Method of fish culture: Fish culture generally means the rearing and takingcare of fishes in internal fresh water bodies. An ideal pond is required to culture fishes successfully. The area of the ponds maybe big or small with a depth of about 3 meters. The culture of major carps is easier and more profitable in an ideal pond. All carps are generally herbivorous.
1. The surroundings of the pond must be open, neat and clean and airy.
2. There should be a large quantity of planktonic food in the ideal pond.
3. The banks of the ponds must be high enough so that no outside water enter into the pond during rainy season.
4. There should be adequate water in the pond and also arrangements change water when it is necessary.
5. The weeds and predatory fishes must be removed from the pond.
6. To keep the water cool, there should be aquatic plants like kalmi, helencha etc. in the summer season.
7. Cleaning clothes and taking baths with detergents must not be allowed in an ideal pond.
It is divided into several specific steps, they are as follows:
A. Preparation of the pond.
B. Collection of fish fry and transportation
C. Releasing the fry in the pond and rearing.
D. Taking care of the rearing fishes; and
E. To apply manure and provide food.
It is prepared by few steps, such as:
(1) According to necessity a new pond can be excavatcd or one can be reclaimed to make it an ideal pond.
(2) Germs of the pond have to be kill ed by using lime and the pond can be made fertile with organic fertilizer. By means of this the production of phytoplankton will increase.
(3) When the pond is ready the desired kind of fries are to be released.
In the life cycle of carps there are 5 stages (i) Eggs (ii) Spawn or egg fry (iii) Fry (iv) Fingerling and (v) adult fish
Three types of ponds are to be used oneafter another for the cultivation of carps in different stages.
(a) Nursery pond: Spawn collected from rivers or fish farms are to be set freein 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. Theseponds 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 toattain 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 grams. 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. The exotic carps, introduced from abroad grow quickly and are quite tasteful to eat, so cultivation of this is profitable but our Bangladeshi carps are liked more.
There may be various types ofdiseases in fishes. The body of a healthy fish is slimy and its colour is bright. If the original colour is changed or discoloured it is to be understood that there is some sort of illness or trouble in the fish. The health of the fish is to be examined once or twice a month by catching them with driftnet, because by checking the health of fish, treatment might be given if required. The fishes may also become ill due to scarcity of food, insufficient or unsuitable food lack of oxygen, fluctuation of temperature, attack of parasites etc. Besides this they may be ill due to attack of different species of protozoa, round worms leeches and lice. Some important diseases of fishes are as follows.
I. Gill rot disease: This disease is caused by the attack of a kind of fungi. 1nthis disease the blood circulation is disturbed in the gills which ultimately rots As a result the respiratory process is affected and the fish dies. This diseae, may become epidemic when the pond is full of rotten or half rotted organic matters and have an excessive heat.
Treatment: The pond has to be treated adequately with lime and the fisheshave to be supplied with artificial food.
(II) Fin or tail rot disease: This disease is caused by the attack of a bacteria.Due to this disease white spots appear and the fin becomes blunt
Treatment: 2000 part water and one part of copper sulphate are to be mixed.and then if the fish is kept in it for one minute, then it might get cured.
(III) White speckle disease: Due to attack of a kind of protozoa minutespeckles appear on the body of fishes.
Treatment: For its treatment the pond has to be purified with salt or powderedrock lime. The fish becomes free from this germ if it is kept submerged for half an hour in 100 parts water with 3 parts salt in it.
(IV) Fish lice: The name of the fish louse is Argulas. This louse is oval inshape and slightly flat. This louse remains attached with the skin below the scale with the help of a pair of suckers on the ventral side. As a result th scales become detached and the body becomes uncovered. This affects the health of the fish.
treatments: (i) The affected fishes are to be transferred from one pond to other(ii) water of the pond is to be removed and the pond has to be dried for least 24 hours in the sun, (iii) The pond water has to be sprayed with 10-30 grams of lime per litre.
(V) Leech of fish: In the body of leech of fish, there are two suckers. With onesucker leech remains clung with the fish and with the other it sucks the blood of the fish.
Treatment: The leech dies if the affected fish is kept in water having 100 partswater and 25 parts of salt for 15 minutes.
Forthe proper growth of the fish, adequate balanced food supply is required. For this, appropriate food that helps in rapid growth of the fish and which food is liked by the fish should be known. Plankton is the chief food for fishes. Both the phytoplankton and zooplankton increase in number by applying manure in the pond. The fishes do not eat manure but the manure assists in growing minute aquatic plants in the bottom of the pond and in this way the food chain of fishes remain active. Fishes grow by eating these aquatic plants or small organisms or even both. Generally manures like nitrogen, phosphorus, calcium, potassium etc. are necessary for the growth of small aquatic plants. If the fish dies, the dead body, being decomposed by bacteria, is again transformed into organic elements. Later from the organic salts phytoplankton or other small plants grow. In this way the food cycle continues in a pond.
Place of collecting fries: The carps lay eggs in the flowing rivers with watercurrents. From there eggs or egg fry are collected. Later these are set free in the pond. In the Halda, the Padma and the Jamuna and other rivers eggs and egg fry are available. Within the covering of the egg the embryos remain available. Theseeggs and embryos are collected in the month of Baishakh and Jaistha. In the Gorai river of Kushtia and the Brammaputra river of Mymensingh the fries are available. The fries are 80 - 230 mm .. long and are available by the end of Baishakh upto Sraban in the river.
Fish farm: Many farms have been established by the Government and Non-Govt. organization in different places of Bangladesh for the production of fish seeds. Among these, there are 96 govt. fish seeds production farms. For the supply of fry, seven Government and 50 non-Government modem nursery ponds or hatcheries have been established. The eggs and fries are collected from these centres, where these are produced by artificial spawning of local and exotic fishes. There is fishery research station for fresh wate fishes at Chadpur. In the Agricultural University of Mymensingh establishments like the artificial breeding centre for local and exotic Shinghi and Magur fishes and in Fishery research institute artificial breeding centre for Magur, Rea Telapia, Pabda, Gulsa etc. have been established.
The amount of fishes that we get in the country is not properly and scientifically preserved, processed and ready for marketing. As a result often lots of fishes become unfit for human consumption. In order to save this loss, transport system, refrigeration, marketing facilities and fish preservation method should be improved. These steps will enable to meet most of the demands of fishes in our country.