Three types of ponds are to be used one after another for the cultivation of carps in different stages.
(a) Nursery pond: Spawn collected from rivers or fish farms are to be set free in nursery ponds which is smaller in size and is shallow. Here the spawn become 20-22 mm long. They are now called fry. After this they are transferred to the rearing pond.
(b) Rearing ponds: This is bigger and deeper than the nursery pond. These ponds are to be supplied regularly with food and manure. Here within 3- 4months the fry becomes bigger and are known as fingerling. When the fingerlings become 100 – 250 millimetre in length they are transferred to the stocking pond.
(c) Stocking pond: These are larger in size and here the fishes grow faster to attain marketable sizes within a short time. Regularly organic and inorganic manures and supplementary foods are to be supplied. At the end of the year the average weight of each fish becomes 700 – 800 grains. In each hector there is a production of about 2,000 – 2,500 kg of fishes. Special care has to be taken for a successful culture of fishes in all stages, particularly the egg fry and fry stage. It is necessary to maintain suitable environment and to supply them adequate food.
Culture of carps in an ideal pond is easier and profitable because (i) they are herbivorous, (ii) they do not harm one another, (iii) there is less competition among them for food and space as they live and feed in different levels of water. Katal takes food from the upper level, Rohu from the middle level and Mrigal and Kalibaus from the bottom level. As a result in the same pond, these four types of fishes if cultivated together are able to use food of all level properly. So this type of fish culture is profitable.
Recently grass carps and silver carps introduced from abroad, are being cultured together in the same pond along with our indigenous major carps. The cultivation of different species of fishes at a time in the same pond is known as composite fish culture.