CHAPTER 12
AQUACULTURE
What is aquaculture?
Aquaculture may be simply defined as the growing of aquatic organisms under controlled
conditions (Bardach, et al., 1972). As such, aquaculture can take many forms. Strictly speaking,
any intervention in the life cycle of an aquatic organism intended to increase the production of
that organism for the benefit of mankind might be classified as aquaculture. Hawaiians, for
example, transplanted juvenile fish from the ocean into confined coastal ponds where the supply
of natural foods stimulated the growth of the fish. The practice of merely transplanting an
aquatic organism to a habitat more favorable for growth is a simple form of aquaculture. A more
sophisticated form of aquaculture would involve manipulation of the pond environment and the
supply of food for the fish. In some cases the natural productivity of the system is stimulated by
the addition of fertilizers, in which case the supply of food for the cash crop is controlled only
indirectly. Such schemes, for example, have been used to culture milkfish and shrimp in
Southeast Asia. Similar strategies may be used to grow shellfish, which feed by filtering
particles out of the water. In other cases the cash crop is nourished directly through the use of
pelletized feed, the culture of catfish and trout being cases in point.
Regardless of the mechanism of feeding the cash crop, control of water quality invariably
becomes an issue in aquaculture. An adequate supply of oxygen and a mechanism for removing
metabolic waste products are essential. Temperature and, in some cases, salinity, must be kept
within bounds favorable for growth. When organisms become stressed they become susceptible
to infection and disease. Stresses created by poor water quality can therefore impact yields both
directly and indirectly.
Strategies for dealing with water quality vary greatly, depending on the availability of
resources and the characteristics of the cash crop. Where there is an abundant supply of clean
water, the simplest way to control water quality is to operate in a flow-through mode. When this
is not an option, water quality is typically controlled by operating what amounts to a polyculture
system in which the role of some organisms is to maintain oxygen levels and/or remove
1
metabolic wastes produced by others. Aquatic plants, for example, often perform this service in
aquaculture systems in which fish or crustaceans are the cash crop.
Table 12.1. Fish production in properly managed ponds under different management regimes.
Source: Hepher and Pruginin (1981)
Yield (tonnes ha-1 y-1)
Management regime
Fertilized ponds, no supplementary feeding
2.4
Heavily manured ponds, some feeding with rice bran
6.2-7.0
Heavily manured ponds, no supplementary feeding (results
4.2-7.1
extrapolated to 240 days)
Fish fed on protein-rich pellets
7.3-10.9
High stocking rates (more than 14,000 per hectare), fish fed on
12.2-35.7
protein-rich pellets (results extrapolated to 240 days)
1,360 – 1,800
Monoculture of carp in running water
Table 12.1 rather dramatically illustrates the impact of different feeding and water quality
management regimes on fish production in aquaculture systems. Yields of a few tonnes per
hectare (ha) per year are achievable in closed ponds in which the food for the cash crop is
provided by the natural food chain in the pond. Production in such cases is stimulated by the
addition of fertilizer or manure. When the fish are fed more directly with protein-rich pellets,
yields are on the order of tens of tonnes ha-1 y-1. Yields increase dramatically when water quality
is controlled through rapid water exchange. It is important to bear in mind, however, that the
effective size of a running water system is actually much larger than the nominal area of the
culture facility.
Aquaculture and Agriculture
It is fair to say that the husbandry of aquatic organisms is de facto a much younger
science (or art) than the husbandry of terrestrial organisms. The origins of agriculture go back
roughly 7,000 to 10,000 years to the Fertile Crescent (Natufian or Sumerian culture), East Asia
(rice), and Central America (maize, squash). Oyster culture can be dated to ancient Rome and
2
Gaul, and there are less certain reports of fish culture in China roughly 2,500 years ago (Bardach,
et al., 1972). Given its longer history, it should not be surprising to find that agriculture has
achieved a degree of sophistication not observed in most aquaculture enterprises. This is indeed
the case, and it is instructive to compare the two husbandry systems to perhaps gain some insight
about the future of the aquaculture industry.
Agriculture is very much focused on the production of plants and herbivores. This
should not be surprising when one considers the inherent inefficiency of food chains in
transferring organic matter from one trophic level to the next (Chapter 1). As in agriculture, the
husbandry of aquatic animals has tended to focus on organisms that would logically be assigned
to a low trophic level. Unlike agriculture, there has been some success in raising aquatic animals
that are not strictly herbivores or detritivores, but the commercial value of carnivorous fish must
be high indeed to warrant the expense associated with feeding them. Most agricultural
production is in fact accounted for by the raising of plants, not animals, and the agricultural
production of crops such as rice, corn, and wheat figures prominently in human nutrition. As
cash crops and sources of human nutrition, plants are much less important in aquaculture than in
agriculture, although they are obviously essential to the production of herbivores and perhaps
less obviously to the maintenance of water quality.
From a nutritional standpoint aquatic organisms differ from terrestrial plants and animals
in some important ways. Aquatic organisms have little need for support tissue, since their
density is similar to that of water.1 Phytoplankton in particular may be as much as 50% protein.
As noted in Chapter 1, fish are also an excellent source of protein, and their lipid tissue is one of
the few good sources of -3 polyunsaturated fatty acids (PFA’s), whose presence in the human
diet is needed to balance the intake of -6 PFA’s from other fats and oils (Chapter 1). Although
it is fair to say that most aquaculture production is targeted for human consumption, this is by no
means true in every case. Pearl oysters and ornamental fish are two cases in point. Some
hatcheries rear and release fish to satisfy the needs of sports fishermen, and bait fish may be
grown in culture for use by both recreational and commercial fishermen. Seaweeds are an
excellent source of colloids such as agar, algin, and carrageenan. Although many uses of these
colloids involve foods, some do not. Carrageenan, for example, is used as a stabilizer in
1
The contrast between whales and dinosaurs is a case in point. It is no accident that blue whales are the largest
animal that ever lived.
3
toothpaste and as a gelling agent in air freshener, and agar is used in the production of solid
growth media for the cultivation of bacteria and microalgae.
One of the important differences between aquaculture and agriculture is the extent to
which breeding has impacted the latter. Virtually every important agricultural crop is the result
of generations of selective breeding of varieties with desirable traits. The history of maize
cultivation (Mangelsdorf, 1947) is certainly a case in point2, but it is only one of numerous
examples that could be cited. Although some selective breeding has been done with
aquacultured species (e.g., tilapia), overall breeding has had much less of an impact on the
aquaculture industry than the agriculture enterprise.
Overview of Aquaculture
Figure 12.1 shows how world aquaculture production has grown since the middle of the
20th century. Compared to capture fisheries, aquaculture production was virtually nonexistent in
1950, but at the present time aquaculture accounts for about 36% of global fisheries production
(capture fisheries plus aquaculture) by weight and about 43% of the economic value of the catch.
The wholesale value of the aquaculture catch is about $1.17 per kilogram, which is substantially
higher than the corresponding figure of $0.84 kg-1 for capture fisheries.
Empirically freshwater fish have proven much easier to culture than marine species, a
fact that is evidenced in the comparison of freshwater and marine fish aquaculture production in
Table 12.2. The reason for the greater success with freshwater fish is the fact that many marine
fish, and most aquatic invertebrates, produce small eggs that hatch into tiny, delicate larvae. In
the case of aquatic invertebrates, the larvae may go through many developmental stages, each
with distinct environmental and nutritional requirements (Bardach, et al., 1972). Rearing such
sensitive and finicky organisms to adulthood is a difficult task. When it comes to reproduction,
freshwater fish are more toward the K end of the r-K spectrum. They tend to produce fewer but
larger eggs, and their larvae are generally hardier than marine fish larvae. A number of the
intensively cultured freshwater fish protect their eggs in some way3, whereas marine fish simply
2
3
Maize is believed to have been domesticated from teosinte by human selection.
Trout, for example, bury their eggs; tilapia hatch their eggs in their mouths; shrimp carry their eggs on their body.
4
release their eggs into the water. The net result is that it is much easier to rear freshwater fish
through the egg and larval stages than is the case with marine fish.
Figure 12.1. World aquaculture production since 1950 and the economic value of the produce
since 1984.
Table 12.2. Breakdown of world aquaculture production by major category of cash crop in 2002.
Category of cash crop
Percentage of aquaculture production
Freshwater fish
42.7
Mollusks
23.0
Aquatic plants
22.6
Diadromous fish
5.0
Crustaceans
4.1
Marine fish
2.3
5
The biological issues associated with a successful aquaculture operation can be
summarized with four words: seed, feed, weed, and breed. Problems and opportunities that arise
within these four areas are summarized in the following paragraphs.
FEED. Any cultured organism must be fed. The easier it is to identify and obtain the
right kind of feed, the more successful the operation is likely to be. There is no question that
some of the species that have been grown with great success in aquaculture systems have been
very easy to feed. Problems associated with feed are minimized if the natural food web in the
culture facility can provide the nutrition for the cash crop. Analogues in agriculture include
animals like cattle and sheep that are content to eat grass. In aquaculture systems, carp, tilapia,
mullet, and milkfish are all capable of using natural pond productivity. Milkfish, for example,
can subsist on a diet of benthic algae and associated protozoa and detritus. Mullet feed on
plankton, benthic algae, and, in ponds, decaying higher plants (Bardach, et al., 1972). Tilapia
will consume phytoplankton, but supplemental feeding of tilapia ponds is desirable. However,
the supplemental feed need not be expensive. Kitchen refuse, rotten fruit, coffee pulp, and mill
sweepings have all been used to stimulate tilapia production.
Carp come in a variety of species that, when used in combination, can make remarkably
efficient use of natural pond productivity. Table 12.3 lists the species of carp used in classical
Chinese carp polyculture and their feeding niches. A well managed carp polyculture pond
requires little or no supplemental feeding. The food web is stimulated with fertilizer and/or
manure. A key to the success of the polyculture system is the right mix of species and stocking
densities. A typical system favors planktivorous fish, since production in these highly eutrophic
ponds is usually dominated by the plankton community. An interesting variation on the fish
polyculture model is an integrated aquaculture/agriculture system in which animal (typically pig)
waste from the agricultural component is used to fertilize the fish pond, and sludge from the
bottom of the fish pond serves as a soil conditioner and fertilizer on the agricultural land.
The significant contribution of mollusks to aquaculture production is accounted for
almost entirely by bivalves such as clams, cockles, oysters, scallops, and mussels. These animals
are all filter feeders that consume suspended particles, including plankton. Feeding bivalve
mollusks can be as simple as locating the aquaculture facility in a productive body of water.
Some of the highest yields ever attained in aquaculture, 300,000 kg ha-1, have involved mussels
grown on rafts in the Galician bays of Spain. This is another case where specification of yields
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on an areal basis is misleading, since currents and mixing effectively transport particles to the
suspended mussels from a much wider area than is occupied by the raft facility. Nevertheless,
from the standpoint of feeding the cash crop, the system is remarkably effective and inexpensive.
Table 12.3. Carp species used in classical Chinese carp polyculture and their associated feeding
niches (Bardach, et al., 1972)
Carp species
Feeding niche
Black carp
mollusks
Common carp
Benthic animals and detritus, including grass carp feces
Mud carp
Benthic animals and detritus, including grass carp feces
Silver carp
phytoplankton
Big head carp
zooplankton
Grass carp
Vegetable tops
Moving to the base of the food chain, we come to aquatic plants. Feeding aquatic plants
is straightforward. They require sunlight and inorganic nutrients. From the standpoint of feed,
manipulation of their growth environment involves nutrient additions, typically nitrogen and
phosphorus. In very intense culture systems, inorganic carbon may also be added, typically by
bubbling in carbon dioxide in a counterflow system. Large scale production of macroscopic
aquatic plants such as kelp may simply involve setting out seed (gametophytes or sporophytes)
on an appropriate substrate in a favorable growth habitat. Aside from habitat selection, there is
no further effort to manipulate the supply of inorganic nutrients and sunlight. Intense culture of
microalgae, sometimes the cash crop and in other cases a source of food for mollusks or larval
stages of fish, often involves addition of inorganic nutrients and in some cases manipulation of
the light regime by artificial illumination.
At the other end of the spectrum (or food chain) with respect to their food requirements
are carnivorous aquatic animals such as catfish and trout. Culture of such organisms can be
justified if the value of the cash crop is sufficiently high and a supply of the right kind of food
available at a reasonable cost. Denmark, for example, achieved preeminence in commercial trout
culture because of the availability of trash fish at Danish fishing ports (Bardach, et al., 1972).
Carefully formulated pelletized feeds are an alternative to trash fish, and in the United States the
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rapid expansion of channel catfish aquaculture during the last two decades of the 20th century
(Fig. 12.2) was due in no small part to the availability of pelletized catfish chow from Purina
Mills. Channel catfish currently account for 50% of U.S. aquaculture production by value and
58% by weight.
Figure 12.2. Channel catfish production in the United States.
One interesting strategy for feeding carnivorous fish is to simply rear the fish to an age
when they can presumably fend for themselves and then release them to the wild. This strategy
is exemplified in salmon ranching. At salmon ranch hatcheries, adult salmon are spawned, the
eggs are hatched, and the juvenile salmon are reared in tanks until they are ready for release.
The salmon find their way to the sea, where they remain until they become sexually mature.
They then return to the hatchery, where they are harvested. The profitability of the operation
requires that at least 3-5% of the salmon in fact return to the hatchery, and therein lies the rub.
Although the cost of feeding the salmon while they are at sea is zero, it is not uncommon for 98-
8
99% of the salmon to be lost to natural and fishing mortality. The result is that privately funded
ocean ranching of salmon has failed miserably. Most commercial aquaculture of salmon at the
present time involves maintaining the salmon in pens or other enclosures throughout their life
and feeding them with fishmeal typically made from clupeids or remnants from fish processing.
As such, salmon farms are actually net consumers of fish. About five tonnes of landed fish are
required to produce a tonne of farmed salmon. Atlantic salmon is the preferred species for
farming because it grows rapidly and is more disease resistant than other candidate species such
as Chinook and Coho. At the present time world production of farmed salmon is dominated by
Norway and Chile (Fig. 12.3).
Figure 12.3. Aquaculture production of farmed salmon by the four major salmon farming
nations.
The impact of feeding regime on fish production is dramatically illustrated by the results
of experiments conducted with carp aquaculture ponds in Israel. In the studies summarized in
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Fig. 12.4, juvenile carp were stocked into ponds at the same initial density (1,000 fish per
hectare), regardless of the feeding regime. The solid line shows the growth rate of the fish when
the supply of food is not limiting.
C
Critical standing crops
B
A
Figure 12.4. Average growth rates of carp at 1,000 fish per hectare under different feeding and
fertilization treatments at the Fish and Aquaculture Research Station, Dor, Israel (Hepher and
Pruginin, 1981). The solid line is the potential growth rate of the fish when food is not limiting.
The dashed lines show the growth rates of the fish when the critical standing crop is exceeded.
Growth rates drop to zero at the carrying capacity. (A) no fertilization or feeding, (B) chemical
fertilization, no feeding, (C) Chemical fertilization, feeding with sorghum.
The critical standing crop (CSC) is the biomass of fish (kg ha-1) at which the growth rate
of the fish begins to drop below the solid line. In the case of the no fertilization and feeding
management regime, for example, the CSC is 65 kg ha-1, which means that fish stocked at 1,000
10
per hectare would have an average weight of 65 grams. As the biomass of the fish continues to
increase, their growth rate slows even further, and eventually growth ceases altogether. This
point corresponds to the carrying capacity (CC) of the system under the given management
regime. Table 12.4 summarizes the information on CSC and CC obtained from the experiments
carried out by Hepher (1975).
Table 12.4. Effect of feeding level on CSC and CC for carp based on studies carried out by
Hepher (1975).
CSC (kg ha-1)
Feeding level
No feeding and no fertilization
CC (kg ha-1)
65
130
Fertilization but no feeding
120
480
Fertilization and feeding with cereal grains
550
2,000
2,400
?
Fertilization and feeding protein-rich pellets
The dependence of growth rates on the size of the fish and the existence of a CSC and CC
for every form of management regime have important implications for the intelligent
management of a fish pond. Figure 12.5 illustrates some of the management implications. In
this example, we envision that the fish stocked into the pond weigh either 100 or 200 grams. The
upper panel shows the growth rates of the fish as a function of stocking density. The solid
curves correspond to no feeding and no fertilization, which means that the CSC and CC are 65
and 130 kg ha-1, respectively. If we stock the pond with 100-gram fish, the CSC will be reached
at a stocking density of 65,000/100 = 650 fish ha-1, and the CC will be reached at a stocking
density of 130,000/100 = 1,300 fish ha-1. In Fig. 12.5, the growth rate of the 100-gram fish
begins to decline at 650 fish ha-1 and reaches zero at 1,300 fish ha-1. The corresponding stocking
densities for the 200-gram fish are 325 and 650 fish ha-1.
Yield or production is just the product of the stocking density and the growth rate of the
fish. Below the CSC, for example, the 100-gram fish grow at a rate of about 3.7 grams per day.
At the CSC of 650 fish ha-1, their production is therefore (650)(3.7) = 2,405 grams ha-1 or 2.4 kg
ha-1 (lower panel of Fig. 12.5). An important point to notice about Fig. 12.5 is that the maximum
production does not occur when growth rate is a maximum. This fact is most apparent in the
simulation of fertilization with no feeding (dashed curves), but it is also true of the simulations of
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no feeding and no fertilization. The maximum yield actually occurs at a biomass greater than the
CSC. The explanation is that for awhile the increase in biomass beyond the CSC more than
offsets the associated reduction in growth rate. In the case of the 100-gram fish with fertilization
and no feeding, the maximum yield occurs at a biomass of 292 kg ha-1, which is more than twice
the CSC of 120 kg ha-1.
Fish weigh 200 grams
Fish weigh 100 grams
Fish weigh 200 grams
Fish weigh 100 grams
Figure 12.5. Effect of stocking density and size of fish on growth rates and instantaneous yields
in carp ponds. Solid curves correspond to no feeding and no fertilization (CSC = 65 and CC =
130 kg ha-1). Dashed curves apply only to fish weighing 100 grams and correspond to
fertilization but no feeding (CSC = 120 and CC = 480 kg ha-1).
Why do growth rates decline when the areal biomass exceeds the CSC? A number of
factors may contribute to the decline. First, animals have a basal metabolic rate that is required
to keep them alive even when they are inactive. The less food an animal consumes, the higher
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the percentage of the consumed food used to support basal metabolism. At some point virtually
none of the consumed food is used for growth. Instead 100% is used for basal metabolism or is
excreted. Under a given feeding regime, the more fish in the pond, the less food available per
fish, and the smaller the percentage of the available food that goes to biomass production.
Second, the more fish in the pond, the more difficult it becomes for the community of
organisms in the pond to maintain satisfactory water quality. Fish produce metabolic waste
products whose concentrations must not be allowed to build up. Fish require oxygen, and
although some exchange of oxygen with the atmosphere will always occur, in a densely stocked
fish pond most of the oxygen required by the fish must be generated in situ. There is a limit to
how much metabolic waste can be processed by the microbial community in the pond, and there
is a limit to how much oxygen phytoplankton and macroscopic plants can produce. It is
important to bear in mind that photosynthesis ceases at night. Both plants and animals consume
oxygen in the dark. If the biomass of organisms in the pond is too great, the whole system may
go anoxic before the sun rises, and a complete loss of the case crop may result. Even if hypoxic
conditions and the accumulation of metabolic waste products do not literally kill the fish in the
pond, the stress associated with these conditions can make the fish more susceptible to disease or
infection. This line of reasoning naturally leads to the second of our four discussion topics.
WEED. Within the context of aquaculture, the term weed covers a variety of potential
problems, including diseases, parasites, predators, competitors, habitat, and water quality. There
is no question that problems related to “weeding” have been a major constraint on the
development of the aquaculture industry. Let’s consider pond aquaculture as an example.
Aquaculture ponds are typically operated at a depth of about one meter. Naively one
might think that it would make sense to operate as deep a system as possible, in effect making
the production system three-dimensional rather than two-dimensional. The problem with
operating a deep system is that most of the oxygen needed by the cash crop is produced in situ by
plants. Furthermore, plants provide a valuable service to the animals by assimilating potentially
toxic waste products like ammonia. In order to produce oxygen and consume waste products at
satisfactory rates, ponds are typically operated in a green-water mode, meaning that there is a
high concentration of phytoplankton in the water. From the standpoint of oxygen production,
there is good reason not to increase the depth of the system, since light penetration in such murky
water is minimal below a depth of roughly 1.0 meter (Boyd, 1979). Another reason for keeping
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the water column shallow is the fact that thermal stratification of the water column can
effectively isolate the bottom waters from the atmosphere. The atmosphere is roughly 20%
oxygen, and although exchange of gases with the atmosphere is not a major source of oxygen in
aquaculture ponds, it is in fact the only source at night. A shallow pond can be effectively mixed
by a light breeze and the entire water column effectively brought into contact with the
atmosphere. Wind mixing at night can therefore allow a pond to effectively absorb oxygen from
the atmosphere when the oxygen concentration in the water is below the saturation level. This is
not the case if the pond is so deep that wind mixing cannot effectively stir the entire water
column.
For these reasons, aquaculture ponds are typically about one meter deep and hence are
more-or-less two-dimensional systems. This imposes a practical limit on the areal density of
organisms in the system. This fact has not prevented overzealous aquaculturists from trying to
push the envelope, but the result of stocking a pond with too many animals is predictable. The
question is not whether disaster will strike. The only question is when.
In addition to issues related to water quality, the verb weed calls attention to a wide
variety of pathogens, parasites, predators, and competitors that can create havoc in an
aquaculture facility. A thorough discussion of such problems could easily be the subject of an
entire book. Here we will discuss some examples to illustrate the nature of the problem.
Shrimp viruses. Species of shrimp that contribute significantly to world aquaculture
production belong to the genus Penaeus and include P. stylirostris, P. monodon (Asia), and P.
vannamei (Latin America and United States). The culture of these and other species has been
plagued for years by problems caused by viral infections (McIlwain, et al., 1997). In the late
1980s Taiwan’s shrimp farm production, at the time ranked number one in the world at about
100,000 tonnes per year, collapsed due to pathogenic shrimp viruses. China experienced a
similar collapse after holding the number one position in 1992 and 1993. Outbreaks in 1995 and
1996 on shrimp farms in the United States caused a 50-95% loss of production at affected farms.
In April of 2004 an outbreak of WSSV occurred at a shrimp aquaculture farm on Kauai. The
company voluntarily drained all 48 of its ponds and buried 20 million shrimp. The economic
cost to the company was estimated to be at least $2 million in lost revenue. Although many
viruses (as well as other organisms such as bacteria and fungi) may cause diseases in shrimp,
four viruses have proven to be particularly troublesome in shrimp aquaculture.
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The Taura Syndrome Virus (TSV) was originally reported in cultured P. vannamei near
the Taura River, Ecuador. The first outbreak resulted in 80-90% mortality of young P.
vannamei. The disease, also known as Red Tail or Blackspot disease, has since occurred in
outbreaks in Hawaii (1994), Texas (1995), and South Carolina and Texas (1996) (Lightner,
1996a). The 1995 outbreak in Texas is believed to have been transferred by birds (i.e.,
contaminated seagull feces). Nearby shrimp packing plants, major importers and re-processors
of large quantities of shrimp from the Far East, are suspected to have been the ultimate source of
the imported virus. The 1996 outbreaks in South Carolina apparently resulted from broodstock
that were contaminated after they arrived at a hatchery (McIlwain, et al., 1997). TSV has been
documented in wild postlarval and adult P. vannamei from fisheries in Ecuador, El Salvador, and
the Mexican state of Chiapas near the border with Guatemala. It has also been reported in U.S.
aquaculture and hatchery facilities in Hawaii, Florida, Texas, and South Carolina (Lightner,
1996a, b)
Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) was first described
by Lightner (1983a; 1983b) in postlarval P. stylirostris and P. vannamei isolated from shrimp
farms in Hawaii. IHHNV has also occurred in Mexico, South Carolina, Texas, and Florida
(Fulks and Main, 1992). IHHNV outbreaks can cause up to 90% mortality in affected
populations of susceptible P. stylirostris. Survivors of IHHNV infections may be carriers of the
virus and pass it on to progeny or other populations. IHHNV is now known to be widely
distributed in shrimp farms in both the Americas and throughout Asia. It is believed to be
endemic in wild penaeids in the Indo-Pacific. IHHNV has been found in wild penaeids in
Ecuador, western Panama, and western Mexico.
White Spot Syndrome Virus (WSSV) was first identified in 1992-93 in China and
Taiwan. It has caused mass mortalities reaching 90-100% in several cultured shrimp species
(McIlwain, et al., 1997). Of great concern is the fact that WSSV can infect crustaceans other
than penaeid shrimp (e.g., amphipods, ostracods, crayfish, copepods). These alternative hosts
appear to have been responsible for transmission of the disease to some Asian shrimp farms.
The existence of such intermediate hosts or reservoirs presents a possible pathway for WSSV to
infect not only native shrimp but also other native marine and freshwater species (McIlwain, et
al., 1997).
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Yellow Head Virus (YHV) syndrome was first reported in tiger prawn (P. monodon)
aquaculture facilities in Thailand in 1992. The virus is believed to have been a major cause of
losses of cultured shrimp in Taiwan, Indonesia, China, and the Philippines during the 1980’s
(Lightner, 1996b). By 1994 it had been identified in India and Malaysia, and in 1995 it was
found in aquaculture operations in Texas. The most obvious clinical symptom of YHV is a
yellow coloration of the shrimp’s head. Juvenile shrimp appear to be most vulnerable to YHV
infection.
In an effort to at least ensure that broodstock are not contaminated, the U.S. Marine
Shrimp Farm Program supports a Nucleus Breeding Center at the Oceanic Institute in Hawaii.
Shrimp grown at that facility are routinely tested for nine viruses, including IHHNV, TSV,
WSSV, and YHV, as well as numerous other pathogens. Shrimp from the Nucleus Breeding
Center are provided to Consortium research facilities, commercial suppliers of postlarvae, and
commercial broodstock multiplication centers. The shrimp must be pathogen-free through two
generations before being used in commercial postlarval production.
Despite these precautions, viral outbreaks have continued to plague the U.S. shrimp
aquaculture industry. The most likely source of the viruses is shrimp imported from other parts
of the world. More than half of the shrimp processed in the United States is imported from
Thailand, India, and a number of other countries where shrimp viral diseases are common. It is
known that some countries deliberately harvest shrimp during the early stages of a disease
outbreak in order to minimize the impact on cultured shrimp sales. The contaminated shrimp are
then exported to other countries, including the United States. Shrimp infected with WSSV,
YHV, and TSV, for example, have been identified in retail stores in the U.S. (McIlwain, et al.,
1997). The importation of infected shrimp for processing by the U.S. shrimp industry
significantly increases the potential for introduction of pathogenic viruses into coastal waters
adjacent to the processing plants.
The problem of shrimp viruses is a serious impediment to expansion of the shrimp
aquaculture industry, and it does not appear to be a problem with a solution right around the
corner. The globalization of the shrimp industry has resulted in the transfer of infected shrimp
from one part of the world to another on a grand scale. While the availability of pathogen-free
broodstock from Oceanic Institute is certainly a step in the right direction, the ease with which
shrimp viruses can apparently be spread from one part of the world to another makes operation
16
of a virus-free shrimp farm for an extended period of time an elusive goal. Infected shrimp are
now routinely found in U.S. retail markets. Major exposure pathways include processing plant
wastes, wastewater from shrimp farms, infected bait shrimp, ballast water, and bird feces. There
is no cure for these viruses, and at least in the United States the response to a viral outbreak at a
shrimp farm is to drain all the ponds and sacrifice the shrimp, obviously at considerable
economic loss to the shrimp farm.
Fish diseases. A wide variety of pathogens have the potential to infect fish (Table 12.5).
In general these pathogens appear to have little impact on fish populations unless the fish are
stressed. However, under intensive culture conditions, a variety of stressors including crowding,
handling, drug treatments, low oxygen levels, and poor water quality can combine to potentiate
infection. Although there are treatments for many fish pathogens and most are not transferable
to humans, some are not treatable and can cause widespread mortality in both hatchery and wild
stocks.
Of the fish parasites, several are treatable in culture facilities with formalin, typically
administered more-or-less indefinitely at 15-25 parts per million (ppm) in earthen ponds or at
250 ppm for up to one hour in raceway systems. In other cases there is no known direct
treatment. For Neascus spp. Clinostomum marginatum, and Posthodiplostomum spp. indirect
control has been attempted by controlling the snails that serve as intermediate hosts to the
parasite.
Myxozoa make up a section of the eukaryotic kingdom protista that includes parasitic
protozoans that inhabit primarily the tissues and organ cavities of ectothermic vertebrates,
especially fish. Many Myxozoa have a two-host lifecycle that involves a fish and an annelid
worm or bryozoan, and this is true of the class Myxosporea, which includes the genera
Ceratomyxa and Myxobolus. C. Shasta and M. cerebralis both infect salmonid fish. The
parasites are transmitted via spores that are released following mortality of the host. There is no
known treatment for either parasite. M. cerebralis is probably the better known of the two
parasites. It causes a disease called whirling disease. Infected fish often become disoriented and
swim in circles – hence the vernacular name for the disease.
Of the viral pathogens, IPN, VHS, and IHN infect salmonid populations. CCV obviously
infects channel catfish, but other Ictalurid catfish may serve as carriers. There is no known
treatment for any of these viruses.
17
Table 12.5. Pathogens responsible for some common fish diseases
Parasites
Bacteria
Protozoans
Aeromonas hydrophila
Ichthyophthirius multifilis (“ich”)
Edwardsiella ictaluri
Trichdina spp.
Pseudomonas spp.
Ambiphyra spp.
Flexibacter psycrophilus
Epistylis spp.
Renibacterium salmoninarum
Chilodonella spp.
Flexibacter columnaris
Trichophyra spp.
Aeromonas salmonicida
Ichthyobodo (Costia) spp.
Proliferative kidney disease (PKD)
Yersinia ruckeri – enteric redmouth
Worms
Trematodes
Gyrodactylus spp.
(ERM)
Viruses
Neascus spp.
Infectious pancreatic necrosis (IPN)-RNA
Clinostomum marginatum
virus
(Yellow grub)
Viral hemorrhagic Septicaemia (VHS)-
Posthodiplostomum spp.
RNA virus
Tapeworms
Infectious hematopoietic necrosis (IHN)-
Proteocephalus ambloplitis (Bass
RNA virus
tapeworm)
Channel Catfish virus disease (CCV)-
Myxozoa
Ceratomyxa Shasta (CS)
DNA virus
Fungi
Myxobolus cerebralis “whirling
Saprolegnia spp.
disease”
Bacteria are ubiquitous in natural waters, and some are pathogenic to fish. Normally
pathogenic bacteria do not cause serious problems for fish populations unless the fish are
otherwise stressed. Treatment for some bacterial infections involves standard antibiotics such as
18
terramycin (Aeromonas spp., Pseudomonas spp., and Aeromonas salmonicida), but in other cases
there is no approved treatment.
Even more than bacterial infections, problems with fungi tend to be secondary symptoms
that occur when other trauma such as injury or disease have compromised the fish. Saprolegnia
spp. are ubiquitous in freshwater systems. Fish eggs are particularly susceptible to this pathogen.
Treatment in the case of fish eggs involves formalin administered at 1-2 mg L-1 for 15 minutes.
There is no approved treatment for control of Saprolegnia on fish.
Certainly one message that emerges from this analysis is that there are a great many
pathogens that can potentially infect animals in an aquaculture system. The pathogens in
question are found naturally in the environment, where they normally do not cause serious
problems. They can become serious problems in aquaculture systems where fish are stressed as
a result of crowding, handling, low oxygen levels, poor water quality, or other factors. While it
is tempting to push the limit with respect to stocking density, the closer one works to the limit,
the greater the probability for disaster to strike. High productivity sustained for weeks, months,
or even years may be moot if one’s entire cash crop is wiped out by a viral infection.
Weeds. Under the category of Weed comes a variety of problems due to aquatic plants.
In pond systems it is customary to maintain a dense population of phytoplankton to produce
oxygen and assimilate potentially toxic waste products, particularly ammonium. However, too
dense a concentration of phytoplankton can cause problems at night, when photosynthetic
production of oxygen ceases. Phytoplankton continue to respire and hence consume oxygen in
the dark. If the phytoplankton population is too large, it is possible that respiration by the
phytoplankton will strip the water of oxygen at night, with disastrous consequences for the cash
crop. One way to keep the size of the phytoplankton population under control is to stock the
pond with appropriate numbers of herbivorous fish. Silver carp (Hypophthalmichthys molitrix)
are frequently used for this purpose in freshwater systems. However, the use of silver carp in
this way is not as straightforward as it might seem. Silver carp are basically filter feeders. Their
gill rakers allow them to remove particles larger than about 40 m, but they have reduced
selectivity for smaller particles and are incapable of filtering out particles smaller than about 10
m (Laws and Weisburd, 1990). Although their feeding activities can certainly decimate the
concentration of large phytoplankton, they may at the same time eliminate many herbivorous
zooplankton. The result in some cases is that the concentration of phytoplankton in the water
19
actually increases, because small phytoplankton are released from the grazing control normally
provided by the herbivorous zooplankton (Smith, 1985a, b, 1988). Satisfactory control of the
phytoplankton population can sometimes be achieved by confining the silver carp to only part of
the pond, in effect providing a refuge for the zooplankton (Laws and Weisburd, 1990).
Grass carp (Ctenopharyngodon idella) are native to large Asian rivers in China and
Russia, where they typically reach sizes of 30-35 kg. Beyond a length of about 7.5 cm they feed
almost exclusively on aquatic plants and, when stocked at appropriate densities, can virtually
eliminate most macroscopic plants from a pond. Effective control has been reported with
stocking densities as low as 5-10 fish per hectare. Although grass carp do not seem to impact
other fish directly through competition or predation, their influence may be strongly felt through
removal of submerged vegetation, which would otherwise provide feeding and nursery habitat
for native species.
Triploid grass and silver carp (fish having an extra set of chromosomes) are functionally
sterile. Because of concerns over the impact of these fish on other species, only triploid fish are
permitted for water quality control in some areas. Silver carp were first introduced into the
United States from China in 1973 to improve water quality in aquaculture ponds, initially in
Arkansas. Unfortunately diploid silver carp escaped from aquaculture facilities within a few
years and were documented in the upper Mississippi River system as early as 1982. Silver carp
are now common in the Mississippi River drainage basin, where they compete with native larval
fish, freshwater clams, and adult paddlefish and bigmouth buffalo. Grass carp were first
introduced into the United States in 1963 by the U.S. Fish and Wildlife Service. Because the fish
were diploid, they were able to reproduce and subsequently spread throughout the Mississippi
and Missouri river systems as well as a number of major tributaries. They are now found in
almost every state in the nation. Triploid grass and silver carp can clearly provide a useful
service by controlling the growth of aquatic plants in some aquaculture ponds. However, their
release/escape into streams and rivers of the United States has been troublesome.
When cultivated with adequate controls, aquatic plants can provide a variety of services
to the aquatic farmer. If the cash crop is a herbivore, plants can obviously be a source of food.
In other cases algae may provide food for larval forms of the cash crop, even if the adults are not
herbivores. In pond systems phytoplankton are an important mechanism for controlling water
quality. In addition to these services, aquatic plants may be the cash crop. Japanese kelp
20
(Laminaria japonica), for example, is by weight the most important aquacultured species in the
world. It is used for the production of alginates and mannitol. In addition, kelp is a good source
of minerals and vitamins, and for this reason it plays a role in the diet of both humans (as dried
seaweed) and livestock (as seaweed meal). Although growing aquatic plants might seem
straightforward compared to animals, there are a variety of problems in the culture of aquatic
plants that fall into the weed category. Epiphytes are probably the most troublesome problem in
seaweed culture (LaPointe and Ryther, 1978). In the case of Japanese kelp, the epiphytes that
cause the greatest trouble are hydrozoa, bryozoa, and polychaete worms. The density of
spirorbid polychaetes, for example, on cultured Laminaria can be as high as 60 individuals per
square centimeter (Ivin, 1997). When present in large numbers, they make kelp unsuitable for
food. Not surprisingly, herbivores can also be a problem. One of the more famous microalgal
predators is the protozoan flagellate that invades outdoor cultures of the marine diatom
Phaeodactylum tricornutum (Raymont and Adams, 1958). The flagellate is capable of
decimating a culture of P. tricornutum within a few days. The principal grazers on cultivated
kelp are snails and amphipods. The most serious problems are caused by the snail Epheria
turrita. Infestations of these mollusks can reduce kelp production by as much as 85%. Kelp are
also subject to disease, fungi being particularly troublesome (Zvereva, 1998).
What can be done to control epiphytes, herbivores, and plant diseases? In the case of
kelp at least, part of the problem appears to be planting thalli at too high a density. Fungal
infections and serious outbreaks of E. turrita are most common in high density cultures (Limin,
1994). Lapointe and Ryther (1978) found that epiphytic growth on Gracilaria tikvihae could be
minimized if the seaweeds were allowed to soak up nutrients in a high-nutrient bath about once
per week and were transferred to a low-nutrient grow-out system for the remainder of the time.
The explanation for the efficacy of the nutrient bath treatments seemed to be that G. tikvihae was
able to store sufficient nutrients during an immersion of several hours to grow for about a week
without further nutrient additions. The troublesome epiphytes, on the other hand, lacked the
nutrient storage capabilities of the G. tikvihae and could multiply rapidly only when grown
more-or-less continuously in a high-nutrient medium.
Laws et al. (1983) reported satisfactory
control of the flagellate that invades P. tricornutum cultures by raising the pH of the culture
system to 9.5 for about one hour. They estimated that the ammonia concentration in their culture
21
at that pH was in the range 0.5-1.0 mM, which may well have been toxic to the flagellate.4
However, ammonia concentrations as high as 0.5 mM have almost no effect on the
photosynthetic rate of P. tricornutum (Avov and Goldman, 1982).
BREED. Crossing one species with a different but sexually compatible species is a
strategy used for centuries by human beings to produce plant and animal hybrid offspring with
more desirable traits than either parent. The cross that has probably been most significant in
human history is the one between a female horse and a male donkey. The offspring, a mule, has
greater endurance and is stronger and less excitable than a horse. Mules have been used by
humans for literally hundreds of years and were the preferred riding animal of gentlemen and
clergy in medieval Europe. Interestingly, mules are almost always sterile, and a line of horses
and domestic donkeys must be maintained to perpetuate their production. In more recent times
the so-called green revolution depended very much on the production of hybrid crops with
characteristics deemed valuable by the farmer. Crossbreeding, for example, has increased corn
yield in the United States nearly five-fold since the Civil War.
In aquaculture as in agriculture, the ability to produce hybrid offspring obviously implies
an ability to control/manipulate the reproductive process. This ability has sometimes been
lacking. The spread of Chinese carps, for example, was effectively blocked for centuries by the
inability of fish culturists to spawn them (Bardach, et al., 1972). Chinese carps naturally spawn
in rivers, and in captivity natural spawning almost never occurs. Chinese farmers who settled in
Taiwan 300-400 years ago brought with them the practice of carp polyculture in ponds, but there
were no rivers in Taiwan large enough to support natural populations. As a result fry had to be
imported annually, at considerable risk, from the Chinese mainland (Bardach, et al., 1972). It
was not until the 1960s that methods involving injection of gonadotropins5 were developed to
induce spawning of Chinese carps.
Table 12.6 summarizes three of the successful hybrid crosses that have been achieved in
the aquaculture industry. Behind various species of carp, Nile tilapia is the most widely cultured
finfish in the world. China is the principal producer, followed by Egypt, the Philippines,
Thailand, and Indonesia. Spawning tilapia has never been a problem. The problem for many
4
EPA water quality criteria for ammonia specify that the one-hour average concentration should not exceed 0.026
mM at a temperature of 25oC and pH of 9.0.
5
hormones produced by the pituitary gland that stimulate the reproductive organs
22
years has been the tendency of the fish to overpopulate a pond. Tilapia become sexually mature
at an age of 2-3 months, and from that time onward they will breed every 3-6 weeks as long as
the temperature of the water is warm enough. In the tropics this means essentially year-round
breeding. A pond stocked with tilapia soon becomes overpopulated, the result being that there
are few large fish and a great many runts.
Table 12.6. Some examples of hybrid animals grown in aquaculture
Species
Hybrid tilapia
Characteristics
ND 21 – all male , docile behavior
ND 41 – all male, docile behavior, spawns at low temperature
ND 56 – all male, docile behavior, red color, grows at high salinity
Hybrid striped bass Desirable characteristics include fast growth rate, increased
harvestable size, offspring vigor, and the fact that they can be
stocked into freshwater reservoirs. Most fish are tank reared and
fed high-protein brain-based diets to prevent rampant
cannibalization. Production is currently about 5,000 tonnes y-1 in
U.S. Successful reproduction of hybrids has occurred in a few
reservoirs. Maximum recorded weight is 10 kg. Lifespan is 5-6
years (similar to white bass). n.b., striped bass may live 30-40
years. Market: food, sport. Hybrid has gained wide acceptance as a
sportfish – stocked into large reservoirs in southeastern U.S., where
it feeds on gizzard shad and threadfin shad. Has potential for
culture in U.S. as food fish. Striped bass maintained in many
reservoirs in U.S. by stocking programs.
Hybrid catfish
Cross between male blue catfish (Ictalurus furcatus) and female
channel catfish (I. punctatus). Desirable traits include faster growth
in ponds (but not in cage, tank, or raceways), better feed
conversion, tolerance to low O2, increased resistance to certain
diseases, tolerance to crowding.
23
Empirically the most practical solution to this problem has been to produce monosex
stocks of tilapia by crossing two different species. There are more than a dozen wild species of
tilapia, and not all crosses have produced desired results. The crosses that have been successful
have produced male offspring, and several of those crosses have produced virtually 100% males.
The percentage is important, since only a few female tilapia in a pond can undo all the effort
involved in producing an otherwise all-male stock. More recent work with hybrid tilapia has
produced offspring (F1 hybrid) with other special characteristics (in addition to being all male)
such as color (e.g., red tilapia, white tilapia), temperature and salinity tolerance, and docile
behavior (Table 12.6).
Channel catfish (Ictalurus punctatus) are by far the most important aquacultured species
in the United States. Research concerning the hybridization of North American catfish has been
ongoing for more than 30 years. To date only one cross shows promise from the standpoint of
commercialization, the hybrid produced by mating a female channel catfish with a male blue
catfish (I. furcatus). The hybrid has a number of desirable characteristics (Table 12.6), including
greater resistance to pathogens such as Flexibacter columnaris, Edwardsiella ictaluri,
Aeromonas hydrophila, Ichthyophthirius multifilis, and channel catfish virus. The greater
resistance to E. ictaluri is noteworthy. E. ictaluri, a bacterium, causes enteric septicemia in
catfish (ESC), economically the most important disease of farm-raised channel catfish. One of
the major obstacles to expansion of hybrid catfish production has been the low success rate of
spawning efforts. Spawning of female I. punctatus and male I. furcatus in open ponds has been a
dismal failure. Greater success has been achieved when the fish are confined in pens and the
female I. punctatus injected with hormones, but the average success rate for pen spawning is
only about 15% (Masser and Dunham, 1998). Substantially greater success has been achieved
with artificial spawning, but this involves sacrificing male I. furcatus in order to remove their
testes. At the present time the major obstacles to expansion of hybrid catfish production are the
availability of blue catfish brood stock and the need for improved spawning techniques. When
and if hybrid egg-fingerling production becomes commercially feasible, it is possible that hybrid
catfish will replace channel catfish as the major aquaculture fish for the United States.
Striped bass are a major sport fishery in the United States. The species, Morone saxatilis,
was originally found along the Atlantic coast from New Brunswick to Florida and along the Gulf
Coast from Texas to Florida (Hodson, 1989). Striped bass were introduced to the Pacific coast in
24
the early 1890s and are now found along the west coast from British Columbia to Baja,
California. Striped bass are anadromous, but landlocked populations are found in some
freshwater reservoirs.
Figure 12.6. Capture production and hybrid striped bass aquaculture production in the United
States.
Although a number of crosses involving striped bass and other species of the genus
Morone have been carried out, none has been as successful as the cross between striped bass and
white bass (M. chrysops), which was first carried out in South Carolina and California in the mid
1960s. If the female in the cross is a striped bass, the hybrid is known as a palmetto bass. If a
female white bass is crossed with a male striped bass, the hybrid is called a sunshine bass. One
obviously appealing aspect of both hybrids is the fact they can be successfully stocked into
freshwater reservoirs. Palmetto and sunshine bass are not capable of reproducing with one
another, but they may back cross with white bass or striped bass. Hybrid striped bass
25
production has exceeded capture production of striped bass in the United States since about 1990
(Fig. 12.6) and currently stands at about 5,000 tonnes per year. The rapid growth in the
production of hybrid striped bass has been in part a response to the market void resulting from
the demise of natural striped bass stocks (Fig. 12.6) and increasing demand for seafood (Ludwig,
2004). Hybrid striped bass cultured in private facilities are supplied to restaurants and
supermarkets as food fish or fresh or frozen-filleted products. Much is sold “whole-fresh” to
high-end restaurants, and some is exported by air-freight to European markets (Burden, 2004).
SEED. Reproducing aquatic organisms and growing the offspring to a size large enough
for stocking in growout facilities has proven to be a conundrum for some species. In the case of
reproduction, control is key. In the southeastern United States, for example, paddlefish
(Polyodon spathula) were given serious consideration for culture in ponds because they feed
primarily on zooplankton. Hence feeding costs would have been minimal. The quality of
paddlefish flesh is excellent, and the roe can be made into a good grade of caviar (Bardach, et al.,
1972). The problem with paddlefish turned out to be the fact that they do not spawn each year,
and they do not spawn at all in standing water. In the late 1950s, interest developed in culturing
buffalofish (Ictiobus spp.) in conjunction with rice cultivation. In order to maintain soil fertility,
rice farmers were rotating crops, and it was thought that rotating buffalo fish with rice might be
more profitable than more traditional strictly agricultural crop rotation. Buffalofish are low on
the food chain and consume primarily plankton, detritus, and benthic organisms. Hence like
paddlefish they would have been inexpensive to feed. There are three species of buffalofish,
bigmouth buffalo (I. cyprinellus), black buffalo (I. niger), and smallmouth buffalo (I. bubalus).
Preliminary studies indicated that largemouth buffalo were clearly superior to the other two
species from a cultural standpoint. They grow faster, become sexually mature at an earlier age,
and are more fecund than the other species. Research showed that spawning of bigmouth buffalo
could be triggered by manipulating their environment, specifically by the addition of freshly
drawn water and a reduction in population density.6 In practice buffalofish and rice were rotated
on a four-year cycle, with two years devoted to buffalofish pond culture and the next two years
to rice cultivation. Several factors ultimately led to the demise of the buffalofish/rice crop
rotation model. One was the increased use by rice farmers of pesticides and mechanized
6
Buffalofish emit a substance that, in high enough concentration, inhibits spawning.
26
methods of cultivation incompatible with fish culture. The second was the development of
channel catfish aquaculture during the early 1960s, initially in Arkansas and later in Mississippi.
Table 12.7. North American catfish (family Ictaluridae)
Catfish
Channel catfish
Pros: aesthetically pleasing appearance, ready adaptation to
(Ictalurus punctatus)
artificial feed, resistance to crowding
Cons: nervous temperament, males tend to fight at breeding
time
Blue catfish
Pros: grow more uniformly and produce fewer giants and runts,
(Ictalurus furcatus)
dress out better, readily learn to feed at surface, less nervous
and easier to seine, males less prone to fight at breeding time
Cons: much poorer growth, poor survival when shipped live,
poorer conversion of artificial feeds, greater age at maturity
Flathead catfish
Pros: ?
(Pylodictis olivaris)
Cons: highly piscivorous and cannibalistic, high mortality of
fry
White catfish
Pros: easily grown to size acceptable to consumers, superior in
(Ictalurus catus)
converting food, hardy – withstand crowding, low oxygen,
turbidity, and high temperatures, feed voraciously throughout
the summer, easier to spawn than any of the other Ictalurids
Cons: smaller than channel, blue, or flatheads, do not grow as
rapidly, do not dress out as well as channel or blue
Bullheads
Brown bullhead
Pros: extremely hardy with respect to the physical environment
(Ictalurus nebulosus)
– oxygen, temperature, turbidity
Yellow bullhead
Cons: smaller than other Ictalurids, reproduce readily in ponds
(Ictalurus natalis)
– hence over-population and stunting frequently occur, more
Black bullhead
susceptible to disease
(Ictalurus melas)
27
There are seven species of North American catfish that have potential for use in
aquaculture (Table 12.7). The species that has come to completely dominate the aquaculture
industry is the channel catfish. An examination of Table 12.7, however, reveals that it is not
entirely transparent that channel catfish are the best choice for aquaculture. Flathead catfish can
obviously be eliminated, as they have several undesirable traits and no superior qualities relative
to channel catfish. Bullheads have the same problem as tilapia. They reproduce naturally in
ponds and hence tend to overpopulate the ponds. In addition, they are naturally smaller than the
other Ictalurids. That leaves channel, blue, and white catfish. All three species have their pros
and cons. This of course underscores the rationale for looking at hybrids (see above). As
already noted, the cross between female channel catfish and male blue catfish appears promising,
but commercialization of the hybrid will require more reliable methods of spawning.
The problem of seed for most catfish farmers has been solved by hatcheries that provide
fingerlings, at a price of course. Very large catfish farms have their own hatcheries, but for
farms that stock fewer than 100,000 fingerlings annually, it is generally more cost effective to
rely on a commercial hatchery for fingerlings (Bardach, et al., 1972).
In the United States, wild channel catfish migrate to the shallows of rivers and lakes in
April and July, and it is there that they spawn. During this time vicious fighting may break out
between males, and wounds received in those fights, if not directly fatal, may lead to infections
that ultimately claim the life of the fish. Male and female channel catfish pair up, and spawning
typically occurs when the water temperature is 20-23oC. Spawning consists of deposition of
successive layers of adhesive eggs by the female and fertilization of each layer by the male. The
eggs are deposited in a crude nest built by the male, usually in a sheltered place. The spawning
process can require as much as 12 hours. If the female is not ready to spawn, she may be
attacked or driven away by the male. In any case, once the female is spent, the male drives her
away and begins to guard and care for the eggs.
Building on this knowledge, catfish culturists have been able to successfully spawn
channel catfish in spawning ponds. The spawning ponds are normally drained in winter and are
not filled with water until 30-40 days before spawning is anticipated. This minimizes the
establishment of predatory insects, which would feed on the young fry. The spawning ponds are
stocked with equal numbers of males and females to minimize fighting insofar as possible.
28
Spawning receptacles are provided in the form of 45-liter milk cans, nail kegs, earthenware
crocks, etc. space 9-12 meters apart with the open end toward the center of the pond.
Channel catfish can also be spawned in pens or aquaria. In the former case, a male and
female are placed in a pen as spawning time approaches. The pair are closely monitored,
because the male may attack and kill the female if she is not ready to spawn. Once spawning has
been completed, the female is removed from the cage to prevent her from eating the eggs or
being attacked by the male. Spawning in aquaria follows a similar protocol except that injection
with pituitary hormones is routinely used to stimulate spawning. Immediately after spawning,
the eggs are removed for artificial hatching and a new pair of spawners placed in the aquarium.
If ponds or pens are used, the eggs may be left to hatch naturally, usually after 5-10 days,
depending on the water temperature. Alternatively, and always in the case of aquarium
spawning, the egg mass is removed and the eggs hatched under artificial conditions in hatching
troughs. A fungicide (malachite green) may be used to control the growth of fungi on the eggs.
Once the eggs hatch, the fry are normally stocked in nursery facilities. Alternatively, the
fry can be returned to the spawning pond if the adults are removed. The fry begin feeding 3-5
days after hatching. They are fed a formulated feed, ground suitably fine. This is a crucial step
in the production of seed. What to feed juveniles or larvae is in some cases by no means
obvious. The fact that recently hatched catfish fry will consume a formulated feed makes their
culture very straightforward compared to some other species with complex life cycles and/or
finicky tastes. The biggest problem in the rearing of flathead catfish fry, for example, is the fact
that very young fry often fail to accept food. Survival rates from the fry to fingerling stage have
been as low as 4-6% (Bardach, et al., 1972).
Although channel catfish fry may be brought to fingerling size in nursery troughs, they
are more commonly transferred to fry-rearing ponds within a few weeks of hatching. Normally
the fry rearing ponds are left dry until a week or so before stocking to avoid the establishment of
predatory insects. Ponds may be treated with SAE-30 motor oil or a mixture of cotton-seed oil
and kerosene to eliminate air-breathing insects. The fry continue to be fed with a formulated
feed, care being taken to ensure that the right-size particles are provided. The typical growout
time to fingerling size is 180 days.
29
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