Spawning and fry production of Clarias spp.
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.
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