by NICK C. PARKER and COLIN E. NASHFishes The Catfishes
More than 2000 species of catfish, representing at least 31 families, occur in marine or fresh water around the world (Kobayagawa, 1989). Species cultured as food fish are primarily in the families Claridae, Ictaluridae, Pangasidae, and Siluridae.
The global production of catfishes as food fish was estimated to be about 320 000 metric tons (t) in 1992 (FAO, 1994). Compared with the total of 8 million t of freshwater fish farmed world-wide that year, the group would not seem to be important. For many individual countries, however, production of indigenous species of catfishes is important in the economies of their national fisheries sectors. For example, the North American channel catfish, Ictalurus punctatus, is the basis of the large and valuable catfish industry which dominates aquaculture in the United States (Parker, 1989).
The success of the channel catfish industry in the southern United States has stimulated interest in catfishes for culture throughout the world. North American ictalurids have been introduced into Europe, Asia, Africa, and South America, but similar successes with this family have yet to be achieved. Only Mexico and Cuba are producing regular quantities each year. For the most part, countries in these continental regions are using their own native species. Europe has been working with the silurids, Africa with the clarids and chrysichthids, and Asia with the clarids and pangasids. Production of these five groups of catfishes is described in this chapter.
The catfish industry did not exist in the United States until about 1960, when there were about 160 ha of ponds in a number of southern states. At that time there were no formulated feeds for catfish, no feed mills, no processing plants, and no specialized equipment, all of which are common now.
The potential for warmwater aquaculture was recognized by the U.S. Congress in the late 1950s and resulted in federal support for fish farming research through the U.S. Fish and Wildlife Service of the Department of the Interior. Federal programmes established to promote warmwater aquaculture at Marion, Alabama, and Stuttgart, Arkansas, augmented research efforts at Auburn University and other research centres to define the basic requirements for the culture of catfishes.
Species of early interest included the white catfish, Ictalurus catus; channel catfish, I. punctatus; blue catfish, I. furcatus; black bullheads, I. melos; flathead catfish, Pylodictus olivaris; and a number of others. Many of these species, and their hybrids, were evaluated as candidates for aquaculture, butthe industry developed around the culture of channel catfish because of its rapid growth rate, early maturity, and ease of spawning. TABLE 4.1 Weight and pond-side value per kilogram of catfish sold to processors in the U.S.A., 1986-1992
Source: Mississippi Agricultural Statistics Service, 1992.
As of January 1993, 1527 catfish enterprises in 15 southern states operated some 62 840 ha of production ponds, as well as 2295 ha of ponds to maintain the broodfish on which the industry relies. About 208 000 t of fish harvested as food fish in 1992 were sold to processing plants and had a pond-side value of US$316 million. The average price per kilogram paid to the producers peaked in 1988 at $1.68 and declined to $1.32 in 1992 (see Table 4.1). Additional sales of fish in the live-haul trade brought total production in 1992 to about 223 000 t.
1.2. Breeding and propagation of channel catfish
About 50% of 2-year-old channel catfish will spawn, but typically fish 3-10 years of age are used; 3-7 years is the preferred choice (Tucker, 1985). Larger and older fish are more difficult to handle and often produce fewer eggs per unit of body weight than smaller and younger broodfish.
In June 1992 the states of Alabama, Arkansas, Louisiana, and Mississippi had 1.36 million broodfish in ponds. These broodfish will produce an average of 18.2 million kg of marketable channel catfish.
Broodfish are maintained in ponds at relatively low densities to ensure that water quality remains optimum. The criteria for good water quality in ponds were first collectively established by Boyd (1979). Broodfish maintained in ponds in which the minimum level of dissolved oxygen is 5 mg/1 spawn more frequently and yield a higher number of viable fry than broodfish from ponds in which water quality is poor and dissolved oxygen is low. Maintenance of healthy broodfish is essential to the production of good-quality fry.
Channel catfish are typically spawned in ponds, tanks, cages, or aquaria; most are spawned in ponds or cages. Wild channel catfish spawn naturally in cavities in river banks, under roots or logs, or in any hollow hidden space of suitable size. Consequently, milk cans, metal drums, clay jugs, and plastic containers have all been used in ponds as artificial spawning habitats (Parker, 1988). The mouth of the artificial container must be large enough to allow entry by the broodfish, and the interior must be large enough to accommodate the male and female fish during spawning. The containers are usually placed in about 1 m of water, with the opening of the can facing away from the bank. A float tied to the container by rope facilitates lifting for inspection and removing the eggs.
The male will enter the spawning container when the water temperature is about 21-29°C (optimum 27°C), which is normally early May in the southern United States, and he will use his tail to sweep silt and debris from the container to construct a nest.
Catfish eggs are adhesive and are deposited in a mass. They are continuously fanned by the fins of the male to maintain a flow of fresh oxygen-rich water over them. Eggs may be left in the spawning container until they hatch, but more typically they are removed to a hatchery trough for incubation. Removing eggs from the nest prior to hatching permits easier capture and counting of fry.
Small cages or pens constructed in earthen ponds are used to hold selected pairs of broodfish to produce strains of known lineage (Stickney, 1993). Pens, about 2 x 3 m in area and in water about 1 m deep, are prepared by placing a spawning container in each and introducing the male catfish first. A female of about the same size as the male is placed in the pen to breed. She is removed immediately after spawning. The male is left in the pen to care for the fertilized eggs, or the eggs are removed for incubation in a hatchery trough.
Standard glass-sided aquaria about 0.6 x 0.6 x 0.3 m deep and provided with a steady flow of well-aerated pond water, are used for mating selected pairs of broodfish and to hold ovulating females for selection in the production of hybrids. Spawning in aquaria or hatchery tanks is not typically practiced by commercial producers but is an effective research tool.
Exogenous hormones are rarely used for fish spawning naturally in outdoor ponds but are often used to induce breeding pairs of selected fish in pens or aquaria. Human chorionic gonadotropin (HCG), injected at a dose rate of 1800 I.U. per kg, is often used to induce spawning in gravid females. Pituitary glands, obtained from sexually mature catfish (or even from common carp, as practiced in the United States during the spawning season and acetone-dried for preservation and storage, are also injected into female catfish to induce spawning. The dosage is about 4.5 mg/kg of female body weight. Dried pituitaries, collected and prepared on site, and a fish hormone analog such as salmon lutenizing hormone releasing factor (LHRHa), are also effective in inducing spawning. They also have an advantage in that they do not carry the human health risks (such as human immuno-deficiency virus, or HIV) associated today with products such as HOG.1.2.3. Controlled incubation The fertilized eggs removed from any spawning container are placed in hatching troughs for incubation. The troughs are fitted with a slowly revolving paddlewheel (about 30 rpm), or any am-driven agitation system, to rock the egg mass slowly and simulate the parental care normally provided by the male (Fig. 4.1). When each egg mass is placed in an individual wire basket suspended in an isolated compartment of a hatching trough, the fry from individual spawns can be collected for evaluation of growth and development, disease diagnosis, or identification of genetic traits.
An alternative method of incubation is to treat the eggs in a 1.5% solution of sodium sulphite for 5-7 minutes to dissolve the adhesive coating on each (Dorman, 1986). This permits the eggs to roll and float freely when placed in the upwelling current of a McDonald hatching jar.1.2.4. Fry and fingerling production Eggs of channel catfish hatch in about 8 days. The newly hatched fry (often called sac fry) swim to the bottom of the trough and remain clustered in a fairly tight ball for 2-3 days as they absorb the energy reserves of their yellow yolk sacs. Once their yolk sacs have been depleted the fry (now called swim-up fry) swim to the surface and begin to feed. Fig. 4.1. Hatching troughs for channel catfish, fitted with paddle wheels to move fresh water across the egg masses.
Swim-up fry have to be fed frequently throughout the 24-hour day, and automatic feeders are used to provide feed continually to fry in the troughs. Fry are supplied with a high protein (40-50%) artificial during the first few days or, more frequently, are stocked in ponds that have been well fertilized and are rich in planktonic organisms. If the fry are maintained in hatchery troughs, the protein level of the artificial diet is reduced to 25-36% as the fry develop into fingerlings.
1.3. Grow-out operations
Early catfish farms were Vertically integrated'; that is, they maintained broodfish, propagated fry, raised fingerlings, produced food-sized fish, harvested and marketed whole live fish, and processed fish (Parker, 1989). Today, the industry has evolved, and many specialist farms provide one or more of these products or services.
Most channel catfish are now raised in shallow warmwater ponds, and farms are typically of three sizesless than 30 ha, 30-100 ha, and over 100 ha (Giachelli et al., 1982). The total cost per kilogram of fish produced decreased as farm size increased from 57 ha to 230 ha (Keenum and Waldrop, 1988). About 182 000 t of farm-raised catfish were produced in the United States in 1991. An additional 2400 t were imported to meet market demand, primarily from Brazil, Thailand, and Canada.
1.3.1. Small-scale farming
The smaller-sized farms are frequently operated as family businesses, with little need to employ non-family members. Many small farms have as few as ten 6-7-ha ponds, and the fish-farming operation is usually one component of a larger agricultural enterprise. On these small farms profits can be high because equipment used in other activities is used to support the catfish operation. Small farms frequently buy fry or fingerlings from specialty producers and stock ponds at relatively low densities, usually less than 10 000 fish per ha.
Owners of small farms market fish to live-haulers for restocking elsewhere or directly supply consumers, restaurants, or processing plants. The profits per unit weight are often higher on small farms than on medium or large farms because production ponds, which range from 0.5 to 10 ha, are formed by using earthen dams to retain water in natural valleys and against hillsides. However, on farms of similar size revenue returns increased as pond sizes increased from 2 to 8 ha (Garrardet al., 1990).1.3.2. Medium-scale farming Farms of 25-100 ha usually have higher production costs than smaller or larger operations. Ponds are especially designed and constructed (with four sides), and special equipment, such as tractors and trucks, is required to operate these farms; however, this equipment is not used continuously.
Fry and fingerlings are typically produced on the farm for stocking the production or grow-out units for marketable fish. The standard production unit is a 9-ha pond, which will typically contain 3600-4500 kg/ha of fish at harvest. Stocking densities for table fish typically harvested at 0.5-0. 75 kg are usually at least 10 000 fish/ha, and in some intensively managed ponds they may be twice that number.
Dissolved oxygen is routinely monitored in intensively managed ponds. Emergency aerators are activated in some ponds by automated on-site monitoring equipment, or they may be manually activated when dissolved oxygen declines to about 2 mg/220.127.116.11. Large-scale farming Well laid-out and specially constructed catfish farms of 100 ha and greater are common in the state of Mississippi, where there are more than 43 000 ha of catfish ponds, representing 475 farms (Brunson and Brown, 1991). Such farms produce fish at the least cost per unit weight because the cost of equipment and operating costs are spread over many units. Almost all the equipment is in use daily. For example, paddle-wheel aerators driven by the power take-off shafts of farm tractors are commonly used for emergency aeration, and on many farms aerators operated by 10-HP electric motors or internal combustion engines are installed in each pond (Fig. 4.2).
Broodfish are maintained on these large farms to produce fry for stocking the production ponds or for sale to other farms. The fish are typically harvested with mechanized seine nets, of a mesh sized to remove individuals of 0.5 kg and above, while leaving smaller fish behind. Almost all fish from these large farms are sent directly to processing plants, where they are prepared and packaged for market.1.3.4. Farming with water re-use systems A number of innovative systems have been built to produce catfish successfully in intensively stocked and well-managed culture units using water filtration and reuse. Although successful as laboratory or pilot-scale units, these systems have not contributed significantly to the annual volume of catfish produced by farming. Most water re-use systems have been designed to operate indoors and produce fish in a series of tanks, with water pumped and circulated from the tanks through various filter units and back. Several high-technology systems have attracted venture capitalists and have been marketed for use in urban areas or as 'backyard' units. Small-scale units are mostly educational and amusing to operate for fresh fish production for home consumption. Large-scale commercial units, on the other hand, have not demonstrated a profitable track record. Most commercial indoor water re-use units have operated only a few short months to perhaps a year or two before failing financially.
A few large-scale water re-usesystems, using large earthen ponds as filtering beds, have been constructed in Arkansas and Texas. These recirculating units reportedly produce up to 7000 kg of catfish per 4.5-m3 cell, each receiving 9.4 m3 of water/mint Water flows from these concrete cells or raceways (Fig. 4.3) containing catfish, through raceways containing tilapia, and then into ponds containing other species, such as carp, buffalo fish, tilapia, and paddlefish. The total water flow in these recirculating systems, which produces about 68 t of fish annually, is about 340 m3/min. The water is then circulated through about 50 ha of ponds which act as filtration units.
High-technology recirculating systems offer several advantages over simple pond farming, such as ( 1) easier accessibility of fish during feeding, harvesting, grading, and treatment; (2) complete protection from predators; and (3) better maintenance of high levels of dissolved oxygen in the water, using gravity flow aeration and injection of pure oxygen. In some of these systems three or four diesel or natural gas combustion engines are used to drive pumps, each delivering about 113 m3/min.1.3.5. Artificial feeds and feeding Formulated artificial feeds for channel catfish are readily available in the United States, so large storage containers for bulk feed are as common on fish farms as they are on dairy, pig, or poultry farms (Fig. 4.4). Daily rations from these silos, or from bags, are loaded into a truck-mounted hopper, from which the feed is controlled by weight as it falls into the air-steam of a high-volume blower. From this contraption, pulled by a farm tractor or vehicle along the berms of the ponds, the feed is broadcast across the water.
The successful mechanization of feed delivery, notwithstanding, the right daily feeding rate to each pond is critical. The correct feeding rate is important for profitability of the farm operations, as feeding costs may be 60-75% of production costs, but also the environment of the pond itself. The environmental balance of the pond can be easily upset by an accumulation of large quantities of uneaten feed on the substrate.
Optimum daily feeding rate and particle size for fry and fingerlings of channel catfish (after Dupree and Huner, 1984)
|Length of fish (cm)||Feeding rate (%)||Particle size (number)||Particle diameter (mm)|
* *10-25 kg/ha equivalent.
Thus, commercially prepared pellets are available in two forms, sinking and floating. Although the unit cost of sinking pellets is less than that of floating pellets, most farmers prefer to mix them in a 5:1 ratio, as the floating pellets enable them to gauge the feeding response of the fish and to avoid overfeeding.
Although the full dietary requirements (essential amino-acids,
carbohydrates, lipids, vitamins, and minerals) for channel catfish at all stages of
development are now known, the composition of the pellets is changed frequently depending
on the natural productivity of the pond and the time of year (Wilson, 1991). In winter,
for example, the protein content is reduced. Typical protein composition of the feed for
good growth under favourable conditions is as follows:
Enteric septicemia of catfish (ESC), caused by the bacterium Edwardsiella ictaluri, is the major cause of mortality in channel catfish. Financial losses to the industry in the United States are between $20 and $80 million annually. Current research is therefore focused on commercially produced vaccines. In a massive field trial in 1990-91, almost half of the 68 million catfish fingerlings distributed in 77 production ponds in four southern states were treated with ESC vaccine. Treatment included immersion as fingerlings or fry, followed by oral vaccination as fingerlings. At the end of 1991, survival was 24.1% greater in vaccinated fish through harvest, and harvest density per acre was correspondingly higher (BioMed, 1992).
1.4. Harvest and transportation
The average annual yield of channel catfish from intensively managed ponds ranges between 3300 and 4400 kg/ha. With standard-sized ponds of 10 ha, harvesting has had to become a highly mechanized process. Because of the variation in the size of fish and the need for conformity in the processing lines,seine nets of differing mesh sizes are used to grade and harvest food fish for market. For example, a grading seine with a mesh of 3.5 cm selects all catfish weighing 0.34 kg and larger, and a 4.1-cm mesh selects all fish 0.45 kg and larger.
The seine nets are pulled by mobile seine-haulers or by tractors. Because catfish are predominantly bottom dwellers, the seine nets are fitted with 'mud lines', bundles of smaller ropes which hold the net down on the bottom but out of the mud. Strong nylon ropes are attached to the mud line for hauling.
Harvesting is carried out in two stages because of the quantity of fish in the pond and the general disturbance of the substrate, which may cause deterioration of the environment. The fish are congregated into one part of the pond with a large seine. They are then fished out in smaller quantities with a smaller seine and transferred to a live car or large net cage. The fish are held for 24-48 hours before shipping, which allows them to evacuate their intestinal tracts. This procedure helps to maintain water quality during subsequent transportation.
Because of the advanced state of the industry, live cars are now specially manufactured for and frequently owned by the larger farms. Each is fitted with a mechanical pump, to move fish as well as circulate the water, and aeration system.1.5. Processing and marketing The principal markets for channel catfish are the buyers of live fish, for further grow-out on farms or for stocking private lakes for sports fishing, and consumers (i.e., the processing industry).
The live fish market is small, but it is the most profitable market. Live fish for sports fishing may be worth more than $4.0/kg (1992 prices), depending on season and supply. This money is easily recouped by the owners of the fish-out waters, who charge anglers one fee for fishing and another for the weight of fish caught and retained.
Most of the fish cultured in the United States are sold to modern processing plants (Fig. 4.5). Again, the price paid to growers, which has fluctuated from $1.32 to 1.67/kg, depends on availability and quality. Off-flavour of the flesh, due to organic compounds that originate in blue-green algae, is a major problem in product quality control. Therefore, before fish are accepted by the processing plant, a sample is taken the week before harvest, when the fish are being held in the live-cars or cages, and another sample is taken immediately before processing. The samples are sent to the plant for analysis and taste testing. Fish that do not pass the test are returned to clean ponds or held under good water quality conditions until the flavour improves. Because of the importance of product quality to the industry, research is being conducted on the lipid content of the fish, and on ways to improve quality and shelf-life.
Fresh frozen catfish are retailed for about $6/kg (1992 prices), but new product forms, such as seasoned, breaded, and pre-cooked fillets, are retailed for about $35/kg.2. CLARIDS AND CHRYSICHTHIDS In Africa the culture of catfishes centres on the clarids and the chrysichthids. The clarids are the most important group, and all species are often simply called 'African catfish'; the chrysichthids, on the other hand, are usually called 'black catfish'.
In different parts of the continent the common name of African catfish under culture is applied to several closely related species or sub-species. For example, in north and central Africa the name is given to Clarias lazera, in the west to C. senegalensis, in the east to C. mossambicus, and in the far south to C. gariepinus. However, for the purposes of conformity in the field, the African catfish is considered to be C. gariepinus, and culture practices are described in this section accordingly. Similarly, the most important black catfish for culture at present is Chrysichthys nigrodigitatus, and reference to only this species will be made.
In Asia the name 'Asian catfish' for culture is often used to identify Clarias batrachus, C. macrocephalus, and C. fuscus. Other common names used for these species are walking catfish, Philippine catfish, and Hong Kong catfish. The pangasid catfishes of Asia are described in the next section.
Production of clarids in African and Asian countries, whether clearly identified or not, is now reported regularly by FAO (1994). In Asia the largest producer is India (with almost 50 000 t annually), followed by Thailand (over 26 000 t) and Indonesia (almost 5000 t). Other countries reporting some production are Cambodia, Hong Kong, Malaysia, the Philippines, and Taiwan.
In Africa the largest producers are Nigeria, with over 5000 t annually, and South Africa (about 500 t). Countries reporting 150 t or less are the Cameroon, Central African Republic, Cote d'Ivoire, Ghana, Guinea, Lesotho, Malawi, and Mali. Outside these two continents, clarid production is reported in the Netherlands (about 1000 t) and on the Pacific island of Guam (10 t). Many other countries produce clarids on farms or in natural water bodies but do not record and report them individually as yet; for example, Bangladesh, Pakistan, Malaysia, and Egypt include them in their collective totals of freshwater fishes. Production of the black catfish is now about 1000 t annually, and confined to the brackish-water coastlines of West African countries, particularly Benin, Cote d'Ivoire, and Nigeria.
In most African countries where farming exists, production of African catfish has been instigated by international technical assistance projects, particularly in the Central African Republic and Cote d'Ivoire. In the past most technical assistance has been given for raising the common tilapia species (see Chapter 7) for improving human nutrition in rural communities. More recently, support for the catfishes has been provided to supplement farmers' incomesas catfishes in Africa have greater commercial value in urban markets. Most of the research and development has been carried out and transferred by the Agricultural University of Wageningen in the Netherlands, supported by the Ministry of Cooperation and Development, and the techniques are now fully described in the literature (Huisman,1986) and practical farming manuals (Viveen et al., 1985).
In Asia development has been through private initiatives, beginning first with the use of young fish captured and then grown-out in prepared ponds or retained in flooded areas and swamps. Today, this practice is supplemented by the propagation of fry and production of fingerlings from captive broodstock.
2.1. Production of fry and fingerlings
Clarids are predominantly tropical and semi-tropical species and therefore mature earlier (at about 200 g body weight) and spawn more readily than ictalurids. Breeding is usually in response to sudden environmental changes, such as heavy flooding in the rainy seasons. Clarids can also withstand the greater extremes associated with drought, such as increasing salinity and low oxygen levels. Consequently, many farmers in Asia continue to rely on the fry found in the most accessible water bodies at the beginning of the annual rains. Others introduce mature fish into flooded rice paddies and let them breed naturally. Traditionally, farmers and villagers collected fry and fingerlings in hand nets, but more recently, many have learned to collect the adhesive egg masses and incubate them in hatching jars or containers to reduce losses to predation by frogs.
Breeding of clarids in ponds in Asia is developing rapidly, but the production of fry in this way has not yet surpassed organized collection. Broodstock are placed in ponds and fed a high-protein diet of trash fish and cereal waste, usually rice bran, at a daily rate of8-10% of their body weight. The pond practices are not unlike those used by the ictalurids. Ponds are about 1 ha, and water depth is about 1 m. The fish are allowed to make natural nests in hollows in the walls of the pond or are offered artificial nesting areas in containers.
In the breeding season the water in the broodstock ponds is simply rain fed, or it is regulated from a holding reservoir or natural source. However, by simulating seasonal flooding through the use of stored water, some farmers are finding it possible to breed catfish almost year round.
Incubation of the egg mass in the nests and parental behaviour of clarids are similar to that described for ictalurids. The males guard the nests, fanning the egg mass with their tails to keep it oxygenated. Depending on water temperatures, the eggs take almost 24 h to hatch. A large female will produce about5000 eggs, but because of early maturity most nests contain much fewer eggs. However, spawning is repeated frequently in the season by manipulation of the water levels.
Emergent fry are gathered by hand nets and placed in smaller net enclosures in the breeding ponds built where there is good water circulation, usually close to the outlet. From here they are transferred to small nursery ponds of 1 are (100 m2), or less. Nursery ponds are specially prepared beforehand to increase natural populations of plankton and small live-food organisms on which the fry feed. As with production ponds for milkfish (see Chapter 11) and marine shrimps, high productivity is established by fertilizing the dry substrate with animal and poultry manures and then slowly building up the water volume in response to increasing phytoplankton growth. However, live-food resources are often quickly exhausted, and an artificial high-protein diet of ground egg yolk followed by small particles of macerated fish is offered. The fry remain in the nursery ponds for about 1 month or a size of3-5 cm.
Some species of clarids are not responsive and are difficult to breed once in captivity. This is true of the African clarids, hence the different background of development by international assistance. Although many farmers in Africa rely totally on the collection of fry during the rainy season, others depend on the propagation of fry and fingerlings through induced breeding.
For induced breeding of clarids the largest females (2 1 kg) are preferred to maximize the volume of ripe eggs, but smaller fish(200-500 g) can be used. Females selected for breeding are retained in broodstock ponds with males, and water conditions are manipulated to simulate those of the breeding season. The ovaries of the females are sampled periodically to determine development of the eggs. When about 90% of the eggs average 1 mm in diameter, vitellogenesis is complete, and fish are responsive to induced breeding (Richter and Van den Hurk, 1982). Each selected female is transferred to a small tank for 36 h. The tank is provided with running water at 23-25°C. The fish is not fed so that all stomach contents are evacuated.
Because of the lack of availability of readily prepared or synthesized hormones, most induced breeding is performed with homogenized pituitary glands taken from donors immediately before injection into recipients. Donors are most frequently other catfishes, but a suspension of carp hypophysis can also be used at a rate of 4 mg/kg of body weight of the female recipient. If available, human chorionic gonadotropin can be used at a rate of 4000 IU/kg, or desoxycorticosterone (DOCA) at a rate of50 mg/kg. These techniques are described by Viveen et al. (1985). Recently, Fagbenro et al. (1992) demonstrated successful ovulation and spawning of the dwarf African catfish, C. isheriensis, with acetone-dried pituitary extracts of the common toad, bull frogs, and chickens.
Response by the female to injection is fast(7-20 h), depending on water temperatures. Final hydration of the eggs and readiness for release are demonstrated by the ease with which eggs can be pressed gently out of the papilla. The eggs are then stripped into a clean container and covered with a solution of milt, prepared beforehand by excising the testes from males, as stripping milt is not possible. The same female can be induced to spawn several times throughout the year, but this is not considered to be practical in terms of egg volume and quality (Hogendoorn and Vismans, 1980).
Breeding of the brackish-water chrysichthids is more difficult, due to their greater sensitivity to environmental conditions. Ambient temperature must be high (above25.5°C) and salinity low (below 2%), and these conditions only occur naturally in the late summer months. Furthermore, maturity of the ovaries is determined by biopsy. The technique for breeding chrysichthids has been described by Hem (1986).
Fertilized eggs of clarids are transferred to incubator trays in the hatchery and treated with prophylactics to reduce incidence of disease from micro-organisms and fungi. Again, depending on water temperatures the eggs hatch in20-57 h. Emergent larvae (5-7 mm) are transferred from the incubators to hatchery troughs supplied with gently flowing, high-quality water. In 3 days the yolk-sac is almost exhausted, and the young larvae require feeding with live food organisms. They are then ready for transfer to prepared nursery ponds, as described above. 2.2 Grow-out
In Africa, pond production of clarids is constrained by the availability of high-quality feeds. The nutritional requirements of clarids have been identified Viveen al., 1985) for fingerlings and grow-out fish and are similar for all catfisha high percentage of animal protein balanced with cereals and fats. In reality use is made of any local waste resources from which a feed is compounded. A typical feed is a mixture of brewery waste, corn bran, cotton cake, sesame cake, groundnut cake, bone meal, blood meal, ground maize, rice bran, salt, and palm oil (Richter, 1976; Janssen, 1984); the proportions vary according to availability. A vitamin supplement is usually added. In areas where resources are not readily available to compound a feed, greater use is made of animal manures to increase natural productivity. Manures may be added directly to the ponds or through integrated farming with poultry and pigs housed adjacent to or above the ponds.
In well-managed experimental production in Africa, where resources are not limited, yields can be over 8000 kg/ha equivalent. However, in the small ponds of typical farms, where there are often constraints of fingerlings, feed, water, and full-time management, production is much lower. Ideally, fingerlings are stocked at a density of 100-200/m2. In monoculture yields are more on the order of 1500-5000 kg/ha, but because of the shortage of fingerlings the catfish are often raised in polyculture with tilapia at a ratio of about 3:2. In West Africa, returns on investment for a well-managed 5-are pond stocked with clarids have been calculated to be 33% in monoculture and 43% in polyculture (Huisman, 1986).
Black catfish are usually grown-out in tidal enclosures in the brackishwater reaches along the coast of West Africa. This method reduces the need to change the water mechanically and hence reduces production costs.
In Asia, catfish production is much more organized and good feed is not usually a constraint. However, fish production integrated with animal husbandry is a common and popular practice throughout most countries, and catfish are included in the mix of species. Yields, again, depend on management and water quality. Catfish in monoculture in fairly static ponds, where disease and mortality are high, may yield no more than 2000-6000 kg/ha equivalent; but in well-managed ponds with high water exchange and continual harvesting, yields may be 12 000-20 000 kg/ha.
Clarids are a sturdy group of fish and resistant to disease. However, unless conditions in the fingerling and grow-out ponds are good, mortality is high due to a variety of diseases. Common diseases include bacterial kidney disease (BKD), infections of the gills and body epidermis by bacteria (such as Aeromonas and Pseudomonas species), fungi (such as Saprolegnia), and a range of parasites (such as the protozoans, Costia and Trichodina species, and the trematodes Daclylogirus and Girodactylus species). Although many effective prophylactic treatments for these diseases and infections are known, often the only economic recourse for treatment by most farmers is a formalin dip for bacterial infections or a malachite green dip for fungal infections.
2.3. Processing and marketing
African and Asian clarids, and the chrysichthids, are high-value species and therefore of great interest to farmers. Less concern about off-flavours in the flesh is given to them than to the ictalurids in the United States, and the fish are usually harvested continually in small quantities for sale in the nearest urban markets.
The fish are sold live by traders, mainly because this is traditional, and ice has not been available. Because of their ability to breath air to survive, marketable fish are often transported considerable distances packed in large leaves in dampened sacks. In Asia, in particular, they are frequently sold directly to restauranteurs who maintain them in live-tanks until required. But in the street markets, because of the large size and therefore high individual cost of catfish, they are usually sold in pieces cut by the fish trader according to the customer's requirements. Where ice is available it is used for shipping and storage, and some fish are frozen if facilities are available. However, they are still preserved 'in the round', and not cleaned.3. PANGASIDS The principal species of pangasids for aquaculture are Pangasius pangasius and P. sutchi. They only occur in Asia, and hence they are frequently included in the general descriptor 'Asian catfish'. The largest production occurs in Thailand (about 13 000 t per annum). Cambodia, Malaysia, and Vietnam produce smaller amounts.
Production of the pangasids is relatively recent and was made possible by the achievements in induced breeding. Adults do not readily breed in captivity, and therefore hypophysation techniques, similar to those already described for clarids, are used.
Because of the control required for induced breeding, considerable use has been made of floating cages, both for the holding of broodstock and production of later stages. The sexes are kept separate in large groups and fed the usual high-protein diet of minced fish and cereal wastes. The density of stocking is about 1 fish/m3. Individuals are only removed as the breeding season arrives. Recipient males and females are injected with a homogenate made from the pituitary glands of pangasids, other catfish species, or, occasionally, carps. A priming injection is given to the females, using the homogenate of one whole pituitary. A second injection, of about one-quarter strength, follows some 12 h later, when the first injection is given to the males.
After ovulation is complete and hydration has occurred, the eggs are stripped into a container and fertilized dry with a solution of milt from the male. No attempt is made to remove the gelatinous cover on the eggs, and the fertilized egg mass is then transferred to a variety of natural or artificial collectors, such as bundles of grasses or fibers, small shrubs, and even clumps of netting. The collectors are then placed in small incubation units or hapas, which are elongated net bags fabricated from muslin cloth and set in the surface waters of a large container or small pond. The fine muslin allows the penetration of flowing water to oxygenate the incubating eggs, while keeping out small predators and ectoparasites.
Incubation time is dependent on temperature but is typically about 24 h. After the eggs hatch, the hapas are inverted and the larvae are released into the container or pond. The larvae live off their egg-yolk resources for about 48 h, by which time they are feeding off the smallest zooplanktonic organisms. They are almost immediately able to consume non-living food particles, and therefore, farmers prefer to raise them in containers rather than release them into nursery ponds, where predation is invariably high. They are provided with a continual daily diet of ground egg-yolk and planktonic organisms, followed by macerated liver and flesh tissues.
Raising fingerlings and grow-outs of pangasids generally follows that described for clarids. The main difference is that many farmers in Asia have adopted cage-culture practices, providing compounded feeds and trash fish to grow-outs held in a variety of fabricated structures able to float in any available inland water body, including rivers, lakes, reservoirs, and, in Malaysia, disused mining pools (Ang, 1990). The mining pools range from 0.3 to 10 ha and have proved to be ideal for a number of freshwater fish culture practices. Cage-farming of pangasids is also practiced in Vietnam because it is highly profitable (Singh, 1990). From a cage of 60 me, an average annual income equivalent to $7500 was obtained. However, shortage of fry continues to be a major constraint to increased development in this country.4. SILURIDS Production of European catfish is limited to Silurus glands, one of only two catfish species found on the continent. It is frequently called the Danubian wels, the wets, or the sheatfish. As its common name implies, it can be found in abundance in the Danube basin and around the Caspian Sea, but it has extended its range into western Europe and some countries of western Asia.
Fig. 4.6. Selecting broodstock of Danubian wels. Courtesy of the Fish Culture Research Institute, Hungary.
Production of the wels has been a tradition in central and eastern Europe for several centuries. It was one of a number of warm-water species reared and released by the Unions of Fishermen under ancient fisheries law in very large ponds specially constructed for the purpose. Unfortunately, with the draining of extensive areas of wetlands for agriculture in the 18th and 19th centuries, interest in fish culture was greatly reduced. Only Czechoslovakia, Hungary, and the former Yugoslavia continued some of the practices into the 20th century, and these are still evident today.
Only about 200 t of European catfish are reported to be produced in Hungary and France (FAO, 1994), but these are mostly farmed-raised stock. Many are raised and released for sports fishing, and hence the data are reported (if at all) as inland fisheries production and not aquaculture.
The wels is a voracious carnivore after its early life stages, eating fish, amphibians, and small aquatic animals. Growth is rapid during the summer season, and the fish attain 100 g by the end of the first year, and 1 kg by the end of the second year. Typical of predatory fish, the wels can live for many years, reaching a length of 2 m and weighing over 30 kg.
Where production is practiced, broodstock are over-wintered in large ponds, where they forage for food, usually small fish that are stocked or occur naturally. Just before May, when water temperatures rise to about 20°C, selected adults are transferred to breeding ponds (Fig. 4.6). These ponds are small, about 1-2 ares, and about 1 m deep. Only two or three pairs of fish are released in each pond. The adults are preferably 4-year-old fish and weigh more than 4 kg. Identification of the sexes is not easy until the early signs of ovarian development are evident in the more swollen females.
|Fig. 4.7. Spawning nest of the wels with eggs being moved to the hatchery. Courtesy of the Fish Culture Research Institute, Hungary.||
Large wooden-framed trestles are placed in the breeding ponds on which are fixed thatches of osiers and roots of trees. The gelatinized egg mass, containing between 25 000 and 125 000 eggs depending on the size of the female, is attached to the thatch and fertilized. The thatch, with eggs attached (Fig. 4.7), is then removed and placed in small hatching basins, measuring about 3 x 3 m and 0.5 m deep, supplied with running water.
Although this traditional method of breeding is reliable, some producers are inducing breeding with homogenized carp pituitaries by techniques previously described for clarids (section B). The eggs are stripped from the female after ovulation, and the solution of milt is prepared from excised testes. The fertilized eggs are processed through a hatchery, using incubation jars or troughs, after the gelatinous covering of the egg mass is dissolved by short submersion in an alkaline protease solution. The disadvantage of hatchery production is that live-food organisms have to be produced in the right quantity and quality to feed the larvae during their first few days.The eggs, which are about 3 mm in diameter, incubate in about 3 days at ambient temperatures, and the larvae carry a large yolk-sac that is absorbed over a period of 7-8 days. Larvae of catfish, like those of marine flatfish (see Chapter 17) and many other species, are sensitive to ultra-violet light and therefore the outdoor hatching basins are covered with fine black fabric.
|Fig.4.8. One-surmner wels ready for further grow-out on the farm or for restocking inland water bodies. Courtesy of the Fish Culture Research Institute, Hungary.||The fry, and subsequently
fingerlings, are then taken through a series of rearing ponds of increasing size or are
reared in a hatchery indoors. Rearing technologies for both systems have been described in
full by Horvath (1979) Each rearing pond, including the
hatching basins, is prepared with fertilizers to maintain a natural productivity rich in
live-food organisms. After 3 days, when the food organisms in the hatching basins are
exhausted, the fry are transferred to the next rearing pond, which may be 2-5 are. The density is controlled, and fry are stocked at about
10-15/m2. They remain in this pond for about 5 weeks when,
measuring about 5-7 cm, they are transferred to the next
prepared pond. This next pond may be 1-2 ha, and the
density is regulated to no more than 10/m2. By this time the fry, or young fingerlings,
are able to eat a variety of artificial feeds, including macerated trash fish, fish
starter ration (crumbs), and then small pelleted feed.
This propagation practice is common for all production. In the autumn, the young fish of one summer (Fig. 4.8), which now weigh up to 100 g, are harvested and distributed as needed. For example, for farming purposes fingerlings are transferred to other grow-out ponds or floating cages, where they are fattened on daily rations of artificial feed. Others are taken and released in small numbers into carp ponds, where they forage for small fish and aquatic animals and eventually contribute to the value of the harvest from the pond the following year. Others are transported to inland water bodies for sport fishing.
By the end of the following year the two-summer fish will weigh about 1 kg and are ready for market. The texture and taste of the flesh of the wels is highly appreciated, and individual fish of 2-4 kg are preferred for market and home consumption.5. REFERENCES Ang, K.J., 1990. Status of aquaculture in Malaysia. In: M.M. Joseph (Editor), Aquaculture in Asia. Asian Fisheries Society, Indian Branch, Mangalore, pp. 265-279.
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Dupree, H.K. and J.V. Huner (Editors), 1984. Third report to the fish farmer: The status of warmwater fish farming and progress in fish farming research, U.S. Fish and Wildlife Service, Washington, DC, 1984, 270 pp.
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Garrard, A.B., J.M. gurney, M.E. Keenum, and J.E. Waldrop. 1990. Effect of pond size on cost of producing farm-raised catfish in the Delta area of Mississippi. Information Bulletin 172, Mississippi Agricultural and Forstry Experiment Station, Mississippi State, Mississippi, 8 pp.
Giachelli, J.W., Coats, R.E. and J.E. Waldrop, 1982. Farm-raised catfish; January, 1982 cost-of-production estimates. Report No. 134, Department of Agricultural Economics, Mississippi Agricultural and Forestry Experiment Station, Mississippi State, Mississippi, 41 pp.Hem, S., 1986. Premiers resultats sur la reproduction controlee de Chrysichthys nigrodigitatus en milieu d'elevage. In: E.A. Huisman (Editor), Aquaculture Research in the Africa Region. Pudoc, Wageningen, 1986, pp. 189-205.
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Huisman, E.A., 1986. The aquacultural potential of the African catfish (Clarias gariepinus, Burchell, 1822). In: E.A. Huisman (Editor), Aquaculture Research in the Africa Region. Pudoc, Wageningen, 1986, pp. 175-188.
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Kobayagawa, M., 1989. Catfishes of the World (In Japanese). English translation by W.E. Burgess (Editor), The World of Catfishes, 1991. TFH Publications, Neptune, New Jersey, 192 pp.
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Richter, C.J.J. and van den Hurk, R., 1982. Effects of 11-desoxycorticosterone acetate and carp pituitary suspension on follicle maturation in the ovaries of the African catfish, Clarias lazera. Aquaculture, 29: 53-66.
Singh, S.B., 1990. Status of aquaculture in the Socialist Republic of Vietnam. In: M.M. Joseph (Editor), Aquaculture in Asia. Asian Fisheries Society, Indian Branch, Mangalore, pp. 371-384.
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