °C. Indian carps evolved in a tropical upriver to spawn when rivers rise during the monsoon
climate and cannot tolerate temperatures below about season and water temperatures are above 25 °C. Fish con
15 °C and have a slightly higher optimum temperature gregate and breed in littoral areas. Females may produce
for growth and a slightly higher critical maximum tem 100 000 to more than 200 000 eggs per kg body weight.
perature near 40 °C. This limits aquaculture of Indian The semi‐buoyant eggs drift downstream and hatch in
major carps to tropical regions. 18–24 hours, depending on water temperature. Larvae
begin to feed on small zooplankton 3–4 days after hatch
Global production of catla and rohu exceeded 1 million ing. Prey size and diversity increases as fish grow but
t in 2014 and these two species are described below. catla remain essentially planktivorous throughout life,
Mrigal (Figure 16.10) production was about a quarter that feeding opportunistically in near‐surface waters on a
of rohu, but its bottom‐feeding habit makes it a valuable variety of suspended matter, including zooplankton,
component of some polyculture systems with the other colonial algae and detritus.
two Indian major carp species (which feed in the water
column) and in so‐called ‘composite’ polyculture systems Catla spawn naturally only in rivers and repro
with Chinese major carps. However, harvest difficulties duction stimulated by environmental cues such as
associated with the mrigal’s bottom‐dwelling habit rising water temperatures and increased river flows.
make it the least preferred of the Indian major carps for In the past, brood catla were induced to spawn in
aquaculture. flooded fields (called ‘bundhs’) where water levels
16.2.3.1 Catla and flow were manipulated during the monsoon sea
Catla are native to rivers of the Indo‐Gangetic plains of son to simulate river conditions. Eggs were collected
northern India, Pakistan, Bangladesh and Myanmar. The from the bundhs by filtering outflow water through
fish has been introduced into waters throughout the sub fine n etting. However, most catla are now produced
continent. The body is stout and laterally compressed, in hatcheries using hormone injections to induce
with a large head and terminal, up‐turned mouth with no spawning.
barbels (Figure 16.11). The eyes are large and placed well
forward. The fish is grey dorsally shading to silvery white, Catla are appreciated as a food fish in India and other
with large, prominent scales. Catla are considered to be countries, and a variety of systems are used in catla
the fastest growing of the three major Indian carps. They aquaculture (Ramakrishna et al., 2013). Most growers
may grow to more than 35 kg in the wild but are usually in India use a 3‐stage system of nursery, juvenile culture
grown only to 1–2 kg in culture. and ongrowing to market size. Nursery ponds are small
(0.05–0.25 ha; 1‐m‐deep) and are prepared by liming
Figure 16.10 Mrigal (approximately 0.5 kg) collected from a and fertilisation (usually with poultry or cow manure)
reservoir in India. Source: Reproduced with permission from
Dibakar Bhakta, 2017.
to stimulate zooplankton communities. Postlarvae are Carps 349
usually offered a supplemental mash feed consisting of
powdered groundnut, rice bran, c ottonseed meal, and Figure 16.12 Rohu (approximately 0.75 kg) from an aquaculture
other by‐products—either alone or in mixtures. Fry are pond in India. Source: Reproduced with permission from Les
grown for 20–30 days and then transferred to fingerling Torrans, 2017.
growout ponds, which are larger (0.4–4 ha) and deeper
than nursery ponds. Fertilisation and supplemental midline. The caudal fin is deeply forked, and the fish is
feeding practices are similar to those used in nursery greyish dorsally, shading to silvery on sides. Rohu grow
ponds Fingerlings may be grown at high densities (up to slower than catla but faster than mrigal in culture. Rohu
0.3 to 1 million fry/ha) to produce a 20–25 g/fish in 2–4 may grow to more than 40 kg in rivers.
months or grown at lower densities to produce large
numbers of ‘yearlings’ of 50 to 125 g/fish after 10–12 Environmental tolerances, spawning behaviour and
months. early life history of rohu are similar to those of catla.
Larvae begin feeding on zooplankton but food selec
The widest variety of culture practices occurs at the tion becomes more opportunistic as fish get older.
ongrowing stage. For example, at least a dozen different Adult rohu feed in the water column on phytoplank
ongrowing systems using catla and rohu have been used ton, zooplankton, submerged aquatic vegetation and
in the state of Andra Pradesh, in southeastern India detritus.
(Ramakrishna et al., 2013). The use of catla in these vari
ous pond polycultures is a consistent feature, but size Rohu, like catla, spawn naturally only in rivers.
and number of fingerlings stocked, culture intensity and Although rohu can also be induced to spawn in flooded
the relative mixture of fishes vary widely. Catla are often fields (bundhs) manipulated to simulate river conditions,
grown in semi‐intensive, fertilised pond systems as a most rohu are produced in hatcheries using hormone
secondary species with rohu (as the primary species), injections to induce spawning.
and perhaps mrigal, usually with supplemental feeding
of mash feeds made from mixtures of de‐oiled rice bran, Rohu aquaculture is broadly similar to that used for
groundnut cake, cottonseed cake or similar ingredients. catla, with a three‐stage system of nursery ponds,
However catla (and rohu) are also grown in polyculture juvenile culture ponds and final ongrowing to
with Chinese major carps, common carp, tilapias, marketable fish. Natural food production stimulated
striped catfish (Pangasianodon hypophthalmus), fresh by pond fertilisation, provides much of the nutriment
water prawns (Macrobrachium rosenbergii) and even in at all stages, although supplemental feeding with pow
low‐salinity polycultures of penaeid shrimp. Of note, dered groundnut, rice bran, cottonseed meal and
commercial pelleted feeds are commonly used to grow other grain and seed by‐products is nearly universal.
striped catfish, with catla or rohu feeding on natural Rohu ongrowing is always conducted as part of a
productivity stimulated by the wastes generated from pond polyculture systems with catla, catla and mrigal,
feeding catfish. or in ‘composite’ culture with other Indian major
carps, common carp, grass carp and silver carp. In
Catla are harvested at 1–2 kg and marketed mostly as India, consumer preference for rohu relative to the
fresh fish. Markets are usually local or regional but may other species has led to emphasis on rohu as the prin
be quite some distance from the production site. For cipal species in these polyculture systems. For
example, much of the Indian major carp production in example, in Andhra Pradesh, rohu production is
Andra Pradesh is traded throughout India. Fish are emphasised by growing the fish in a simple, two‐spe
washed, packed in crushed ice and shipped in insulated cies combination with catla, with rohu constituting
trucks or vans through the country. more than 70% of the harvest biomass (Ramakrishna
et al., 2013). Rohu are harvested at 1–2 kg and
16.2.3.2 Rohu m arketed locally or distributed as iced, fresh fish
The native range of rohu is similar to that of catla and the regionally (Figure 16.13).
fish has been introduced for aquaculture throughout the
subcontinent and in several countries in Asia, Africa and
Oceania.
Rohu are handsome, moderately elongated fish with
conspicuous scales (Figure 16.12). The mouth is rela
tively small and subterminal, with a snout projecting
beyond the mouth. The eyes are lateral, dorsal of the
350 Aquaculture carp was originally summarised in the still‐useful text by
Jhingran and Pullin (1985).
Figure 16.13 A harvest of rohu transported to a central location
for transport to a processing facility in Myanmar. Source: Initially, high efficiency of ovulation was achieved
Reproduced with permission from Sena De Silva, 2017. using hCG either alone or in combination with carp pitu
itary extracts. This protocol was replaced by the use of
16.3 Artificial Propagation gonadotropin‐releasing hormone analogue (GnRHa) to
stimulate reproduction. But GnRHa alone is not entirely
As for many cultured fish species, the critical break effective and has to be accompanied by the administra
through in carp aquaculture was the development of tech tion of a dopamine receptor blocker. A better under
niques for artificial propagation of the major species. standing of neuroendocrine regulation of gonadotropin
Before this, carp culture depended on the collection of secretion has led to the development of new effective
natural seed for stocking. In fact, specialised fisheries techniques for induced ovulation and spawning of cul
developed in the flood plains of major rivers in mainland tured fish species. Basically, modern techniques of
China and India to collect the natural seed during the inducing ovulation and spawning use a combination of
spring and early summer spawning runs. In the case of drugs, one of which blocks the inhibiting action of dopa
catla and rohu, seed could also be collected from flooded mine within the neurohormonal systems (the Linpe
fields that were manipulated to simulate river conditions. method; section 6.2.1). Details of various effective com
binations of GnRHa (LHRHa, luteinising hormone‐
The traditional and most commonly used technique of releasing hormone analogue) or sGnRHa (salmon
induced spawning in carps is injection of either a) crude GnRHa)] and a dopamine antagonist (e.g., domperidone)
extract of the pituitary gland of common carp (or from for induced ovulation and spawning of Chinese major
other mature fish species that are phylogenetically close carps are summarised in Table 16.4. The Linpe method is
to carp) or b) partially purified human chorionic gonad known to be more effective in many ways, ensuring a
otropin (hCG). Use of hormones and/or analogues for high rate of ovulation, consistency between broods,
inducing spawning is referred to as hypophysation. complete ovulation and that the time lag between injec
Details of the hormonal control of reproduction in fish tion and ovulation is short and predictable.
and artificial spawning induction are given in Chapter 6
(section 6.2.1; Figure 6.1). The use of various techniques Ovulation and spawning by the Linpe method do not
for spawning induction of Indian and Chinese major influence the subsequent reproduction cycles of the
same brood fish. The Linpe method uses synthetic drugs
that are cheaper and more stable and, because only one
injection is needed, brood fish are stressed to a much
lesser extent.
Spawning induction of carps in India has been under
taken using commercially available kits (marketed
under the trade name ‘Ovaprim™’), which utilise the
Linpe method.
Another important advance in induced spawning of the
major carps has been the development of the ability to
spawn brood fish twice in a calendar year. A second
spawning is now achieved for most major carps and is
commonly practised. This development has enabled farm
ers to maintain fewer brood fish and has enhanced seed
availability almost year round. This has almost completely
eliminated the need for dependence on natural seed.
After hormonal treatment, broodstock are put into
spawning ponds in a ratio of three males to two females.
They usually spawn at dawn following a second injection
of hCG or pituitary extract and are removed from the
spawning ponds after spawning. The floating eggs are
moved from the spawning pond, by movement of the
water through the tank, into a collection box. Alternatively,
the brood fish may be stripped and a dry fertilisation per
formed. The process of stripping gametes from brood fish
and dry fertilisation is described in detail in Chapter 15
Table 16.4 A summary of the Linpe method of ovulation and spawning of cultured carp in China. Carps 351
Species Temperature (°C) Domperidone (mg/kg) Treatment sGnRHa (µg/kg) Time to ovulation (h)
Silver carp 20–30 8–12
5 LHRHa (µg/kg) – 8–12
Mud carp 22–28 5 10 6
Grass carp 18–30 5 20 – 8–12
Bighead carp 20–30 5 – – 6–8
Black carp 20–30 5 10 – 6–8
3 10 – 6–8
7 50 –
10
15
Figure 16.14 A commonly used
cylindrical spawning tank cum hatching
tank for carps (as well as other species)
known as the ‘Chinese design’. By
removing the polythene nets and concrete
stairs it could be converted into a
spawning tank. Source: Reproduced with
permission from Sena De Silva, 2017.
(section 15.3.3 and Figure 15.3). This method is becom generally about 13 cm in diameter and 60 cm long, with con
ing increasingly common in major carp culture. ical bases. Each jar is supplied with water up through its
conical base to create vertical water movement. The basic
Specially designed spawning tanks are now commonly concept of all hatchery designs for major carps is to provide
used for spawning, hatching of fertilised eggs fry produc a water current of sufficient strength to maintain the eggs in
tion of Chinese carps (Figure 16.14). They are usually the water column and to remove metabolic waste products.
circular or elliptical cement tanks about 1.2–1.5 m deep,
containing 50–60 m3 of water. The tank bottom usually Common carp and crucian carp are important to
slopes towards the centre, where an outlet leads to an egg Chinese aquaculture and technical advances in the type
collection chamber. Incoming water is directed to create of facilities used in artificial propagation of these species
a circular flow within the pool at a rate of 200–400 L/s. have been achieved in the last two decades. In the case of
common carp different modes of operation may be used
Fertilised eggs are transferred into incubation tanks or for producing viable larvae (Figure 16.15).
hatching pools which are circular, 3.5–4.0 m in diameter
and 1 m deep. Water flow is maintained at approximately The recent emphasis on the development of culture of
0.2–0.3 m/s. Eggs are usually incubated at a density of native species has led to concerted efforts to artificially
around 700 000–800 000 eggs/m3. Under these condi propagate cyprinid species that are of value to certain
tions, a hatching rate of about 80% is achieved. In China, countries in Asia. In this regard the development of arti
150 000–200 000 eggs are incubated in 150-L clay jars or ficial propagation techniques for two, high‐valued
in funnel‐type incubators with vertical water movement. Malaysian mahseer species, Tor tambroides and T. dou-
After 4–5 days, when the larvae have absorbed the yolk ronensis, stand out (Ingram et al., 2007). The above trend
sac, they are removed to nursery ponds. is being linked to introduce acceptable broodstock
m anagement procedures in order to maintain the genetic
Glass hatchery jars are also commonly used for hatching diversity of wild stocks and for conservation purposes.
both Indian and Chinese major carp eggs. The jars are
352 Aquaculture
Broodfish selection
Artificial spawning
Natural spawning
No With nests?
Smooth Yes
Cement tank
Sifting Hanging up
Incubation pond
Nests (Palm leaves or aquatic weeds)
Incubation tank
Nests (Nylon bolting cloths)
Incubation jar
Circular incubation pool
Figure 16.15 Different modes of Common carp artificial propagation in China. Source: Reproduced with permission from Dr X. Hu, 2017.
16.4 Nutrition nutrient requirements and basic nutritional research on
the group has lagged behind that of other cultured fish.
Of the commonly cultured cyprinid species, the nutrient Only recently has emphasis shifted, for some species, to
requirements of common carp are best known. This is to feeding commercial feeds that have been compounded
be expected because it was one of the earliest species to and formulated, and research on nutrient requirements
be cultured and examined experimentally. The nutrient of selected carp species is increasing. But with recent
requirements of most Chinese and Indian major carps are controversies on the use of fishmeal and fish oils in aqua
incompletely documented when compared to our under culture (see Chapter 5) there has developed a significant
standing of cultured salmonids and many other fish. In awareness on the use of these ingredients in feeds for all
part, the relative lack of research is due to the manner in aquatic animals, including carps.
which most carps are cultured. That is, most carp are
grown extensively or semi‐intensively in ponds, and The dietary protein requirement of the major carps
derive much of their nutriment from natural foods. As has been investigated. Most of these investigations
such, there has been little need to understand their exact have been carried out with fry and fingerling stages, and
there is considerable variation in the results of different
investigators. This is mostly a result of variations in Carps 353
experimental protocol. Based on available information,
the dietary protein level that results in maximum growth ●● Rearing fry to fingerling stage—most effectively done
of major carps is 45% and the economically optimal die in well‐fertilised rearing ponds.
tary protein content is 31% (Webster and Lim, 2002; see
also section 8.7.3 and Figure 8.2). It is, however, not uncommon to combine the above two
stages in one pond.
The protein, amino acid and carbohydrate require
ments of common carp are known (National Research Preparation of nursery and rearing ponds often involves
Council, 2011) and partial information is available for sowing a short‐term crop of a leguminous plant (e.g.,
catla and rohu (Webster and Lim, 2002). The dietary beans, clover), and ploughing and levelling the pond once
fatty acid requirements of cultured cyprinids are not well the plant crop has grown to 6–10 cm. This process is
known. Indeed, apart from the early work on common known as green manuring and is believed to enhance pond
carp which demonstrated that this species requires equal productivity. In most instances, unwanted organisms in
amounts of dietary linoleic acid (18:2n‐6) and linolenic the ponds are eradicated using a biodegradable toxicant or
acid (18:3n‐3), no studies have been conducted on the quicklime. This procedure is carried out at least a fort
fatty acid requirements of carps. It is plausible that the night before stocking. Commonly used toxicants are
originally identified requirement is true for all carps and derris powder (4–20 mg/L), oilcake of the plant Bassia
this conforms to the basic notion that freshwater fish latifola (mahua oilcake; 200–250 mg/L), tea‐seed cake
require the two fatty acids (section 8.9.7). They have the (525–675 kg/ha); or quicklime (CaO; 900–1000 kg/ha).
capability to elongate and desaturate these to longer‐
chain polyunsaturated fatty acids, such as eicosapentae The next stage is to prepare the pond with a view to
noic acid (20:5n‐3), docosahexasenoic acid (22:6n‐3) and ensuring a good production of small zooplankters, such as
arachidonic acid (20:4n‐6), among others. rotifers, which provide a food source for growing larvae.
Ponds are often treated with either organic or inorganic
Information available on the requirements of other fertilisers. The quantity of manure to be used is related to
nutrients is scant and all the nutrient requirements for the toxicant used earlier. For example, if mahua oilcake
any one species of major carps are not known. This situ was used, a dose of dry cow manure at the rate of 5000 kg/
ation is changing rapidly however, as production intensi ha two weeks before stocking and a similar dose 1 week
fication has made formulated feed use increasingly after hatching are desirable. However, with toxicants that
common for some species, including common carp, have no fertiliser value, doses of 10 000–15 000 kg/ha ini
grass carp, Crucian carp, and even the mollusc‐feeding tially, and 5000 kg/ha later, are desirable. These manuring
black carp. The increased emphasis on intensive, feed‐ doses are sufficient for 1–2 million larvae/ha.
based culture has stimulated considerable research into
the nutrient requirements of certain major carps. Fertilisation with a mix of organic and inorganic fertilis
ers is undesirable, as more often than not it results in
16.5 Culture Practices harmful plankton blooms. Despite the early preparation
of the ponds, undesirable predatory insects such as water
16.5.1 Larval Rearing spiders and water skaters may colonise the ponds.
Induced cyprinid spawning is discussed in section 16.3. Therefore, the ponds have to be regularly treated for insect
After the eggs are fertilised, hatching occurs in 2–3 days at control, particularly before stocking. Jhingran and Pullin
23–27 °C. The yolk sac continues to provide nourishment (1988) recommend any one of the four treatments:
for a further 3 days or so, at which time larvae require an
exogenous food supply. It is desirable, if not essential, to ●● spraying an emulsion of 56 kg of mustard or coconut
expose the larvae to an external food source before yolk oil and 18 kg of washing soap per hectare;
sac resorption is complete. This involves removing larvae
from the hatchery jars and introducing them into a fry‐ ●● spraying an emulsion of 56 kg of mustard oil and
rearing facility. The young hatchlings, which mostly move 560 mL of Teepol (detergent) per hectare;
vertically, tend to change to a horizontal movement, which
indicates their readiness to ingest food particles. ●● a 0.01 ppm dose of pure gamma isomer of benzene
hexachloride dissolved in ethyl alcohol; or
Larval rearing in carp culture has two distinct phases:
●● Rearing postlarvae to the fry stage—usually carried ●● application of 0.25–3.0 ppm organophosphate such as
fumadol, sumithion or diptrex.
out in nursery ponds or in hapas (fine mesh enclo
sures) suspended in ponds or channels The prepared ponds are stocked when it is certain that a
substantial zooplankton population (particularly small
zooplankters such as protozoans and rotifers) is estab
lished. Abrupt changes in quality and temperature
between hatchery water and nursery water are avoided
when stocking. Stocking is best done in the evening,
which hopefully gives the larvae sufficient time to accli
matise themselves before any possible predation. The
354 Aquaculture 16.5.2 Ongrowing to Market Size
Three distinctive features characterise culture practices
stocking rate depends on the proposed management commonly used for ongrowing major carps to market
practice and, if the following conditions are met, a stock size (Ramakrishna et al., 2015; Wang et al., 2015):
ing rate of 1 million fry/ha can be used: ●● ponds are the most commonly used culture system;
●● continued and repeated fertilisation to produce and ●● practices are predominantly semi‐intensive, relying
maintain good plankton production; on natural foods supplemented with some type of
●● supplemental feeding; and feed; and
●● equipment to remedy oxygen deficiencies that may ●● polyculture is almost always used.
Cage culture in reservoirs is also common, particularly
occur. in China. Carps are also grown as a component of ‘cul
Carp post‐larvae are voracious grazers. Supplemental ture‐based fisheries’ which is essentially a form of stock
feeding and manuring, when carried out concurrently, enhancement. In culture‐based fisheries, reservoirs or
result in better survival and growth. The commonly used lakes are fertilised and stocked with various fish or crus
supplemental feeds in carp culture are rice bran and taceans which utilise natural productivity for growth.
oilcakes of peanut (groundnut), coconut and mustard Major carps are important contributors to culture‐based
in India. These are also used in China, together with fisheries (section 16.8) wherever it is practised.
soybean milk and meal, and egg‐yolk paste. It is rare for In the past, carp culture was often integrated with
juvenile carp culture to be based on complete formulated other poultry, duck, or swine farming wherein waste
feeds, except in the case of common carp culture in some from animal production were used to fertilise fish ponds.
countries. Feeds are often dispersed as a crude mix, in Integrated farming, despite its appeal as an extraordinar
either dry (e.g., meals or pellets) or moist form. A sum ily efficient farming system, is rapidly disappearing
mary of commonly used feeds and feeding schedules for (Edwards, 2015) due to the emphasis on maximising
fry of Chinese carps is shown in Table 16.5. However, it profits from the aquaculture component as well as public
should be noted that in the case of common carp, which health and quality‐control concerns. Indeed, a recent
is cultured fairly intensively on a small scale in ponds in governmental decree in China—the global leader in
Israel and in cages in China, formulated diets (pellet aquaculture production and carp culture—bans inte
feeds) are used. grated fish‐livestock farming.
Polyculture is thought to have originated in China,
The fish in rearing ponds are harvested with sieve nets when various combinations of species with widely differ
when they reach 4–6 cm. Periodical harvesting may be ent food habits were cultured together, e.g., black carp
carried out to avoid overcrowding. Often, rearing post (feed on snails), grass carp (feed on coarse vegetable
larvae to fingerlings is undertaken in larger, earthen matter), silver carp (feed on phytoplankton), bighead
ponds and polyculture is practised. Stocking densities of
fingerlings in polyculture range from 100/m2 to 2500/m2
with a mean of about 800/m2 (Table 16.6). Size at har
vesting usually ranges from 7–20 cm, with grass and
black carps tending to be the largest.
Table 16.5 Some feeds and feeding rates used for Chinese carp fry and fingerling rearing.
Country Species Pond area Depth Fish length Age SD Feeding rate
(m2) (m) (mm) (days) (per m2) Feed
China Bighead, – 0.5–1.0 Up to 20 Up to 30 100 Egg yolk paste or 1 egg/2500–7500 fry/
grass carp soybean milk + peanut day or milk from
cake after 10 days 300–500 g beans/
All species – 0.5–1.0 23.1 Up to 30 – Soybean meal 50 000 fry/day
All species 1000 Soybean milk and 45 kg/5000 fry/
Hong 0.8 8–30 (3 mg to 1 g) Up to 150 peanut cake meal month
Kong All species 1400 25–30 Peanut cake, rice bran 100 kg soybean milk
or soybean cake or 200 kg peanut
1.0 31 (1.5 g) 30–70 35 cake meal/month
Start at 1.5 kg/day,
build up to 5 kg/day
Source: Jingran and Pullin (1988). Reproduced with permission from Worldfish.
Carps 355
Table 16.6 Examples of stocking rates and size at harvesting of carp fingerling in polyculture (gc, grass carp; bc, black carp; sc, silver carp;
bhc, bighead carp; cc, common carp; wb, Wuchang bream.
Stocking density (×100/m2) Size at harvesting (cm)
gc Bc sc bhc cc wb gc bc sc bhc cc wb
4–6 – 20–25 – –– 13–15 – 8–10 – ––
10–25 – 4–5 – –– 8–13 – 11–13 – ––
2–4 – – 8–12 –– 16–20 – – 11–13 ––
4–6 – – 15–20 – – 13–15 – – 8–10 ––
10–25 – – 4–5 – – 8–13 – – 11–13 ––
– 5–6 4–5 – – – 13–15 13 – ––
– 5–6 – 4–5 – – – 13–15 – 11–13 – –
1 –– 4 5–6 – 13 – – 13 8–10 –
1 –– 0.08–0.1 – 15–20 16–20 – – 0.25–0.5 kg – 7
carp (feed on zooplankton and omnivorous) and mud region‐specific culture protocols. This is best exempli
carp (a bottom scavenger). A typical species combina fied in Andhra Pradesh, a coastal state in south‐east
tion used in a polyculture practice with an approximate India. In this state, many different combinations of fish
indication of the niches occupied by each of the species have been used in polyculture but currently the most
is shown in Chapter 2 (Figure 2.4). The number of spe common approach is to co‐culture only two species of
cies used, and the ratio of each species, varies from Indian major carps, catla and rohu (rohu being the domi
region to region. Polyculture, apart from ensuring that nant species at about 80% of production). Ponds often
most of the food resources in the system are efficiently exceed 1 ha and are generally stocked at a density of 5000
utilised, offers other advantages, including higher yields, fish/ha with 6‐ to 12‐month‐old (100–150 g) juveniles.
reduced incidence of infectious diseases and better Ponds are fertilised with poultry manure and inorganic
growth rates of some species than in monoculture. fertiliser, and are provided with supplementary feed,
Polyculture maximises the synergistic fish–fish and fish– often consisting of simple mixtures of rice bran (de‐
environment relationships and minimises antagonistic oiled) and oilcake (mustard, peanut). The feed mixture is
relationships. suspended in perforated polythene bags from bamboo
poles at a number of locations in the ponds (20–25 poles/
Despite various experimental findings in both China ha), from which the fish soon learn to feed. In this region,
and India, farming activities tend to depend on the indig production averages about 8000 kg/ha with a range of
enous species of each country. This trend is primarily 5300–14 620 kg/ha. Fish are harvested when they are
influenced by the preferred consumer acceptance of over 1.5 kg.
indigenous species. In China and India, where carp cul
ture is the predominant form of fish culture, two or three Another, less common, approach to carp culture in
species of either Chinese or Indian carps are polycul India uses both Chinese and Indian carps. This concept
tured. In these polyculture systems, the dominant spe was termed ‘composite fish culture.’ The basic species
cies in China is silver carp, and in India it is rohu. The combination in Indian composite polyculture are catla,
actual culture practices vary from region to region and rohu, mrigal, silver carp, bighead carp and common
country to country. The primary variables are the size at carp. When stocked at a density of 5000/ha (120–250 kg/
stocking, stocking density, fertilisation regimes, and the ha) the annual yield was nearly 9 t/ha when ponds are
nature and quantity of supplementary feeds. In Chinese fertilised, and fish are provided simple supplemental
systems, for example, fingerlings are generally stocked at feed, such as a mixture of rice bran and oilcake.
a size of about 15–20 g (>10–12 cm total length). In
Andhra Pradesh, India, rohu are stocked in grow‐out In China, polyculture is practiced with Chinese carps
ponds when they are more than 2 years old (between 80 in conjunction with common carp. There are also signifi
and 100 g), as it is believed that this is when they approach cant differences in regional culture practices within
their maximum growth rate. China. The most important difference is the dominant
species in polyculture systems. For example, grass carp is
Since about 1990 farmers and scientists in the main the main species used in southern China, whereas silver
carp‐producing countries have developed country‐ or and bighead carp dominate in central China. There has
356 Aquaculture impossible to assess the potential yield from any one
practice. A summary of practices and production from
been a trend towards increasing the proportion of graz a survey of 348 fish farms in Jiangsu Province is pre
ing fish—such as grass carp, black carp and Wuchang sented in Table 16.7. Farms in the survey were classi
bream and a corresponding decrease in filter‐feeding fied as belonging to one of four models based on
fish such as bighead and silver carp. This change in cul the primary and secondary species in the system.
ture emphasis is attributable to changing food prefer Production of common carp in one system ranged
ences, with silver and bighead carp becoming less between 14 250–17 475 kg/ha, and the total yield of all
popular, especially with the growing Chinese urban pop species combined was 18 000–18 750 kg/ha—a very high
ulation. There is also an increasing tendency to use production for a pond system relying mostly on primary
Chinese major carps in low stocking numbers in con production to support fish production. A detailed study
junction with the culture of species of high economic on carp farming systems in Andhra Pradesh, India, was
value, such as crayfish (Procambrus clarkii) and mitten conducted by Ramakrishna et al. (2013) and reported
crab Eriocheir sinensis (Wang et al., 2015). These prac average annual production from several production
tices are relatively new and also provide improved water systems used in the region. As described above, most
quality and subsidiary income. ponds were stocked with rohu and catla and operated
The wide range of culture practices adopted in carp
culture, within and between regions, makes it almost
Table 16.7 Data on stocking and harvesting of four different polyculture farming models based on primary and other species used
in each of the practices in Jiangsu Province.
Stocking Harvest
Species Size (g/ind.) Density (ind./ha) Size (kg/ind.) Yield (kg/ha)
Crucian carp 50 22 500–30 000 Model 1 7500–12 250
Silver carp 50–100 2250 0.35–0.45 2025–2625
Bighead carp 100–167 750 1–1.5 1225–1350
Wuchang bream 50 1500 1.5–2 600–650
Total 0.45 12 250–18 000
167–250 3750
Grass carp 50–100 2250 Model 2 7500–8250
Silver carp 100–167 750 2.5 4050–5250
Bighead carp 50 4500 1–1.5 2250–2700
Crucian carp 1.5–2 2250–3000
Total 100–125 18 000–22 500 0.33 11 625–13 500
50–100 2250
Common carp 100–167 750 Model 3 14 250–17 475
Silver carp 250 300 0.9 2025–2625
Bighead carp 1–1.5 1125–1350
Grass carp 50 18 000 1.5–2 625–750
Total 50–100 2250 3 18 000–18 750
100–167 750
Wuchang bream 50 3000–4500 Model 4 9750–12 000
Silver carp 0.6–0.75 2025–2625
Bighead carp 1–1.5 1125–1350
Crucian carp 1.5–2 900–1500
Total 0.35 14 1 250–17 250
Source: Data are from the unpublished 2006 Master’s Degree dissertation ‘Pond Culture Technology Investigation and Study in North Jiangsu’ by
Yongguang Luo, Nanjing Agriculture University). Models 1–4 Reproduced with permission from Pr Y. Luo, Nanjing Agriculture University.
semi‐intensively with supplemental feeding. Average Carps 357
annual production was about 6000–10 000 kg/ha.
16.5.3 Food and Feeding
Not all carp are grown in extensive or semi‐intensive 16.5.3.1 Natural Food Availability
pond polycultures and exceptions to this paradigm are In view of the fact that the great bulk of carp culture is semi‐
becoming more common. Common carp and grass carp, intensive, increasing the availability of natural food types in
for example, are often intensively cultured in ponds and the culture systems plays a crucial role in enhancing yields.
cages in China, as is the case of rohu in Myanmar. A typi As pointed out earlier, the most common method used for
cal carp farm in China can be rather large and ponds increasing natural food supply in carp ponds is through the
often have supplemental aeration using paddlewheel application of inorganic fertilisers and/or organic manures.
aerators (Figure 16.16). In some areas, cage culture of The commonly used organic manures include cow dung,
carp is relatively common. One example is in West Java, poultry litter and pig dung, and the inorganic fertilisers are
Indonesia, where common carp are grown intensively in superphosphate and ammonium sulphate.
cages in three reservoirs. The cage systems used in
Indonesia—and being adopted elsewhere— are interest There have been many studies conducted on the effects
ing. They are locally referred to as ‘apis dua’ and are of fertilisation and manuring in carp polyculture prac
multi‐layered systems consisting of an inner cage with tices. However, apart from the fact that such practices
the primary (fed) species surrounded by a much larger result in increased algal production, it is impossible and
cage with one or more secondary species that feed on impractical to make a set of general conclusions from the
feed wastes produced by the primary species or natural findings. This is for a number of reasons, the foremost
foods swept through the cage by currents (Wang et al., among these being:
2015). For example, a possible combination might be ●● many different fertilisation and/or manuring regimes
common carp fed pelleted feed in the inner cage and Nile
tilapia in the outer cage. are used;
●● many different stocking densities and species combi
nations are used; and
Figure 16.16 A typical farm for carps in Hubei Province, China. Note the grass planted on the banks of each pond, which is used for
feeding grass carp. Source: Reproduced with permission from Sena De Silva, 2017.
358 Aquaculture feed through the perforations. It is almost impossible to
assess the food lost in the latter practice, and indeed it is
●● responses to fertilisers and other culture practices vary possible that the food has a greater effect as a fertiliser
with climate, water source, soil type and other physic than providing direct nutrition to the fish. All of the above
ochemical factors. aspects, particularly in relation to Indian major carp cul
ture systems are described by Ramakrishna et al. (2013).
Pond fertilisation is discussed in more detail in Chapter 4
(section 4.4.2) and Chapter 9 (section 9.9) and a summary Mixed feeding schedules, i.e., use of different feeds at
of the principles of pond fertilisation, including carp different feeding times, have been found to have benefi
ponds, is presented by Mischke (2012). cial effects on growth and food cost reduction, and a
reduced discharge of nitrogen and phosphorous, without
16.5.3.2 Supplementary Feeds compromising growth performance. This concept was
The supplementary feeds used in carp culture are diverse. originally developed for Nile tilapia but has been found to
Most supplementary feeds are simple mixes of agricul be effective for carps also. However, and rather unfortu
tural by‐products, which are readily available at a relatively nately, the experimental findings have not been extended
low cost. The most common of these are brans of rice and to on‐farm practices, and as such missed the opportunity
wheat, often mixed with cakes or meals of various oilseeds to reduce feed costs and nutrient levels in the effluent.
such as mustard, canola and soybean. Most farmers tend
to use some sort of supplementary feed, which could be 16.5.4 Harvesting
either a single ingredient or a mixture of two or three, at Pond‐cultured carps are harvested when individual fish
most. The quantity of feed as well as the amount of indi reach a weight of about 1–2 kg unless they are destined
vidual ingredients used in the feed mixes could vary for a specialised market. Harvesting in most instances is
greatly. Obviously, this is an area that needs further done by seining and is labour intensive. Major carp cul
research, which in the long run could reduce feed use and ture ponds are rarely flow‐through and are rarely com
thereby increase profitability. In addition, it could also pletely drainable. These factors together with the large
lead to improved water quality in the ponds and cleaner size of ponds make harvesting by seining almost an
pond effluent. This trend is indicative of a potential con imperative. Major carps are generally marketed fresh. It
straint to expansion of culture activities due to increasing is not uncommon to retain portions of the catch, live, in
competing demands for the same food ingredients from temporary net pens, to minimise market saturation
other animal husbandry activities and from other users. within a short period of time. This practice is followed in
most rural areas when the distances to population cen
In Chinese polyculture, a wide range of ingredient tres are high and the total production in an area does not
mixes is also used as feed—the type and quantity often justify transportation to such centres.
being dictated by availability and price. Soybean meal,
sesame cake, silkworm pupae powder and canola meal 16.6 Diseases
are more commonly used in major carp farming systems
in China. Carps, like most fish, are susceptible to a variety of infec
tious (viral, bacterial, fungal and parasitic) as well as
As mentioned above, the use of manufactured pelleted environmentally induced disorders caused by poor water
feeds has become common in many types of carp aqua quality (Chapter 4). A somewhat dated, although still
culture. For example, Chiu et al. (2013) surveyed 351 useful, comprehensive monograph on carp diseases,
farms in three provinces of China (Hainan, Shandong, diagnostic procedures and treatments is provided by
and Zhejiang) to evaluate production practices and feed Hoole et al. (2001) and readers are recommended to refer
use. Most farms engaged in some form of carp polycul to this rather extensive work.
ture and more than 95% of the farms used commercial,
manufactured feeds to some degree. 16.7 Genetic Improvement
16.5.3.3 Feeding As described in Chapter 7, apart from the genetic
In general, feed management is relatively poor in carp improvement of salmonid stocks and, more recently,
culture. The main reason for this is that practices often Nile tilapia and channel catfish, genetic improvement of
depend on supplementary feeds, which are simple mixes cultured fish and shellfish species has lagged far behind
of agricultural by‐products. Perhaps the only exception is
the feeding practice adopted in common carp farming in
eastern Europe and in some parts of Asia. A wide range of
feeding practices are used by carp farmers, from simple
hand broadcasting to tying perforated bags containing
food to sticks in the pond and allowing the stock to obtain
that of farmed terrestrial animals. There is a gradual Carps 359
change however, with an increasing emphasis on the
genetic improvement of cultured groups in Asia field studies and mass selection to improve perfor
(Nguyen, 2016). mance of superior strains. For example, common carp
strains selected for improved performance are widely
Some of the cyprinid species, in particular common used in China. There has also been interest in produc
carp and crucian carp, have been domesticated for cen ing interspecific hybrids to produce populations with
turies. Domestication and consequent selection have heterosis for important production traits or to produce
resulted in the development of a number of strains, gen sterile populations. More than 100 different hybrids
erally selected for aesthetic purposes rather than for pro have been made in China among common carp,
duction characteristics useful in aquaculture, such as Crucian carp and various Chinese major carps.
growth rate, meat yield and tolerance to diseases and Likewise, more than 40 hybrids have been produced in
environmental stressors. Examples of this are discussed India among common carp, Chinese major carps and
in section 7.2.1 in relation to selection for scale type in Indian major carps. Promising hybrids have, for exam
common carp. ple, been produced between rohu and catla that have
superior meat quality and faster growth in culture.
In view of the importance of common carp to Chinese
aquaculture there had been a concerted effort to pro Despite considerable recent research activity in genetic
duce strains that possess improved traits that are impor improvement and demonstration that considerable gains
tant in aquaculture (Penman et al., 2005). In China, a can be made by using improved germplasm or hybrids,
number of varieties of common carp have been identi there has not been widespread adoption in the commer
fied and used in breeding programs. For example, the cial sector. Penman et al. (2005) and Nguyen (2016) both
Xingua red carp C. carpio singuonenis is a common carp conclude that increased efforts to transfer technology to
variety used in culture for hundreds of years. Beginning the commercial sector need to be a priority for aquacul
in the 1970s, a program of mass selection has increased ture development in carp‐producing countries.
growth rate by about 10% per generation and the
improved strain has been used as the parent strain for 16.8 Culture‐Based Fisheries
several other important varieties. Similarly, a cold‐
resistant strain of Chinese Purse red carp was produced Culture‐based fisheries are, in essence, a form of stock
by hybridising Heilongjiang common carp and Purse enhancement where fish are added to a water body that
red carp C. carpio wuyuanensis. Systematic selective is incapable, for one reason or another, of sustaining a
breeding of F1 sibs produced a variety that retained the fishery. For example, a water body may not have suitable
cold resistance of the Heilongjiang carp but also toler spawning areas and regular stocking is required to
ance to hypoxia and desirable red colour of the Purse replenish populations. In the traditional view of stock
carp. An important intraspecific hybrid—the Jian carp, enhancement, the resulting fishery is commonly acces
C. carpio var. jian) was produced in a complex program sible. What distinguishes culture‐based fisheries from
of family selection. The Jian carp grows about 50% faster traditional stock enhancement is individual or collective
than either of the two parental strains and is perhaps ownership and management of the fishery. As such,
the single most widely‐used common carp variety in culture‐based fisheries can easily be defined as a type
Chinese commercial aquaculture. of aquaculture.
Genetic resources of Asian and Indian carps are Culture‐based fisheries are usually established in rel
reviewed in depth by Penman et al. (2005). This syn atively small water bodies although larger lakes and res
thesis included genetic resources and improvement ervoirs partly or completely managed as culture‐based
fisheries are increasingly common. In most cases cul
programs in India, China, Bangladesh, Thailand and ture practices are limited to stocking of juvenile fish
Vietnam and summarised the status of genetics that use the natural productivity of the waterbody for
research up to the time of publication. Nguyen (2016) growth. Occasionally, however, productivity is
enhanced by fertilisation. Culture‐based fisheries use
more recently reviewed the status of carp genetic existing water resources and therefore do not compete
research. Since about 2000, there has been considera for land and water with other uses. Culture‐based fish
ble progress in certain areas of genetics research. eries are also ecologically efficient because they use
Selective breeding programs in China have produced water non‐consumptively and have minimal input of
superior strains of common carp and breeding pro resources, such as feed.
grams for other major carps exist in India and other Culture‐based fisheries are considered to have very high
countries where carp aquaculture is important. potential to contribute to food resources, particularly in
Differences in production traits clearly exist among the light of increasing demand for primary resources
strains of the same species and genetic improvement
programs are usually based on strain comparisons in
360 Aquaculture 16.9 Conclusions
such as land and water. They are often recognised This chapter highlights the importance of carp culture,
as an important avenue for increasing inland fish some of the key features of carp culture practices, and the
production, particularly in developing countries potential of carp culture as a food source. Living stand
(De Silva, 2016). ards are generally increasing throughout the world, and it
is often suggested that the demand for carp will decline as
Apart from all of the above, culture‐based fisheries a result. However, production trends do not support that
have significant relevance for carp culture, because the contention. To the contrary, in light of increasing envi
majority of such fisheries are based on Chinese and ronmental concerns related to the culture of carnivorous
Indian major carps, occasionally augmented with tilapia fish species, it may be that carp culture could become
and other minor species. The variety of feeding niches even more important in the future. In this regard China,
offered by the various carps, and their generally low the main producer of cultured carp species, continues to
trophic level, provides opportunities for efficient use of improve the carp culture techniques, including reducing
natural food produced in ponds and lakes. Also, the fact the environmental impact of production (such as nutrient
that the Chinese and Indian major carps need flowing pollution associated with aquaculture facility effluents).
water to reproduce offers an advantage in some situa
tions; that is, the absence of natural reproduction means One of the major constraints to further intensification
that populations can be carefully managed for optimal of carp culture is increasing competition for supplemen
fish production relative to the waterbody’s potential tary food sources, which are primarily agricultural by‐
productivity. products. As such, a concerted effort may be required to
develop suitable feeds and to develop more prudent strat
Culture‐based fisheries in China are the most devel egies of feed management. Apart from yield increases
oped in the world. This fishery is confined to small‐ and through intensification and better pond culture prac
medium‐sized reservoirs throughout the country. tices, popularisation and development of culture‐based
Estimated production from culture‐based fisheries in fisheries using carp species appears to have the greatest
China have increased from about 100 000 t in 1981 to potential to augment inland fish production. This is
more than 3.3 million t in 2012 (Wang et al., 2015). particularly important to developing countries, thereby
Annual productivity is estimated at about 1800 kg/ha, making available a good‐quality source of animal protein,
which is amazing for extensive systems that rely only on at an affordable price, to the poorer sectors of the com
seed addition (De Silva, 2016). munity. It is also important to note that selective breed
ing for faster growth, disease resistance and cold‐ and
Culture‐based fishery practices in China are based pri warm‐tolerance of selected carp species is proceeding at
marily on stocking grass carp, bighead carp, silver carp many research centres in China, India and elsewhere. It
and common carp. In addition, species such as the is possible that such strains will become available to
Wuchang bream, black bream (Megalobrama terminalis) farmers in the not too distant future and will help main
and mud carp may be used. In southern Asia, culture‐ tain economic viability of carp farming. It certainly will
based fisheries are primarily based on a combination of be hard to replace the role of carps as a group of fish
Chinese and Indian carps, the latter being predominant. possessing so many desirable attributes for farming.
Success of the culture‐based fishery practices in China
are based on the following: 16.10 Summary
1) Consideration (at the planning stage of reservoir con ●● Carps (family Cyprinidae) are the most commonly cul
struction) of those factors that enhance fishery tured fish in the world. Total carp production in 2014
production; was almost 28 million t, accounting for 65% of all inland
finfish aquaculture, 56% of total finfish aquaculture,
2) Relatively large and uniform size of fish at stocking; and almost 40% of total animal aquaculture produc
3) Minimising the number of escapees; tion. Carps are grown throughout the world, but China
4) A staggered but complete harvesting of the stock; and and India dominate cyprinid aquaculture production.
5) Adopting marketing strategies that minimise an over
●● Many carp species have traits that make them good
supply of fish within a narrow time‐frame. candidates for aquaculture but six species (grass carp,
silver carp, common carp, bighead carp, catla and
Wang et al. (2015) and De Silva (2016) review develop rohu), each with an annual production of more than
ments in culture‐based fisheries in China and else
where and point out the changes in culture‐based
fisheries practices following restrictions of using ferti
liser in open water bodies in China. These changes
include, for example, a shift to higher‐valued species
such as Chinese mitten crab and Mandarin fish
Siniperca chuatsi.
1.5 million t in 2014, constitute 91% of total carp aqua Carps 361
culture. Three additional species—Wuchang bream,
black carp and mrigal—represents an additional 6% of ●● Most carp aquaculture is conducted in ponds using
total carp aquaculture. As such, 97% of carp aquacul various combinations of fish in polyculture. Typically,
ture is represented by nine species. there is a primary species (for example, common,
●● Although the important carps in aquaculture are very Crucian, or grass carp) that is fed a supplemental diet
fecund, they do not spawn in captivity without using of agricultural by‐products or pelleted manufactured
hormones to induce spawning. feed. Feeding waste stimulates production of natural
●● Feeding habits of the major carp species vary considera foods that support growth of secondary species (for
bly, including opportunistic bottom‐feeding omnivores example, the filter‐feeding bighead or silver carp).
(common carp and mrigal), opportunistic water‐column Increasingly, however, carp aquaculture relies on sim
planktivores (rohu and catla), water‐column filter feed pler intensive systems, such as monocultures, based
ers (bighead and silver carp), aquatic plant herbivores on pelleted commercial feeds.
(grass carp) and molluscivores (black carp). The variety
of feeding habits provides opportunities for polyculture ●● Carps are also cultured in cages and pens in lakes and
where fish with complimentary feeding habits are rivers and in culture‐based fisheries. Culture‐based
co‐cultured together to make efficient use of the wide fisheries are a form of stock enhancement where water
variety of foods available in aquatic habitats. bodies are periodically stocked with juvenile fish which
grow by using the natural productivity of the water.
The fishery is owned and managed individually or
collectively.
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363
17
Salmonids
John Purser
CHAPTER MENU 17.7 Maturation, Sex Reversal and Triploidy, 384
17.1 Introduction, 363 17.8 Fish Health, 386
17.2 Biology, 364 17.9 Harvesting and Products, 387
17.3 Freshwater Farming, 367 17.10 Environmental Issues, 387
17.4 Marine Farming, 376 17.11 Summary, 388
17.5 Feeds, 381
17.6 Grading and Stocking Densities, 383 References, 388
17.1 Introduction aquaculture production of salmonids in developed coun-
tries than in developing countries, which correlates with
There are numerous species and strains of salmon, trout the high level of industrialization in its production.
and charr within the family Salmonidae. Most of the
commercially important species belong to the genera Freshwater facilities are usually used for:
Salmo, Oncorhynchus and Salvelinus. They are all either ●● rearing salmonids to fingerling or smolt stage for subse-
anadromous (ascending rivers from the sea to breed) or
complete their entire life cycle in freshwater. No species quent transfer to freshwater or sea farms for grow‐out;
is solely marine. The major species produced in aqua- ●● freshwater raceways, which are typically used for
culture are listed in Table 17.1. Capture fisheries contrib-
ute widely varying proportions of total market supply rainbow trout grow‐out, and which require very rapid
depending on species. Atlantic salmon and rainbow pumping from a generous source of freshwater
trout are the two most important species in aquaculture (surface water, ground water or springs);
and, together with brown (sea) trout, nearly all their ●● as ponds for rainbow trout grow‐out.
production is derived from aquaculture. Aquaculture Sea cage1 culture has proved to be the most successful and
accounts for most Chinook and coho salmon production widely used strategy for marine production (Figure 3.6).
for human consumption although capture fisheries also The alternative pump‐ashore system, where seawater is
contribute significantly. Pink salmon, sockeye salmon pumped into land‐based culture facilities, is not widely
and chum salmon are important commercial species but used for marine culture. In recent years, however, large
are mostly derived from capture fisheries. Arctic charr recirculating shore‐based marine systems have been pro-
(Salvelinus alpinus) and brook trout (Salvelinus fontinalis) posed as a method of controlling the culture environment
have received attention for commercial aquaculture but and avoiding threats from predators, algal blooms and
have yet to develop significant levels of production. pollution experienced in cage culture. Similarly, sea cages
based on solid-bag net configurations offer protection
Salmonid production from both aquaculture and fish- from threats experienced by conventional mesh nets, but
eries, especially the former, is increasing significantly. they rely on close monitoring and low‐head pumping to
Atlantic salmon sea cage production started relatively circulate water. The solid bag excludes algae, parasites,
recently in the 1960s and developed into a major indus- predators and jellyfish, and allows the control of lighting
try in the 1980s and 1990s. Unlike many of the major
products from aquaculture, there is substantially more 1 The fish are enclosed in a flexible bag‐like structure, usually mesh,
suspended below a floating ring or other shape.
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.
364 Aquaculture
Table 17.1 Global salmonid aquaculture production in 2014*.
Common name Scientific name Aquaculture production (t)
Atlantic salmon Salmo salar 2 326 288
Rainbow trout Oncorhynchus mykiss 812 939
Pink (humpback) salmon Oncorhynchus gorbuscha 495 986
Sockeye (red) salmon Oncorhynchus nerka 164 222
Coho (silver) salmon Oncorhynchus kisutch 132 576
Chum (dog) salmon Oncorhynchus keta 48 638
Brown (sea) trout Salmo trutta 25 554
Chinook (king) salmon Oncorhynchus tschawytscha 20 448
* FAO data.
using covers and temperature by altering the depth of Table 17.2 Countries with annual production (t) of Atlantic salmon
seawater intake. or rainbow trout exceeding 10 000 t in 2014. The major three
countries producing each species are marked in bold. It is notable
In this chapter, general husbandry practices and the Chile is in the first three for each species and the greatest
techniques used for rearing salmonids are outlined. It is producer of rainbow trout. (The totals include production
acknowledged that many variations exist and that strate- from unlisted countries with annual production <10 000 t.)*
gies differ between farms, countries and species cultured.
The basic life cycle and production techniques for salmo- Country Atlantic salmon Rainbow trout
nids as a group are, however, similar. One important 41 591
difference is their survival following spawning. Atlantic Australia 78 979 151 773
salmon, trout and charr tend to survive spawning (iter- Canada 28 141
oparous) while Pacific salmon (including Chinook, coho, Chile 644 459 35 750
pink, sockeye and chum salmon) die after spawning China 86 454
(semelparous). The life history characteristics of three Denmark 10 732
commercial species are used below to illustrate these Faroe Islands 1 258 356 34 000
similarities and differences. That information is followed Finland 126 515
by a discussion of freshwater and marine husbandry tech- France 165 000 34 400
niques for salmonids using Atlantic salmon production in Iran 18 719 15 400
Tasmania, Australia, as the primary example. Italy 68 910
Mexico 2 323 232 32 923
Approximately 60 countries are involved in culturing Norway 14 363
salmonids. The major countries producing Atlantic Peru 25 005
salmon and rainbow trout, the two species that together Poland 15 111
constitute almost 70% of salmonid production, are Russian Federation 112 345
shown in Table 17.2. It is notable that Chile, while not Spain 14 800
being of the top ten aquaculture‐producing countries Turkey 21 979
(Table 1.4) is the top producer of rainbow trout and one United Kingdom 8 12 940
of the top three countries producing Atlantic salmon. USA
TOTAL
International salmonid production and general culture
techniques are covered comprehensively by Coates * FAO data.
(2006), Haworth (2010) and Lien (2015).
recreational fishing and aquaculture. The major species
17.2 Biology and their native regions are:
●● Atlantic salmon (Salmo salar): Atlantic Ocean from
17.2.1 Distribution
The native ranges of the Salmonidae include freshwater the latitude of northern Spain to Iceland;
lakes, rivers and marine coastal regions of temperate
zones in the northern hemisphere. However, their range
has been greatly extended as a result of a demand from
●● rainbow trout (Oncorhynchus mykiss): Pacific coast of Salmonids 365
North America and north Asia;
17.2.2 Life Cycles of Three Farmed Species
●● Pacific salmon species (Oncorhynchus species): Pacific A generalised Atlantic salmon production cycle is
basin between 35°N and 65°N; illustrated in Figure 17.1, which outlines the life history
and terminology describing the various stages. Details of
●● brown trout (Salmo trutta): Eurasia and North Africa; the life history of wild fish have been added below.
●● brook trout (Salvelinus fontinalis): north‐eastern
17.2.2.1 Atlantic Salmon
North America; The life cycle of Atlantic salmon is typically anadromous,
●● Arctic charr (Salvelinus alpinus): highlands of central although there are some landlocked strains. Mature fish
Europe, northern Britain, Scandinavia and the Arctic
basin.
Brood fish
Female Male
Eggs
Eyed eggs
Alevin
Fry
S1 or S2 smolt Parr Mature
Smolt male parr
Freshwater
Seawater
One sea winter
Two or more sea winters
Grilse
Salmon
Harvest Harvest
Figure 17.1 Production cycle of Atlantic salmon, showing specific life stages. Most anadromous salmonids have this generalised cycle
although the duration of stages is specific to species and environmental conditions. Source: Forteath et al., 1996. Reproduced with
permission from R. Byrne, Seafood Training Tasmania Inc. (formerly Fishing Industry Training Board of Tasmania Inc.).
366 Aquaculture The parr metamorphose into the smolt stage during
smoltification or the parr‐smolt transformation. The
following their homing instinct will return from sea to process is initiated several months before the seaward
their natal rivers during the 12 months before spawning, migration, usually at an age of 1–3 yr. Smolts, which may
which usually takes place in late autumn (fall) and early weigh up to 60 g, or more, enter the sea in spring and
winter. Fish migrate upstream and remain in pools until summer depending on latitude (spring at the more
adequate water flows allow them to ascend rapids until southerly distributions in the northern hemisphere). It is
they reach areas of suitable water flow and substrate during this downstream migration that stream imprint-
type. Migrating fish do not feed but rely on body reserves ing takes place allowing them once mature to return to
laid down at sea. Atlantic salmon do not show the mass their natal river for spawning. At sea, fish migrate to rich
migrations displayed by Pacific salmon species. feeding grounds consuming crustaceans (e.g., krill) and
fish (e.g., sand lances, herring, flatfish, cod, capelin).
Once in an area of the stream suitable for spawning, A number of these feeding grounds have been discovered
the female uses lateral body movement to make a ‘redd’ off western Greenland and the Faroe Isles.
(a hollow of 10–30 cm in depth) in silt‐free gravel. The
redd is positioned to receive a relatively rapid water flow Some fish will return to their natal streams and rivers
of at least 0.5 m/s. Male fish compete with other males after 18 months in the sea. These fish are known as ‘grilse’
and smaller precocious male parr releasing milt into the (one sea‐winter fish) and weigh between 1 and 7 kg. The
water column to fertilise the eggs as they are released by remaining fish may spend 2–5 winters at sea, returning
the female into the redd. Female salmon produce about as adult salmon of 4–30 kg.
1500 eggs/kg body weight. Once ova are released the fish
moves slightly upstream of the redd and with sideways Wild Atlantic salmon populations are declining, even
body movement dislodges substrate which is carried into to the point of local extinction in some areas. Pressures
the redd by water current and covers the eggs. The on populations include climate change (influencing food
female may repeat this behaviour a number of times and chains and water temperature), pollution, river altera-
can overcut redds created by other pairs. Once spawning tions and obstacles which impact migration, diseases,
is complete the fish are referred to as ‘kelts’. The female and dilution of gene pools through breeding with farm
fish slowly move back downstream and out to sea to escapees.
recondition, returning in subsequent years to spawn 2–3
times. Males continue to spawn until they also move 17.2.2.2 Rainbow Trout
downstream, although proportionally fewer males tend Rainbow trout may undergo marine‐freshwater migra-
to survive than females. Although many survive, a num- tions as ‘steelhead’ or may stay solely in freshwater for
ber of male and female fish—exhausted and with body their entire life. Rainbow trout spawn between autumn
reserves depleted—will die. (fall) and spring, with females producing about 2000
eggs/kg body weight. Eggs hatch 4–7 weeks post‐
The eggs remain buried in the redd for a number of fertilisation, depending on water temperature; e.g., eggs
weeks dependent upon water temperature. On hatching, will hatch within 30 days at 10°C. Considerable egg mor-
the alevins also remain within the redd, utilising their tality may occur at temperatures above 15°C. Eggs are
substantial yolk reserves and showing a negative photo- deposited in gravel (redds), usually in or near riffles.
taxis. Their movement within the redd is probably negli-
gible and little, if any, energy is spent maintaining their Optimum temperatures for growth after hatching are
position. This permits maximum growth on yolk 12–18°C. Males mature as 2‐yr‐olds, although preco-
reserves, an important factor with respect to subsequent cious males mature at 0+ yr. Most females mature as
survival. 3‐yr‐olds. Age at maturity depends on growth rates
and genetic strain. Females maturing as 2‐yr‐olds are
Emergence of fry from the gravel occurs in the spring common in some strains and environments.
about 800 degree‐days post‐fertilisation (e.g., 80 days at
10°C or 100 days at 8°C). The fry are carnivorous, feeding 17.2.2.3 Chinook Salmon
on stream insects and other small invertebrates, and they This species has several common names, including
display territorial behaviour. As they grow, fry develop Chinook, king, spring, tyee, masunosuka, chavycha and
‘thumb‐print’ lateral markings that are characteristic of quinnat. It is the largest Pacific salmon and the least
the parr stage. abundant. Chinook salmon has been the focus of salmon
aquaculture in New Zealand (Haworth, 2010) and the
The parr stage may last for up to 8 yr in cold environ- industry uses that fact as a marketing tool to differentiate
ments, such as northern Norway, or as little as 1 year it from Atlantic salmon. New Zealand is the largest pro-
towards the southern part of the natural range of the ducer of cultured Chinook. The species was introduced
species. During the parr stage, some males may mature into New Zealand from California in the late 1800s for
early as ‘0+ fish’ (‘precocious’ male parr) and participate
in spawning over the redds by fertilising ova from adult
females.
recreational fishing (along with rainbow trout, sockeye Salmonids 367
and Atlantic salmon) and soon established wild runs.
While the aquaculture industry is based on freshwater cages. However other functions include selective breed-
and sea cage operations, sea ranching was also trialed in ing programs, fish‐out and production of broodfish,
the 1980s. Chinook salmon is also cultured in Chile. caviar, eyed eggs, fingerlings and plate‐size fish.
In the wild adult fish spawn as water temperature falls 17.3.2 The Hatchery
below 12°C. An average female weighs 10 kg, although Water quality in the hatchery is of paramount impor-
40–50 kg specimens have been recorded. They lay tance. Heavy metals, suspended solids and inadequate
between 3000 and 14 000 ova, depending on size of the dissolved oxygen levels must be avoided. Cadmium, cop-
female. Eggs are deposited in a redd. per, lead and zinc are all toxic to salmonid eggs, although
the toxicity of these metal ions can be reduced by increas-
Chinook salmon are able to migrate to sea as small fin- ing the hardness of the water. In Europe and Scandinavia,
gerlings, within 3–4 months of emerging from the substrate adding lime to the hatchery water supply has been used
(Ocean‐type: southern distribution in North America), to counteract pollutants such as acid rain and associated
but some stocks remain in freshwater for 1 or 2 yr as parr release of heavy metals in acidic waters.
(Stream‐type: northern distribution in the Pacific). Also,
there are precocious males that spawn and some then Suspended solids accumulate rapidly during egg incu-
migrate to the sea while others remain in freshwater. bation and may smother and kill the eggs. Solids removal
by filtration or settlement may be necessary for the influ-
The Chinook salmon is a coastal species and remains ent water. Ideally the influent water should be at oxygen
at sea for 1–7 yr; the duration of the sea‐going life stage saturation and effluent water should contain at least
is determined by strain and geographical distribution. 6 mg/L dissolved oxygen (50–55% saturation at 8–12°C).
Ocean‐type Chinook salmon return to spawn in autumn, As eggs hatch, vitelline fluids and old egg shells released
a short period of time before spawning, while the stream‐ during hatch will increase oxygen demand. Hatching
type Chinook salmon enter their natal river in spring/ troughs should therefore be cleaned regularly, although
summer, months prior to spawning. this becomes difficult during hatch and while maintain-
ing alevins. Every effort should be made to avoid disturb-
Female fish of the northern‐most rivers remain at sea ing newly hatched fish. It is not uncommon to increase
for the longest periods. Many males may return prema- hatchery water temperatures to enhance the developmen-
turely as precocious adults after only one winter at sea. tal rate of fertilised ova, alevins and first‐feeding fry or to
These are called ‘jacks’ (grilse). They are rarely more cool ova to delay development to optimise incubator
than 30 cm in length and weigh less than 1.5 kg. All space usage in hatcheries and extend fry production.
adults die after spawning and hence the returns are year‐
class related. Supersaturation of dissolved gases—especially nitro-
gen—is dangerous as it causes gas bubble disease (see
17.3 Freshwater Farming sections 4.3.6 and 4.4.6). This condition is characterised
in its later stages by emboli (bubbles) in the vascular
17.3.1 Establishing a Freshwater Salmonid Farm system, which can cause a variety of physiological prob-
During early industry development, hatcheries were lems, possibly leading to death. Gas supersaturation can
located on water sources with adequate supplies of high‐ be managed by de‐gassing the water before it enters the
quality water and with access to utilities, suitable land hatchery. Elevated levels of highly soluble carbon dioxide
topography and available workforce. Systems were config- originating from fish respiration may occur in recircu-
ured largely on a flow‐through strategy with some capac- lation systems. Carbon dioxide can be removed and
ity for water recycling. More recently, hatcheries are using maintained below 10 mg/L through de‐gassing pro-
advanced recirculating aquaculture system technologies cesses. Pure oxygen may also be added to the water
due to increasing pressures from other users (such as before delivery to the fish, especially where high stocking
hydro‐electric schemes, irrigation, municipal water sup- densities or when water recycle/recirculating conditions
plies), climate change (lower water flows, for example), are used or where dissolved oxygen concentrations fall
public perceptions of negative environmental impacts), below 80% saturation.
government policy and logistics. Recirculating systems
provide more control over water quality and environmen- 17.3.3 Broodstock and Spawning
tal conditions, and the flexibility to control fish produc- 17.3.3.1 Broodstock
tion by manipulating photoperiod and temperature. Broodstock should be cultured as part of a selective
breeding program based on family lines possessing desir-
Most hatcheries and freshwater farms are geared to the able traits. In Tasmania, such traits include amoebic gill
production of smolt to be on‐grown after transfer to sea
368 Aquaculture Figure 17.2 Hand stripping ova into a dry bowl after the rainbow
trout brood fish has been anaesthetised. Source: Reproduced with
disease (AGD) resistance, good growth, and proper flesh permission from John Purser, 2017.
colouration and body shape. Atlantic salmon broodstock
are maintained in freshwater for security and biosecurity advanced and delayed by a couple of months using
purposes with the unidirectional movement of fish from photoperiod manipulation. Most brood fish are used
freshwater to seafarms (as smolt), minimising the risk of over one to two seasons at 3–5 years of age, with salmon
disease spread from seawater production facilities to reaching 3–5 kg as first spawners and up to 7–10 kg as
freshwater hatcheries. second‐year spawners. Although hand stripping after
anaesthesia is the most common method, ova may also
On unrestricted sites, fish are simply selected from the be expressed by injecting air into the body cavity using a
current production run and conditioned before spawning. needle (air stripping) and through dissection after eutha-
Where sea‐grown fish are used, they are normally condi- nasia. About 1200–1500 eggs/kg of fish at a diameter of
tioned in freshwater for a few months prior to stripping. 4–7 mm are removed from the female stock of 4–14 kg.
Mature fish remaining in seawater will usually die, Fish are moist‐towelled before stripping to remove
although increased survival and reconditioning may be excess water from the surface of the fish to avoid
undertaken by strategies such as photo manipulation. accidental water contamination of eggs and premature
Male and female broodstock develop asynchronously activation of milt. Sperm cells are inactive in expressed
under some environmental conditions in freshwater, and milt, and it is important that activation through contact
hormone preparations such as Ovaprim™ are used to syn- with water is avoided before egg fertilisation. Sperm
chronise gamete production. During the conditioning motility, once activated, occurs over a short duration of
period, males and females may be kept separately or only 0.5–2 min. Consequently, to avoid sperm activation,
together in culture tanks or raceways (some of which are milt may be stored during the day in syringes or small
designed as continuous raceways) at low densities and in sealed bottles until it is needed. Alternatively, sperm
high-quality water. Chemicals (pheromones) released by may be kept in Cortland’s saline or modified Mounib’s
fish into the water may assist spawning synchrony and extender in an oxygenated container in a refrigerator for
so some systems use water recycling rather than flow‐ a few days. Longer term storage up to 4–6 weeks is
through to expose fish to these pheromones. Elevated
holding temperatures commonly encountered in late
summer and early autumn in salmonid broodstock tanks
in Australia can be detrimental to the quality of the devel-
oping oocytes, producing atresia pre‐ovulation or poor
fertilisation rates of ovulated eggs (Pankhurst, 1996). To
optimise egg viability under these conditions it may be
necessary to cool the water for brood fish. The carotenoid
astaxanthin included in conditioning diets will be mobi-
lised to the ova during egg development not only providing
the characteristic orange colouration but enhancing the
fertilisation and hatch rates of eggs, and survival of fry.
Broodstock are valuable fish and must be sedated and
handled carefully when checking for egg release. Stock
are normally checked every few days and eggs can be
strip‐spawned (Figure 17.2) within about 4 days post‐
ovulation. The majority of eggs are removed during a
single stripping with a subsequent hand stripping used to
remove the remaining eggs from the cavity to avoid con-
tamination by egg shells of the following year’s eggs.
After stripping, many species can be reconditioned for
the next spawning season. The exceptions are fish deemed
to be showing reduced fecundity and egg quality (spawned
2–3 times) or species of Pacific salmon, which may be
euthanized before egg removal as they die after spawning.
17.3.3.2 Spawning and Egg Fertilisation
Spawning of Atlantic salmon in Tasmania occurs in May
(autumn), whereas rainbow trout are spawned from the
end of May to early July (autumn–winter). Times may be
possible with the addition of antibiotics such as penicil- Salmonids 369
lin and streptomycin to inhibit bacterial growth.
be counted by sub‐sampling using a variety of techniques
Under standard fertilisation techniques, ova and ovar- based on egg volume or weight or using counting
ian fluid are stripped into a dry bowl and milt is expressed paddles laid down in the egg incubators. Salmon have
onto the eggs at a ratio of ~1 mL for every 1500–10 000 larger eggs than trout, numbering ~7000/L compared
eggs and mixed before freshwater is added a few moments with ~9000/L for trout. Eggs may be handled gently for a
later. Variable fertilisation rates are achieved using this few hours after water hardening, but then not until the
technique, and some farms remove the ovarian fluid eyed stage occurs at about 23 days for Atlantic salmon
from the ova, wash the eggs with a sodium bicarbonate and 17 days for rainbow trout at 10°C. Egg fertilisation
solution to minimise the effects from ruptured eggs and rate and survival rate may be checked at 2 days by the
then replace the solution with an artificial ovarian fluid detection of the blastodisc or about 7 days for the neural
(saline diluent) to maintain consistency between batches streak after addition of a preservative to a sample of eggs.
before the addition of the milt. While it should be Stockard’s solution (40 mL glacial acetic acid + 50 mL
avoided, contamination by small amounts of blood, formalin + 60 mL glycerol + 850 mL distilled water) is
faeces and anaesthetic do not significantly affect egg commonly used for this purpose.
fertilisation—unlike ruptured eggs which may impact
negatively on fertilisation rates. A level of about 1–3% 17.3.4 Incubator Systems
ruptured eggs in a batch caused by excessive pressure Choice of incubator depends on the production goals of
during hand stripping may reduce the fertilisation rate the hatchery. Upwellers (vertical cylinders; Figure 17.3)
by as much as 90%. The presence of ruptured eggs is are preferred when large numbers of ova are being pro-
detected by a vivid white colouration when water is duced for sale in the ‘eyed’ state or when large numbers
added to the egg/milt mixture, indicative of the yolk pre- are incubated prior to transfer to troughs for hatching.
cipitation which prevents spermatozoa from entering Upwellers require minimal floor space and can hold up
the micropyle of the egg. to 100 L of eggs. Water is introduced in the base under a
stainless‐steel mesh floor and flows upward through the
Milt and ova are mixed gently by hand and allowed to eggs (but without moving the eggs) to overflow over the
stand undisturbed for 2–3 min. After fertilisation, eggs lip at the top into a drain. Eggs may also be incubated and
are washed gently with water and left undisturbed for hatched in trays and trough systems. Trays such as the
45–60 min to water harden. During this stage, the eggs Heath incubator system (Figure 17.4) can be stacked in
increase in size by about 30% and become hard to the banks of up to 20 per unit and are space‐efficient. Water
touch. While taking up water they become slightly enters at the top of the stack, where it is first forced up
sticky for a short time. The water‐hardened eggs may
Figure 17.3 Up‐welling jar incubators
with rainbow trout eggs. Source:
Reproduced with permission from John
Purser, 2017.
370 Aquaculture
Figure 17.4 Removing dead
Chinook salmon eggs from a
Heath-type egg incubator at the
Dworshak National Fish Hatchery in
Idaho, USA. Each tray hold about
5000 eggs. The facility has more
than 425 individual trays in vertical
stacks of two units of eight trays
each. Source: Angela Feldmann, US
Fish and Wildlife Service.
Figure 17.5 California tray‐and‐trough
system for incubating rainbow trout eggs
and holding alevins. Source: Reproduced
with permission from John Purser, 2017.
through a perforated base supporting about 1 L of eggs or 9000–10 000 rainbow trout eggs); although higher
spread two layers in thickness. The water is then numbers are often used. The eggs rest on a perforated
c ollected, and gravity fed to the tray below and so on. base (Figure 17.6) and water is forced up through them
Oxygen levels are maintained by water turbulence at the by a flange on each basket that is flush with the base of the
point of entry to each tray. trough. Upon hatching, the alevins move down through
the perforations to be on‐grown in the trough below
Troughs containing egg baskets are often the pre- (Figure 17.7) and the hatching tray can be removed.
ferred method of hatching, but these take up a consider-
able amount of space. Furthermore, sluggish water flows Saprolegnia fungus is a constant problem in hatcheries:
may cause detritus to accumulate on the eggs, and this it spreads through spores and hyphae in the water from
may reduce oxygen availability. The Californian trough dead eggs to viable eggs. It has in the past been controlled
system (Figure 17.5) contains up to seven baskets each by the bi‐weekly application of zinc‐free malachite green
holding about 1 L of eggs (6000–8000 Atlantic salmon at a concentration of 2 mg/L for 1 hr. However, this drug
Figure 17.6 Eggs incubating on slotted substrate in California Salmonids 371
tray. Source: Reproduced with permission from John Purser, 2017.
from non‐viable (opaque) ones at rates of 100 000 to
Figure 17.7 Rainbow trout alevins crowded on the smooth floor 200 000 eggs/hr. Eyed eggs may first be subjected to
(without textured substrate) of a California trough. Source: physical shock—such as striking the trays of eggs against
Reproduced with permission from John Purser, 2017. the sides of troughs or poured into a bucket to rupture
the shell—which turns the eggs white. This, in conjunc-
has been banned for use in many countries, being replaced tion with mechanical sorting, aids the hatching process,
by alternative treatments such as formaldehyde, hydro- avoiding a prolonged hatch period.
gen peroxide, or ozonated water. Many other fungicides
have also been tested. None of these alternatives have 17.3.4.1 Water Flow
proven as affective as malachite green (Thoen et al., 2016). Eggs have a high requirement for dissolved oxygen,
which increases through eyeing and hatching. Oxygen is
Picking out infected eggs is possible in Californian provided by adequate water flow in the incubators.
troughs and reduces chemical use, but care must be Water flow rates through the incubator systems require
taken to minimise disturbing eggs before the eyed stage constant monitoring. Upwellers require considerably
because they are susceptible to handling shock and less water than trays or troughs. Indeed, only 25 L/min
bright light (especially UV in fluorescent lights). Also, water may be sufficient in an upweller holding up to
manual egg‐picking using either forceps or a siphon hose 100 L of eggs, but this will only be adequate if water
is labour intensive. Egg‐picking is quite safe at the eyed quality is particularly good. Most hatcheries use
stage. Mechanical pickers are used to sort healthy eggs 3–4 L/min water flow per 4–5 L of eggs. Water demand
in a 7‐basket trough system or a 16‐tray incubator is
approximately 15–20 L/min. Water flow in some incuba-
tors may be impaired by the accumulation of egg shells
on screens, requiring frequent cleaning.
17.3.4.2 Temperature Control
It is becoming increasingly popular to manipulate water
temperature in hatcheries to modify spawning time or to
accelerate or delay embryonic development in fertilised
eggs. Egg development and hatching times are based on the
concept of degree‐days, which is the number of days post‐
fertilisation multiplied by the water temperature. For exam-
ple, 10 days at 8°C is 80 degree‐days. Availability of warmed,
ambient and chilled water for egg incubation provides flex-
ibility in hatch times and subsequent fry production and
improves efficiency of hatchery tank usage. Temperature
can be lowered to delay development of rainbow trout eggs,
but temperature manipulation is avoided for the first 10–12
days. Subjecting the eggs to water temperatures of 2°C or
3°C between days 13 and 70 results in an extended hatching
period of about 80–145 days. By comparison, eggs will
hatch in 30–35 days at a constant 10°C.
Egg development in Atlantic salmon can be accelerated
by increasing water temperatures within acceptable
limits during egg incubation. A typical regime is 8°C for
eggs to eyed stage, and 10°C for eyed eggs to hatch.
Hatching will then take place in about 45 days. Care is
taken to avoid accelerating development too rapidly as
this may result in increased egg loss, deformities or
poor‐quality alevins.
17.3.4.3 Light
Salmonid eggs in the wild are buried in gravel and
exposed to low light levels, so care is taken to ensure that
lighting in the hatchery is limited to a few dim sources or
372 Aquaculture are the onset of normal dorso‐ventral swimming attitude
(alevins rest on their sides) and reduction of the yolk sac
to red lights. Ultraviolet light is harmful and daylight to about 20% of its original size. Regular sampling of the
bulbs are avoided. Consequently, egg trays and troughs alevins is recommended to determine the developmental
are covered and exposed to very low light and are exposed stages and to observe food particles in the gut. Moribund
to moderate light levels only during brief periods of fry close to the water surface are often a sign of starvation.
inspection. Starving fish are obvious, having large heads, emaciated
bodies and dark body colouration. At a water tempera-
17.3.5 Alevins and First‐Feeding Fry ture of 12°C, healthy fish will start stacking in the water
column by day‐10 of feeding.
Under natural conditions, salmonid eggs are deposited
in a redd and fry do not emerge from the substrate until First feeding may be established in troughs or fry tanks.
the yolk sac is all but depleted. Alevins are stressed by Fry tanks are favoured by large Atlantic salmon hatcheries.
bright light and frantically seek sanctuary beneath each These tanks are usually circular, but rounded‐corner
other. As with eggs, dim or red lights are used. It is square (Rathbun) tanks have been used extensively. Fry
important to ensure maximum growth during the alevin at the point of taking artificial food are carefully
stage. First‐feeding fry are much easier to culture if they immersed in the tank and allowed to swim out of the
are about 0.2–0.7 g. Best growth is achieved by providing shallow net or tray used for transfer. More recent designs
a substrate that allows alevins to lie quietly and be physi- have incorporated hatching trays and alevin incubation
cally supported for long periods, thereby optimising substrate within the alevin/fry tanks to reduce the
yolk‐sac absorption. Corrugated sheeting, meshes, artifi- frequency of handling and transfers.
cial turf, plastic grid, gravel and plastic biomedia have
been tested as substrates. Better growth is achieved Water temperatures between 12–15°C are ideal for
when fish are supported in this manner relative to those growth, but can be difficult to achieve unless artificial
forced to lie on a smooth substrate. Complex substrates heating is available. Groundwater at 12°C may be availa-
also allow small fish to distribute themselves more evenly ble in some areas, making heating unnecessary. However,
throughout the trough, thereby protecting them from this water may need to be aerated to remove high levels
suffocation caused by overcrowding. Fish are transferred of nitrogen, carbon dioxide or dissolved iron and to
from substrate in trays to larger tanks (with substrate) at improve oxygen levels. The farmer must strive to achieve
about 0.16 g. maximum growth from a very early stage in order to
maximise economic production in the grow‐out phase.
Rainbow trout and Pacific salmon fry swim to the Water temperature should ideally remain constant during
water surface to inflate their swim bladder (‘swim‐up’) the first 2–3 weeks of feeding. Light levels remain critical
when ready to commence feeding. Water depth at this and must be <50 lux at the water surface. Lights not only
time must be shallow and not exceed 10 cm. As a rule of serve to permit feeding, but also ensure a relatively even
thumb, small quantities of feed are delivered at frequent distribution of the fish. In complete darkness, fry tend to
intervals after half the fry have ‘swum up.’ A light with a form dense congregations, which can result in damage to
25‐W red bulb suspended over the trough is often used. fins and opercula.
Once fry are feeding, starter feed must be fed frequently
to satiation (or in excess) or at a rate of ~8% body weight/ Stocking levels used in Rathbun tanks are between
day. Furthermore, feeding over a 20‐ to 22‐hr period with
2–4 hr of darkness is beneficial as it maximises the time 15 000 and 20 000 fry/tank or ~250 000 fry in larger 6‐m
available for food intake and hence growth, while main- diameter tanks. First‐feeding fry benefit from moderate
taining light‐dark synchronicity. The consequence of crowding, which seems to encourage feeding in these
such an intensive feeding regime may be the deteriora- early stages. The water flow rate initially is about 25 L/
tion of water quality. Low water velocities do not permit min. Stocks of rainbow trout and Atlantic salmon fry
waste to be deposited in the drain at this early feeding may need to be graded 4–5 weeks after first feeding.
stage and gentle brushing of the tank bottom is necessary
to remove waste materials. The fry can be damaged easily; At this point there is a major divergence in farming
therefore, violent brushing or sudden increases in aims and objectives for the grow‐out phase of Atlantic
water flow are avoided. Cleaning is necessary twice per salmon and trout. Rainbow trout farmers usually aim to
day to remove organic solids that may express an oxygen
demand and to reduce bacterial loadings. grow fry to market size according to a plan designed to
satisfy weekly production demands or to produce fish of
In contrast to trout, Atlantic salmon feed prior to swim‐ 300–800 g for grow‐out in sea cages. The Atlantic salmon
up and need to be weaned onto artificial diets before their farmer will endeavour to ensure that as many fish as
yolk sacs are absorbed. It is difficult to determine the
best time to start artificial feeding, but useful indicators possible will become smolts to be transferred to sea
cages. In both cases, husbandry of stocks in the first 2–3
months after feeding is of great importance if subsequent
goals are to be achieved.
17.3.6 Culture Systems for Juveniles Salmonids 373
17.3.6.1 Vessels suggested to ensure self‐cleaning. Circular tanks require
Many ‘low‐cost’ rainbow trout farms keep their stock in lower flows for self‐cleaning, which may be optimised by
facilities based on the Danish earthen‐pond system. using combinations of inlet/outlet configurations and
Although colloquially called ‘ponds’ because of their sloping tank floors. Only carefully designed ponds will
earthen construction, they are not true ponds in the avoid the need for regular mechanical removal of organic
hydrological sense but rather they are earthen raceways mud. Indeed, it is good management practice to leave
with lower water flow than used in typical concrete some ponds fallow for several weeks each year to permit
raceways. Construction is inexpensive but little atten- cleaning and facilitate disease control.
tion is sometimes paid to design efficiency. Current flow
can be irregular, uneaten food and faeces accumulate on Smolt production in cages in freshwater lakes (lochs) is
the bottom and the sidewalls are susceptible to erosion. a strategy used in Europe but not supported in Tasmania.
The recommended design dimensions for ponds It overcomes some of the logistical difficulties associated
are 30 m × 10 m × 1 m, sloping to 1.5–2 m depth at the with supply and also reduces problems associated with
outflow. Stocking rates in earthen ponds are less than in water quality in the growing system. The cage systems
concrete raceways or tanks. used are similar to, but smaller than, those used in
marine grow‐out.
Raceways are typically used for on‐growing rainbow
trout, Pacific salmon and, to a lesser extent, Atlantic Recirculation aquaculture systems (RAS; see section 3.37;
salmon. Typical dimensions are ~30 m × 2.5 m × 0.7 m: Figs 4.14 and 4.15) have become popular because they
they are long and shallow (Figure 3.11). Raceways are may be sited closer to markets than traditional flow‐
often used in series as one raceway empties into another, through systems and afford greater control of water
making water quality in successive races more difficult to quality and fish production. Broodstock, egg and fish
maintain. Consequently, fish densities are managed to culture tanks may be serviced by RAS, enabling stocking
reflect the changing water quality towards the down- densities up to ~100 kg/m3 during juvenile production
stream end of each raceway and subsequent downstream in systems supported by oxygen injection. Temperature
raceways. Fish tend to aggregate in the upper portion of control assists with optimal oocyte development in
the raceway and fin damage may occur due to nipping at broodstock, controlled hatching in eggs and regulated
feeding time or from rubbing on the concrete walls. Fish growth in fry and fingerlings. Photoperiod manipula-
must be graded more frequently in raceway systems than tion regulates feeding times and out‐of‐season smolt
in ponds as the aggregations of fish hinder even food production.
distribution, which results in unequal growth. Organic
solids accumulation can be minimised by increasing 17.3.6.2 Grading
flow velocities using baffles and a smooth sloping floor. An experienced farmer knows when to size‐grade stock
Although concrete is commonly used for raceway con- for optimum growth efficiency and, most importantly,
struction, earthen and lined races are also used. to provide marketable fish throughout the year. Fry are
usually graded before grow‐out in ponds, raceways or
Well‐designed tanks offer several advantages over tanks, as grading affords benefits such as more consist-
raceway and ponds, including better water mixing and ent fish size, use of consistent pellets sizes and removal
more homogeneous water flow and quality. This allows of size‐related aggression between individuals.
fish to use the entire water volume, permitting efficien- Subsequent grading will depend on the market plan.
cies in water use and food distribution. A further Handling stresses fish and increases oxygen demand
advantage is better disease control than in either pond or and is generally avoided during hot weather, low water
raceway systems. Crowding fish using screens to facili- flows, immediately before transport or while treating
tate pumping and transfer is easier in raceways; in some disease. Grading may also impair growth for a period
systems fish are released from tanks into races for this through handling stress and the need to deprive fish of
purpose. Although raceways and ponds are used pre- food for 1–2 days before handling. Fish pumps and ele-
dominantly to rear rainbow trout (Figure 3.11), tanks vators deliver fish more efficiently to the graders and
are the preferred system for Atlantic salmon. reduce the handling stress associated with manual
transfer such as netting. Compact, manoeuvrable grad-
Water exchange is an important consideration regardless ers are available and these handle fish from 1–5 g
of the system. In tanks and raceways it is usually suffi- upwards. Separation of fish into 2–4 size classes is based
cient to ensure a minimum of 1–2 volume‐turnover per on the variation in fish width (girth), using tapered or
hour, although this depends on fish loading, water quality set spaces between grader bars or belts. Counters con-
requirements and currents to move solids and self‐clean figured with graders provide an accurate inventory of
the system. In raceways, a flow of at least 2 cm/s is fish numbers in the specified size grades.
374 Aquaculture where fish activate a wire, bead or rod to release food
into the water. Such devices may be simple in design or
17.3.6.3 Feeding quite sophisticated computer‐based monitoring systems
Every grower is mindful of the high cost of food in inten- that record the number of actuations over time to
sive salmonid culture. Surprisingly, wastage may still describe feeding patterns. Pellet sizes used in hatcheries
occur as a result of overzealous feeding, inefficient range from 300 µm as starter feed to ~3–4 mm for
grading leading to inability of small fish to consume the juveniles.
pellets, poorly designed feeders and poor feeding
regimes. Although overfeeding is generally to be avoided, 17.3.7 Production of Atlantic Salmon Smolt
slight overfeeding can be used to maximise growth dur- The primary focus of a salmon hatchery is the produc-
ing the early phases in the hatchery when a relatively tion of quality smolt to be transferred to marine farms
low volume of feed usage does not result in significant for grow‐out. Smoltification is a process in which juve-
additional cost. Underfeeding may result in poor feed nile salmonids (parr; Figure 17.8) in freshwater undergo
conversion ratio (FCR), low growth and possible hierar- morphological, physiological and behavioural changes
chy formation, which in turn can lead to greater size that enable them to migrate to, and then survive and
variation. A periodic sampling program can be estab- grow in, marine conditions. Smoltification is complex
lished to determine growth and FCR. Samples can be and involves a number of physiological processes to
taken monthly during the colder months when growth is respiration, osmoregulation, excretion, circulation, and
slower, but in summer once every 2 weeks is prudent. growth. Progressing over several months, it culminates
In practice, sampling may be more infrequent due to in a ‘smolt window,’ which is the optimal time to transfer
concerns of negative impact of sampling on growth. Feed to seawater with minimal stress.
conversions vary between species but should ideally be
close to or less than about 1.0 in freshwater, with trout Fish undergoing smoltification display a silvering of
usually producing less efficient conversion (i.e., higher the scales (Figure 17.9) due to the deposition of purine
FCRs) than salmon. The use of poor‐quality feed crystals from protein metabolism, obscuring the lateral
increases production of solid and dissolved wastes and parr marks. The scales are delicate and easily dislodged
reduces feed efficiency. Quality, high‐energy diets are during handling. The fins show darkened margins and
available and are very favourable from growth, environ- the body shape changes with a resultant reduction in
mental and economic standpoints. body condition index. In the final stages of smoltification
there are numerous physiological and biochemical
Most feeding strategies are based around high frequency changes that allow fish to adapt from freshwater to sea-
delivery and satiation feeding, with feeding tables acting water. Oxygen consumption and metabolism are higher
as feeding guides. Although automatic feeding systems in smolts and changes to lipid metabolism, buoyancy,
(auger, spinning disc, rotating plate, pulsed air and belt eye pigmentation, and thyroid concentration also occur.
feeders) can distribute feed at pre‐set intervals through- Should fish be confined to freshwater through the smolt
out the day, they are supplemented by hand feeding to
assess feeding response, with subsequent feeding being
adjusted accordingly. Some operations use self‐feeders
Figure 17.8 Atlantic salmon parr. Fish at
this stage are adapted to life in freshwater
and have characteristic ‘parr marks’ (bars
and spots) that help camouflage the fish
in streams and rivers. Source: Per Harald
Olsen/NTNU 2012. Reproduced under the
terms of the Creative Commons
Attribution Share Alike license,
CC‐BY‐SA 3.0.
Salmonids 375
Figure 17.9 Atlantic salmon smolts. Fish
at this stage begin losing the distinctive
parr marking and become silvery as they
undergo physiological, morphological and
behavioural changes that enable them to
adapt to seawater. Source: E. P. Steenstra,
US Fish and Wildlife Service 2009.
window they will undergo desmoltification or parr‐ to an artificial representation of a natural cycle. Both
reversion to a physiological state that allows them to incorrect photoperiod and high water temperatures will
continue growth in freshwater. adversely affect salinity tolerance in potential smolts by
diminishing enzymatic activity and shortening the smolt
Under a natural thermal regime, Atlantic salmon parr window (usually ~300–400 degree‐days). Should smolts
must attain a threshold size by the start of their first win- be transferred after the smolt window (as post‐smolts
ter to successfully undergo smoltification in spring. and during the desmoltification process) the fish may
Under cooler water environments hatcheries may observe show reduced feeding response, disease susceptibility
a bimodality in length/frequency distribution in parr and an inability to tolerate stressful conditions.
populations, the upper mode completing smoltification
during the first spring (S1) and the lower mode becoming One strategy to allow year‐round harvests involves the
smolt a year later (S2). In general, S1 smolts at the com- use of out‐of‐season smolts. Under natural light condi-
mencement of winter are approximately 10–15 cm in tion, smolts will be available only in spring, when daylight
length, whereas S2 fish are small in the first spring, but is increasing. It is possible to artificially advance autumn‐
much larger the following spring, at the time of transfer. winter‐spring changes in light‐dark photoperiod to
In contrast, Pacific salmon species have a less‐defined produce off‐season smolts that are available in autumn of
smolt window or windows, are usually smaller in size at their first year. Transfer of out‐of‐season smolts to marine
transfer, cannot always tolerate direct transfer to seawater conditions in autumn spreads the smolt intake period
and may require an acclimation period in brackish water. during the year thus allowing a corresponding spread of
production on the marine farms. An additional strategy
The aim of the majority of Atlantic salmon hatcheries is used by some farms is the extension of the grow‐out to
to produce yearling smolts (S1). Unfortunately, S1 and S2 S1½ smolts which are larger (~300 g) on transfer in
are similar in size until summer, when S2 fish become autumn, allowing a shorter grow‐out period in seawater
anorexic. However, S1 smolt production can be promoted and a quicker conditioning period post‐transfer (com-
by manipulating light and particularly temperature (and bined larger size and smoltification).
feeding) to optimise growth during the early stages of
development. These two environmental variables play an In Tasmania, the most popular smolt types produced
important role in the smolting process. Seasonally are as follows:
changing temperature regulates metabolism, which ●● all‐female diploid and all‐female triploid out‐of‐season
affects appetite, consumption of food and growth rate,
whereas photoperiod signals day length and seasonal smolts;
cycles. If the light and temperature are held constant in ●● all‐female diploid and mixed‐sex diploid marine pre‐
the early stages to optimise growth, fish must be resyn-
chronised with natural photoperiod a few weeks or smolts; or
months before the smolt window (i.e., in time to enter the ●● mixed‐sex and all‐female triploid spring smolts.
sea when salinity tolerance is at a peak) or resynchronised In recent years hatcheries have moved to all‐female
stock.
376 Aquaculture Although the latter may be due to genetic factors, there
are other parameters, acting alone or in combination,
While the majority of fish undergo the parr‐smolt which also control growth. These include water temper-
transformation, some male parr mature instead, a reflec- ature, oxygen levels, tank or pond design, flow rates,
tion of a genetic trait and favourable growing conditions density, feed, grading (efficiency and interval) and health
in the hatchery. Maturation is a contrasting physiological maintenance.
process which impairs osmoregulatory capability, hence
early maturing parr will not survive transfer to seawater. 17.4 Marine Farming
Smoltification is not evident in some landlocked strains Great advances have been made in the technology and
of salmon or trout. For example, rainbow trout stocks husbandry techniques required for farming salmonids in
used for marine farming in Tasmania to produce ocean sea. Cage structures have been developed to withstand
trout are a non‐smolting strain and are transferred at a more exposed sites (e.g., offshore and open ocean aqua-
relatively large size of 300 g, or more. They are grown in culture), and they have increased in size and carrying
brackish waters or are acclimated to marine conditions, capacity. Feeding, grading and fish pumping systems
although higher temperatures together with full seawater have become more technologically advanced incorporat-
salinities may reduce growth and survival in summer. ing computer and sensor systems reducing the labour
requirement, improving farm efficiencies and fish
17.3.8 Grow‐out of Rainbow Trout performance. Losses to disease and parasite infestation
In Freshwater have decreased and growth rates improved by the devel-
opment of new diets and food distribution strategies,
Many rainbow trout are farmed in sea cages: more than one fish handling and transfer techniques, reduced stocking
might expect. There is, however, substantially more aqua- densities, site rotation and fallowing, development of
culture in freshwater. Of the ~813 000 t annual production vaccines and better understanding of environmental
of rainbow trout in 2014, ~577 000 t was from freshwater conditions in terms of fish performance. Together these
aquaculture and ~225 000 t from ocean culture. factors comprise the important field of fish welfare, a
major underlying principle of fish aquaculture (Van de
The freshwater aquaculture of rainbow trout is typically Vis et al., 2012). The following sections will discuss the
in raceways. These are narrow elongate tanks with a strong general issues involved in salmonid farming in sea cages.
water flow that must be continuous and unidirectional It is primarily based on the Tasmanian Atlantic salmon
(Figure 3.11). Water is supplied at the upstream end of the industry but has broader applicability to other salmonid
tank and discharged at the downstream end. There is industries.
usually a once‐through flow of water without recircula-
tion and high water quality is maintained. There must be a 17.4.1 Site Characteristics
permanent source of high‐quality water at appropriate While the majority of salmon sea farms are located
temperature in the vicinity of the farm to provide an inshore in high‐salinity waters (32–35‰), the increasing
uninterrupted supply through pumping or gravitation. incidence of AGD has shifted the focus to less saline
This is a limiting factor on the location of raceway farms. waters or inshore areas with access to freshwater. In
The unidirectional flow and high water quality are well Tasmania for example the industry developed sites in the
suited to the culture of rainbow trout, which naturally stratified brackish water of Macquarie Harbour on the
swim upstream against water currents in streams. west coast, an area used over many years for rainbow
(Ocean) trout production. Placing cages in sheltered
European farmers are able to extend egg production coastal waters (bays, fjords and lochs) is coming under
over at least a 9‐month period. Photoperiod manipula- increasing pressure worldwide from recreational users
tion of broodstock permits about a 6‐month supply of and groups concerned by organic and inorganic enrich-
eggs. Temperature control of the hatchery environment ment from the farms. On inshore farms in Tasmanian
may add an additional few weeks, and the importation of waters, there is regular monitoring of water quality,
ova from Australia or other southern‐hemisphere coun- benthic changes and phytoplankton assemblages as
tries adds a further 2‐ or 3‐month supply. It is feasible, part of the government regulations pertaining to lease
then, to operate a hatchery for 12 months in Europe. permits (section 17.10).
It is possible to manipulate availability of rainbow trout Alternative offshore farming technologies are being deve
fry to ensure an extended supply of marketable fish and loped to address these concerns, but they are expensive
there are two further strategies:
1) Farmers can use compensatory growth, whereby
growth is slowed then increased to coincide with
market demand.
2) Farmers can take advantage of variable growth
between fish stocks.
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