Propagation of Pangasius
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 1000m2 surface 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.