Selection and segregation of brood stock
Proper selection of brood fish is very important in obtaining best results in breeding as well as in later grow-out. Many farmers select the largest, fast-growing fish, or the ones with the desired body shape, on the assumption that these characteristics will be inherited by the progeny. It is not advisable to select from the same brood stock or their offspring, as this may lead to inbreeding, depression of growth rate and a predominance of deformed fry. As far as possible, brood stock should be selected from divergent sources.
Generally, two-year-old fish weighing about 2–3kg are used for breeding, and they can be bred every year for several years. Larger fish may be more difficult to handle, but large females spawn more and larger eggs, and the hatchlings are also larger and survive better. About 100000–150000 eggs are produced per kg body weight.
At least three to four months before the breeding season, the brood stock are removed and stocked in segregation ponds. Males and females can be distinguished by external features during the spawning season. The female has a swollen abdomen due to the developing ovaries and in the males the milt runs freely when the abdomen is gently pressed. Chinese farmers identify older males from the tubercles on the sides of the head and on the pectoral and ventral fins. It is desirable to segregate the males and females into separate ponds, to avoid unwanted spawning. In segregation ponds, the stock is maintained under uncrowded conditions and fed on protein-rich natural food and supplemental feeds to assist faster gonadal development. High carbohydrate feeds have to be avoided, in order to prevent the accumulation of fat. During the period of segregation, it is necessary to prevent stress through netting or stimulation of spawning through the flow of fresh water into the pond. The pond is cleared of any substrates for eggs, such as weeds and grass, to avoid stimulating wild spawning.