Although catfish are widely distributed in Asia, culture of Clarias spp., (walking catfish) particularly of C. batrachus and C. macrocephalus, is especially well established in Thailand. The two species look very much alike but they can be easily distinguished from each other by the shape of their occipital process, which is pointed in the case of C. batrachus and rounded in C. macrocephalus. They feed naturally on worms, insects, shrimps and organic detritus, but they also accept artificial feed easily.
Seed production
The spawning season starts with the onset of the monsoon, the peak period of which is May-September. The rains facilitate migration to the rice fields where these fish normally construct nests in the grassy bed, in shallow water 20–30 cm deep. Females build the nest in the form of round holes measuring 20 cm in diameter and 25 cm deep in the case of C. batrachus, and 30 cm diameter and 5–8 cm deep for C. macrocephalus. Males take care of the eggs after spawning.
Clay soils with a pH of 6–8 are desirable for spawning ponds, which are normally constructed near the irrigation canals so as to obtain a ready supply of water during breeding. Ponds are usually rectangular in shape and vary in size from 1 to 3 hectares. Along their margin a broodstock holding channel, 2–3 m wide and 1–1.25 m deep, is dug out. In the middle of the pond raised platforms about 2 m wide are constructed in rows with 50 cm wide channels in between them. Grasses are grown on these platforms and holes are made, measuring 20–25 cm in diameter and 15–25 cm in depth. Such holes may number about 100 to 120 in about 0.16 hectares.
The broodstock (25–30 cm long; 150–200 g each) are stocked in November/ December in the holding channel at a stocking rate of 625 kg/ha. They are fed daily rice bran and trash fish in the ratio of 1:9 at the rate of 5–6% of body weight.
During the spawning season, when breeding starts, feeding is stopped and water is pumped in to inundate the holes. Normally 25–35 cm of water is maintained above these holes. The broodfish are stimulated by the arrival of fresh water and move to the holes to spawn. Spawning takes place within 1 to 2 days.
After hatching, the larvae remain huddled together in the holes for 9–10 days, after which they come out of the holes and start wandering. Therefore, they are caught using a hand scoop net while they are still in the holes. Then, the water level of the pond is lowered to bring the broodstock back into the holding channel. Feed is given for another 12–15 days before the operation starts again. This is done on average about once a month. In October, the broodstock is sold out and a new batch is introduced into the holding tank.
Spawning of C. macrocephalus is generally not particularly successful under the above conditions, and therefore seed production is undertaken with hypophysation. The broodstock (about 250 g each) are injected 13–40 mg of pituitary extract at a water temperature of 25°–32°C. The injections are given in two doses at an interval of six hours. Eight to ten hours after the second injection, the eggs are stripped and fertilized with sperm obtained from the macerated testis of freshly killed males. The eggs hatched within 20 hours at a water temperature of 26°–33°C.
Nursery ponds are 800–3 000 m2 in size, with a 50 cm water depth. A channel 2 m wide and 50–60 cm deep is constructed in the middle of the pond to facilitate collection of fry during harvesting. Prior to stocking, chicken manure is applied to the pond at about 30 kg/100 m2. Stocking rate is 250–350 fry/m2. Fish are fed with ground trash fish and rice bran in the ratio of 9:1 at the rate of 1 kg of food/ 1 000 fry, twice a day. The fry reach 2–3 cm in 15 days. Survival rate is usually 30–50%.
The C. macrocephalus fry can also be reared in cement tanks on Moina, which is regarded as the best diet for catfish larvae. A recently developed artificial diet consists of fish meal (56%), peanut meal (12%), rice bran (12%), alfa starch (14%), fish oil (4%), vitamins/minerals (1.6%) and a binder (0.4%). This feed is given daily at the rate of 2–3% of the body weight.
On-growing walking catfish
Clarias can be cultured in stagnant or running water ponds, either in monoculture as in Thailand, or in mixed culture with other species as in India. Because of their air-breathing capabilities they are generally reared at extremely high densities. The development of Clarias farming in Thailand is considered one of the major achievements of aquaculture in the region.
Catfish farms are normally constructed in areas with heavy clay soils near a perennial source of water. The size of farms ranges from 0.1 to 1 ha with pond size varying from 400 m2 to 2 000 m2. Each pond has its separate inlet and outlet channel. Preparation of ponds involves drying and occasional application of lime at 600–700 kg/ha. Two to three days before stocking, the on-growing ponds are filled with water to a depth of about 40–60 cm. Stocking rate is 100–300 fingerlings of 3–5 cm long/m2. Ponds are stocked during February-April and harvested during May-August. For the first two weeks after stocking the fish are fed with ground trash fish and a little rice bran. Subsequently a mix of trash fish, rice bran and boiled broken rice is made into a sticky paste. The proportion of rice bran in this feed mix is gradually increased as harvest time approaches, since rice bran helps in fattening the fish. Vitamin and mineral premix, and at times antibiotics, are sometimes added to this feed mix. Feeding rate varies from 120 to 400 g/m2/day during the culture period. Water is changed frequently. The fish are harvested after 4–5 months when they have grown to 100–350 g. On average, the ponds yield 3–10 kg/m2.
During the culture period, Clarias have been found to suffer from a number of diseases. The most important protozoan pathogens are various species of Trichodina and Trichodinella. These parasites are found on the skin and gills of all sizes of Clarias, but they pose a severe problem mainly to the fry and fingerlings. Normally formalin is applied in the pond at 25 ppm, which appears to be effective in controlling these parasites. Ponds with heavy silt and stagnant water have myxosporidian parasites which encyst in the gills and muscles of the catfish. No medication has yet been very successful against them.
Bacterial septicaemia caused by Aeromonas hydrophila also commonly affects Clarias. Affected fish show swelling at the base of the pectoral fin and a distended abdomen with the peritoneal cavity filled with fluid. Haemorrhagic areas also occur at the base of the fins and on the body. No medication has yet been found successful. Other diseases are of viral and fungal origin. Nutritional diseases may also occur.
Other parasites which usually affect the fish are Gyrodactylus and Dactylogyrus spp. They are found on gills, body and fins. They can be controlled by Dipterex (0.25 ppm) or formalin (50 ppm).
This species is commonly known as the river or silver striped catfish. It occurs in Thailand, Lao, Democratic Kampuchea and Viet Nam where it is extensively cultured in ponds, cages and pens. It is an omnivorous fish with a preference for a meaty diet. It spawns naturally in rivers between June and September and at times continues breeding up to November. Eggs are adhesive in nature. P. sutchi is quite a fast growing fish and can attain 3–4 kg in two years.
Induced spawning
In Thailand, river catfish broodstock are maintained in ponds about 400 m2 in size stocked with 100 broodfish. Feeding consists of boiled broken rice, rice bran, fish meal and aquatic weed in the ratio of 2:3:1:4 respectively. In some places, 1% of vitamin E is added to this feed. Feeding is done daily at the rate of 1–5% of the body weight. To facilitate proper growth and egg development, the pond water is occasionally changed. Induced breeding is usually undertaken with fish of over 2 years of age. Males are easily distinguished by their orange hue on pectoral and ventral fins; oozing of milt takes place when gently pressing the abdomen: Females show distended abdomens and pinkish vents. On examination by catheter the females in full ripeness show yellow, loose eggs of uniform shape and size.
An extract of fresh or preserved pituitary gland is normally injected to females, the pituitaries from 4 donors of the same weight as the recipient being regarded as the full dose. This is divided into one part as the first injection and 3 parts as the second. Males are normally not injected. Ovulation occurs between 8 and 12 hours after the second injection. Female fish of 2 kg weight lay 200 000–300 000 eggs. About 1 ml of milt is sufficient to fertilize about 1 million eggs. Eggs hatch within 24–26 hours at water temperatures of 28°–30°C. On absorption of the yolk on the second day, the larvae are given hard boiled yolk of chicken eggs until the fourth day, when they are transferred to nursery ponds.
The stocking rate in nursery ponds varies from 100 to 500 fry/m2. They are fed four times a day with fish meal and rice bran in the ratio of 1:1. In some places they are instead reared in prepared nursery ponds enriched with a Moina bloom. Fry grow to about 8–10 cm within 6 weeks, when they are harvested.
On-growing of silver catfish
Culture of Pangasius sutchi in ponds and floating cages has been practised for a long time. However, development of seed production under controlled conditions has recently helped spread its culture. The species is also reared in integrated farming systems.
On-growing of river catfish is undertaken in ponds varying in size from 1 600 to 2 400 m2, with water depth of about 1.5 m. The ponds are stocked in November and harvested in September the next year. However, the grow-out period may be even shorter than this, depending on the growth of fish, stocking rate and market price. The stocking rate varies from 3 to 5 fingerlings/m2. Prestocking management involves eradication of predators, but no fertilizer is applied to the pond. Fish meal or trash fish are normally not given as supplementary feeds because of prohibitive cost. Instead, kitchen refuse is mainly used. The pond water is replenished daily. The fish grow to about 2 kg in 8–10 months. Disease problems are not as common as with Clarias.
On-growing of river catfish is also undertaken in wooden cages of 15–20 m3 each. Cages are suspended from bamboo poles in the water of rivers. Sometimes, the cages are sited below the floating homes of the farmers. Stocking is done with 50 fingerlings of 8–10 cm/m3. The survival rate is quite high, up to 80–90%. The fish grow to about 1 kg in 12–15 months, the average production rate reaching about 40 kg/m3 of cage. The fish are fed with a wet mix of cow dung and kitchen waste or rice bran in the ratio of 3:1. Management involves monitoring of fish health and removal of fouling organisms.
River catfish is also cultured using pig manure. In chicken-cum-fish culture or duck-cum-fish culture, P. sutchi is usually associated with Clarias.
There are several species of snakehead belonging to the genus Channa (syn. ophiocephalus) but only one species (Channa striatus) is cultivated.
Seed procurement and production
Seed is available throughout the year, but the peak season for their availability is during May-September/October. The fry are collected from the natural spawning grounds by simple netting. Seed production through artificial spawning is also undertaken. Broodstock are provided with a muddy bottom and water of about 30–100 cm deep. Aquatic weed, mostly water hyacinth, is introduced and the fish start spawning after 1–2 months.
Snakehead farms normally have a continuous water supply. Some farms pump water for 24 hours a day, others up to 8–10 hours. Sometimes water is pumped directly into the ponds; in others water is first pumped to a ditch and sub-sequently flows by gravity. Generally pond size ranges from 800 to 1 600 m2. The depth of the pond is usually between 1.5 and 2 m.
Prestocking management between two successive crops involves draining and repair of the pond, cleaning of the bottom mud and lining the bottom with sand. The pond is left to dry for fifteen days. Then it is filled up; water is left in it for three days and then drained. Fresh water is then introduced and 1–2 cm fry about 10 days old are stocked in July/August. Stocking rate is highly variable, ranging from 75 to 460 g/m2. Fry are fed with finely chopped trash fish. Unconsumed floating feed is recovered after several hours. Feeding rate is regularly adjusted according to left-over feed. Intially, the fry are fed three times a day but this is gradually reduced to once a day.
Later, a mix of trash fish and rice bran is preferred although broken rice, vegetable and kitchen waste may also be fed. The ratio of trash fish to rice bran ranges from 8:1 to 13:1, depending on the availability of trash fish. These ingredients are well mixed and made into strands with an extruder. The fresh strands are put on a wooden platform or directly thrown into the pond. The feed sometimes contains also vitamins and minerals. However, no artificial pelleted diet is yet available. The rearing period extends for 7–10 months depending on stocking rate, fish growth and feeding. Usually cropped fish weigh 700–1 000 g at harvest. Larger size fish are very expensive and thus are not in high demand. The fish are harvested about April/May using seine netting over 4–5 days. Production ranges from 9 to 16 kg/m2.
Fish is preferably sold live, since the price is reduced by 30–40% when dead. Not much is known about disease, but heavy mortalities occur during stocking. Some farmers incorporate antibiotics in the feed to prevent disease.
Thus snakehead is reared using methods similar to those for Clarias. While monoculture is common in Thailand, culture with other fish such as Anabas sp. is practised in India. In an experiment in Thailand, snakehead and tilapia were cultured together. The former gave a recovery of 53.7% while the latter propagated to more than 22 times the original number stocked within a rearing period of 290 days. The total production of harvestable fish was much better than in monoculture of tilapia. However, such an experiment in India did not give encouraging results. It appears that tilapia, being a fish with compressed body and spines, is not easily preyed upon by snakehead.
In an experiment in India, the natural production of snakehead from a control swamp varied from 129 to 146 kg/ha/year. In managed swamps where manuring and liming were done snakehead production reached from 713 to 895 kg/ha/year.
