The most important aquaculture species of Asian catfish is Clarias batrachus (family Claridae) (fig. 18.3). Commercial culture of this species is undertaken on a large scale in Thailand. In South Asian countries, it forms a major species in the so-called ‘live fish’ culture in swampy areas. Another important Clarias species is C. macrocephalus, which is preferred by the consumer for its appearance and eating quality. However, because its growth rate under culture is comparatively slow and there is a scarcity of fry, its culture has received less attention.
The ability to adapt to fresh and brackish waters with a very low oxygen content and to grow under generally poor environmental conditions makes Clarias extremely valuable for small- and large-scale rural fish farming. They are usually cultured to market size in pond farms, but fry are sometimes grown in floating
baskets. In eastern India and Bangladesh, partly improved swamps are used for growing these species, along with another catfish, Heterop-neustes fossilis, the climbing perch Anabas tes-tudineus and the snakehead or murrel, Channa spp. High-density tank culture in recirculating water systems has been tried on an experimental scale and found to be profitable (Tarncha-lanukit et al., 1982), but has not yet been adopted for commercial production.
As mentioned earlier, another important Asian catfish Pangasius sutchi (family Pangasidae) has been cultured in ponds and cages for many years in Thailand and Kampuchea. Two other species of the genus, P. larnaudi and P.pangasius, are also cultured in ponds on alimited scale.
Clarias batrachus will readily spawn in pondsand other confined waters, if the necessary environmental conditions are available. Even though methods of pond breeding have been developed, farmers still depend to a large extent on fry collected from natural waters, irrigation canals, rice fields, etc. Fry collection is performed during May to October. The fry are found in nests on the margins of water bodies, about 50cm below the water surface. About 2000–15000 fry can be found in a nest. Small, fine-meshed hand nets are used to transfer the fry from the nests, and they are then transported to nursery ponds for rearing.
Natural spawning of C. macrocephalus is very similar to that of C. batrachus. They spawn in rice fields during the rainy season. The females make small, round hollow nests, about 30cm in diameter and 5–8cm deep, in the grassy bottom in shallow waters. The eggs are deposited in the nest and, being adhesive, stick to the surrounding grass. The male guards the nest and the female stays nearby.
Pond breeding starts with the procurement of brood fish from wild or captive stocks. They are held in small brood ponds or in holding sections of spawning ponds until they reach maturity. The brood stock is given a high protein diet every day, consisting of a mixture of 90 per cent ground trash fish and 10 per cent rice bran, at the rate of about 10 per cent of body weight.
The fish attain maturity in about one year, when they weigh around 200–400g.
Spawning ponds are usually 8000–16000m2 in area. Nests in the form of 30cm deep hollows (20–30cm in diameter) are made on the banks about 20–30cm below the water line, to resemble nests built by the species in natural waters for spawning. If the brood stock has to be held in the spawning pond, a section of about 20–30 per cent of the pond is dug deeper by about 1m.
The spawning season is generally during the rainy months and extends from March or April through to September or October. But it has been observed that the species can spawn all the year round, if the pond water is changed with fresh water from outside sources. Brood fish are stocked at the rate of about one pair per 4m2 of the holding area of the pond, prior to commencement of breeding. The sexes can be distinguished easily when the fish have grown to a size of 20cm: the anal papilla of the male is pointed, while that of the female is oval in shape. During the spawning season, the abdomen of the female is comparatively more distended.
Initially only the holding area of the pond is filled. Feeding is stopped and after one or two days the pond is filled to the maximum level with fresh water from an outside source. This stimulates the fish to spawn within a day or two, in the nests on the pond margins. If the spawning pond has no holding area, brood stock held in brood ponds can be introduced into the spawning ponds directly after filling with fresh water. The eggs are round, yellowish-brown in colour and 1.3–1.6mm in diameter. They adhere to the soil or grass and are guarded by the male, as in the case of channel catfish. Hatching takes place in the nest within 18–20 hours at a temperature of 25–32°C. The hatched fry remain in a school in the nest and are removed with small scoop nets within six to nine days after spawning. Each female produces about 2000–5000 fry. The fry are held in net enclosures for transfer to nursery ponds or for sale to nursery pond operators.
The same brood stock can spawn again after a 10-day period. In spawning ponds with holding areas, the brood fish can be made to return to the deeper areas by reducing the water level. Feeding is resumed for a 10-day period, after which the water level is raised again and feeding stopped. The fish can spawn again and the cycle can be repeated several times. Up to 13 crops of fry from a brood stock have been recorded (Kloke and Potaros, 1975).
Clarias macrocephalus do not seem to spawnin the confined waters of fish ponds and so hypophysation has to be resorted to. Intramuscular injections of common carp pituitary at the rate of 26–39mg per fish of 23–30cm length are reported to have been successful in inducing spawning in aquaria or hapas (cloth tanks) between June and July. The females can be stripped 10–12 hours after the injection, at temperatures of 29–32°C. After fertilization of the eggs with milt stripped from males, incubation is carried out in cloth tanks installed in ponds with flowing water. The eggs hatch out within 24–30 hours at the temperature range mentioned above. Yolk is fully absorbed within two days after hatching.
Nursery ponds for Clarias spp. are 400– 1000m2 in area, with depths of 0.8–1m. Before releasing the hatchlings for rearing, the ponds are fertilized with chicken manure and rice bran to produce an adequate quantity of food organisms. Initially the ponds are filled to only about 50cm depth. Stocking is generally performed at the rate of 1000–3000 fry/m2, but if it is planned to grow the fry for a longer time a lower stocking density has to be adopted. The rearing period varies from 15 to 35 days, depending on the size of fry required. Larger fry are preferred for stocking, as survival and production will then be higher. Steamed poultry eggs are considered to be the best feed for fry soon after the yolk sac is absorbed. For the first day or two, they are fed twice a day at the rate of 10 eggs per 100000 fry. After that they are fed on ground trash fish twice a day, at the rate of 1kg per 100000 fry. Every two or three days the quantity is increased by about 1kg. The ponds are filled to their normal depth of about 1m a few days after stocking. No addition or exchange of water is generally practiced. The fry are harvested with scoop nets after the ponds have been drained and the fry concentrated in a catching sump. The fry are usually grown to only about 3–5cm length, even though there is a greater demand for larger fry, because of lower survival rates during prolonged rearing.
Pangasius sutchi does not spawn in captivityand so hypophysation techniques have to be adopted for its propagation. Brood fish can be obtained from wild stocks or from culture ponds. Usually, three-year-old fish that weigh about 4–5kg are selected. The brood stock can be held in ponds or in floating cages. Brood ponds of about 1000m2surface area are stocked at the rate of 1kg fish per m2 surface. The males and female spawners are separated well ahead of the spawning season, which generally starts in June and continues through to September. The brood stock is fed with a high-protein diet (about 35 per cent protein), similar to that given to C. batrachus, consisting of ground trash fish and rice bran. An alternative feed is a mixture of fish meal (35 per cent), peanut meal (35 per cent), rice bran (25 per cent) and broken rice (5 per cent). Addition of a vitamin premix (1 per cent) or ground fish, once a week, is recommended three or four months before the spawning season. Occasional exchange of pond water is also recommended.
Floating cages used for brood stock rearing measure about 5 x 3 x 1.5m and are stocked at the rate of two fish for every 1–2m3 of the cage. The cage is installed in running water or in a water body where there is sufficient current to remove waste products from the cages. Covering the cages with aquatic weeds or similar material offers an additional protection for the fish. The feed given in cages is the same as in ponds.
Pituitaries of the same species or of C. batrachus have been used for hypophysation. Thesexes of mature fish are distinguished by the distended abdomen of the female and the easy emission of milt by the male on gentle pressure near the genital pore. Before injection, selected brood fish are removed and males and females held separately in tanks or cloth hapas. Pituitary extract is injected between the dorsal fin and the pectoral fin or at the base of the pectoral fin. The male is given one injection and the female two injections. The first does for the female is the extract of one gland of a fish of about equivalent size. The second dose, given after about a 12-hour interval, is about one and a half to three times that of the first. The dose for the male is about a quarter of the dose for the second injection of the female, and is administered at the same time that the female is given the second. Ovulation takes about 8–12 hours after the second injection, at a temperature of 28–32°C. The eggs are gently stripped and fertilized by the dry method. After a couple of minutes, the fertilized adhesive eggs are spread on special egg collectors in the form of mats of palm fibres, roots of aquatic plants or fine-meshed netting. One litre of eggs would need about 10m2 of egg collectors, which are transferred to hapas made of fine-mesh cloth and held in flowing water for hatching. Spraying of water into the hatching hapa improves the oxygen supply.
Most of the fertilized eggs hatch out in 24–26 hours at the temperature range of 28–32°C. After the eggs have hatched out, the collectors are removed. The swim-up larvae appear in 10–12 hours and the yolk sacs are absorbed in about two days after hatching. It is important to provide adequate quantities of live food like Moina or other zooplankton, as otherwise theybecome cannibalistic. From the third day after hatching, the larvae can be fed on ground boiled egg yolk and waterfleas or other zoo-plankton, in small quantities, several times a day. Five-day old larvae will eat ground liver or cooked feeds. They can then be removed to rearing ponds. Methods of fry rearing are very similar to those for C. batrachus.
For grow-out of C. batrachus, small ponds of about 200–1000m2 area are used in Thailand (fig. 18.4). Normally the ponds are not ferti-lized, but between crops the ponds are dried and occasionally treated with light doses of lime. They are filled to a depth of 50–80cm. Stocking is generally performed in March or April and the duration of culture is usually three to four months. After the first harvest in July or August, the ponds are stocked again between July and September for a second crop. In order to avoid the period when wild-caught catfish are available in the market, the second harvesting may be delayed until the following February or March.
To offset high mortality in commercial ponds, most farmers stock about 200 fingerlings/m2, although the recommended stocking rate is lessthan half of that. The main feed is ground trash fish with rice bran (in a ratio of 10:1 by weight), which forms a sticky paste. By the fourth month the proportion of rice bran is generally increased to two parts, sometimes with one part of cooked broken rice added. The feeding rates are by no means standardized, and each farmer depends on his own experience. Soon after stocking, 2kg feed is given every day in two equal quantities for every 10000 fingerlings. Every two weeks the quantity is doubled, through gradual increases. Many of the problems of poor water quality, diseases and mortality in catfish ponds are due to the heavy rates of feeding. Water in many ponds gives the appearance of a pea soup, with quantities of uneaten feed and blooms of algae. Feeding with more than 0.5kg feed per m2 has been found to result in water pollution and fish mortality, in spite of the hardy nature of catfish and their ability to breathe atmospheric air. Frequent water exchanges, when possible, are an efficient means of pond sanitation and maintenance of water quality standards. The average yield of Clarias in Thailand is 29–32.6 tons per year in a1600m2 pond.A production of up to 100 tons/ha per four-month period has been reported.
The first harvesting of Clarias is done within three to four months, around June/July, when the fish have grown to 25–30cm, weighing 200–300g. The second harvest between February and May, after five to six months’ rearing, produces fish of 35–40cm length, weighing 400–450g. The fish are transported alive to markets in metal boxes (180l capacity) with very little water.
Pangasius ponds are usually somewhat largerthan Clarias ponds, but the methods of grow-out are generally similar. Pangasius is stocked at the rate of two fingerlings of 10–15cm size per m2 pond surface. The duration of culture is 12–15 months.
Cage culture of Pangasius is common in Kampuchea,Thailand and, in more recent years, in Vietnam (Pantulu, 1979). In Kampuchea, cages are made of bamboo poles and splints.
They are box-shaped when installed separately but when trailed behind a fisherman’s boat, as is often done, they are arranged to fit the shape of the boat.
The sizes vary considerably, but the larger cages are 40–50m long, 4–5m wide and 2.5–3.0m high. Small cages are 4m x 4m with a depth of 2.5m. A number of small cages may be lashed together to form a floating cage farm, buoyed with airtight metal drums and walk-ways for feeding, harvesting, etc. In Vietnam, box-shaped cages are made of wooden planks with mesh-wire panels on the sides through which a free flow of water is possible. There is a floating cabin on the cage farm for the owner or caretaker to live in. The whole installation is moored in the river near the shore, or secured directly to the shore.
The cages are usually stocked with wild fry or fingerlings. In Kampuchea, a large cage of the dimensions mentioned above may be stocked with 6000–10000 fry during the period June to August. They are fed with cooked vegetables like pumpkin, banana and a combination of cooked rice bran. As the fish grow in size, they are fed on live and dead weed-fish and kitchen refuse. Harvesting is carried out during the months February to May, when the fish have grown to 1.5–2.5kg each.
In Vietnam, cages are stocked with fry 38– 63mm in length from August to October. Sometimes advanced fingerlings, obtained from rice fields or rivers, are stocked. The stocking density of fry in cages is about 93 fry/m3.
Pangasius are fed on vegetable matter, suchas chopped leaves, rice bran and forage fish. This may be supplemented with cooked or uncooked meat of mussels, snails, etc. Harvesting is usually carried out in March to August, after a culture period of over 10 months on average. The production ranges from 3000– 25000kg/year per cage of capacity 1600m3.
As mentioned earlier, the environmental conditions in high-density culture ponds of Clarias are conducive to a heavy incidence of disease and mortality, which often decimates almost half the fish stocked. The three most common diseases of cultured catfish are Trichodinainfection of the gills, bacterial infection of the kidney andGyrodactylus infection. Infections by Aeromonas spp., Flavobacterium spp., Flexibacter columnaris, Pseudamonas spp. And Edwardsiella tarda, have been identified in C. batrachus and C. macrocephalus. Most of theseinfections are believed to be brought in with the fry or fingerlings. Treatment with 25–50ppm formalin in the pond or a one-hour bath of 250ppm formalin in tanks is recommended, before the fry or fingerlings are stocked.
Accumulation of H2S is another cause of mortality in Clarias ponds (Colman et al., 1982). Dissolved oxygen levels do not appear to be so critical for the survival and growth of the species.
Studies of the economics of Clarias farming in Thailand have shown that, in spite of risks involved, most farms make substantial profits (Kloke and Potaros, 1975). For a single crop, the net income to gross returns averaged 37.7 per cent, the net income to the total cost ratio averaged 71.4 per cent and on an annual basis the return of the total capital averaged 108.1 per cent (Kloke and Potaros, 1975).