Indian aquaculture has been growing at a fast pace over the last two decades, with freshwater aquaculture contributing over 95% of the production. The three Indian major carps, namely catla (Catla catla), rohu (Labeo rohita) and mrigal (Cirrhinusmrigala) contribute to the bulk of production amounting to about two million tonnes annually (FAO, 2003). Silver carp, grass carp and common carp formthe second important group for fish production. Average national production from pondfisheries has increased from 0.6 tonnes/ha/year in 1974 to 2.2 tonnes/ha/year in 2002 (Tripathi, 2003).
COMPOSITE FISH CULTURE
The three major carps cultured in India, namely, catla(Catla catla),rohu(Labeo rohita) and mrigal(Cirrhinus mrigala), contribute as much as 87 percent of the total Indian aquaculture production. Three exotic carps were also introduced, namely, silver carp (Hypophthalmichthys molitrix); grass carp (Ctenopharyngodon idellus) and common carp (Cyprinus carpio). There are also several other medium and minor carp species, namely,Labeo calbasu, L. fimbriatus, L. gonius, L. bata, L. ariza,Cirrhinus mrigala, Puntius sarana, Hypselobarbus pulchellus, H. kolus and Amblypharyngodon mola, which are important in aquaculture. Among catfishes, magur(Clariasbatrachus) is the only species that is widely cultured, while the catfish, ‘Singhi’ (Heteropneustes fossilis) is cultured to some extent in the eastern states.
Attempts have also been made to culture the other catfishes like Pangasius pangasius, Wallago attu, Sperata seenghala, S. aor and Ompokpabda. The finfish species of importance include climbing perch (Anabastestudineus), murrels (Channa striata and C. marulius) and tilapia (Oreochromismossambicus and Oreochromis niloticus).
PREPARATION OF PONDS
Pond preparation involves making the ponds weed and predator-free and generating adequate natural food for the survival and growth of fishes. Control of aquatic weeds, removal of undesirable flora and fauna and improvement of soil and water quality are important aspects of fish management.Weeds have to be removed from the ponds first, after which the tank is fertilized with both organic and inorganic fertilizers, such as Oil Cake and raw Cow Dung @ 5,000 kg/acre.
The PH of pond water should be 7.5 – 8.00, for which lime is added in the tanks @ 200 kg/acre per annum. The lime increases pH and also helps in eradicating fish parasites.The organic fertilizer in the form of raw cow dung is added in the tank @ 500 kg/acre per annum. This is followed by the application of inorganic fertilizers like Super Phosphate @ 120 kg/acre and Ammonium Sulphate @ 200 kg/acre, in spaced intervals.
Because of constant temperature and favourable weather conditions, carps spawn all the year round in India. Spawning takes place early in the morning when the water surface cools down to about18 degrees. The female carp swims near the water surface followed by the male carp in nuptial swimming and rubbing each other’s bodies. Female lays egg and male releases its milt and eggs are fertilised.
Three days after fertilization, the eggs begin to hatch. The newly hatched larva is about 5.5 mm long, delicate and transparent, with a yolk sac attached to the belly. It rarely swims but settles on the bottom or on some floating object. On the second day, the larva starts swimming and on the third day swims actively from surface to bottom. During these stages, the larva or fry gets its nourishment from the yolk sac, which disappears on the third day and the fry now must search for food and eat. Supplementary fry-feed in the form of hard-boiled egg yolk or powdered milk can be applied on the water surface at this time.
Carpscan feed on almost anything like insects, shells and worms and can also eat aquatic plants, bread crumbs, rice bran and fish meal made from corn, copra and soybean.
Both female and male brood fish should be carefully tended for 2-3 months before induced spawning operations are carried out and males and females should be segregated and kept in separate ponds.
To be good brooders the fish must be more than one year old and 150 gm in weight. Sex can be determined by the shape of the genital papilla which is pointed in male and oval in female. When the female is ready for induced spawning operations, It should have a bulging abdomen that is soft to touch. The cloaca is reddish and prominent, and the contour of the enlarged ovaries can be seen on both sides of the abdomen. The head should be small and the snout pointed.
Nursery ponds are constructed to rear carp fry or larvae. A normal sized nursery pond measures 5 x 10 m, with a depth of 0.5 m. Before filling up water the pond should be cleaned thoroughly to get rid of predators and parasites that may be destructive the larvae.About 1,500 to 3,000 fries can be stocked in the nursery pond andfed with milk, wheat flour or boiled egg yolk by spreading it on the water surface. This feed can be supplemented with rice bran, bread crumbs or fish meal, which can be given twice a day, in the morning and in the afternoon.
Rearing ponds, where adult carpsare cultured until they reach marketable size, are needed, which have dimensions of 15 x 50 m and depth of 1.5 to 2 m. Rearing ponds should also be thoroughly cleaned before filling them with water. This is done by exposing the bottom and letting it dry thoroughly.Next step involves application of fertilizers, which encourages growth of aquatic plants, moss and algae, which are important natural food and also lead to growth of micro fauna. Manure in the form of chicken dropping is the most commonly used being cheaper and more readily available in large quantities. When carp fry reaches the length of about 5 to 7 cm, they are transferred from the nursery pond to the rearing pond and allowed to grow to adult stage.
STOCKING OF PONDS
Ponds are stocked with fish fries of appropriate size. Fingerlings of over 10 cm in size are recommended for stocking in culture ponds. Stocking of smaller fishes may result in higher mortalities and slow growth during the initial months. In fish polyculture a fingerling size of 50-100 g is preferred for stocking to ensure higher survival and better growth. Generally, a density of 5,000 fingerlings is kept as a standard stocking rate per ha for carp polyculture, which will give a yield of 3-5 ton/ha/yr.
Prior to stocking, the fish fries should be dippedin 3-5% potassium permanganate solution for 15 seconds to kill parasites. In composite fish farming, a combination of six species are cultured, namely, Catla, Rohu, Mrigal and exotic Carps like Silver Carps, bass and common Carp.Supplementary feeds like Groundnut Oil cake and Rice Bran are fed to fishes during culture. At the end of the culture period of say 12 months, the fish will reach marketable size and fetch attractive prices.
POST-STOCKING POND MANAGEMENT
While fertilizing the carp ponds, 20-25% of the total amount of organic manures is applied a fortnight before stocking and the remaining amount is applied in equal instalments on a bimonthly basis. Other commonly used organic manures include poultry manure, pig dung, duck droppings, cow dung, domestic sewage, etc. Azolla, a nitrogen-fixing fern is used as a bio-fertilizer for aquaculture at the rate of 40 tonnes/ha/yr, which supplements nutrients required for intensive carp culture. The bio-processed organic manure, biogas slurry has also been used as manure in carp culture.
The supplementary feed in carp polyculture is usually restricted to a mixture of groundnut/mustard oil-cake and rice bran. Grass carps are fed with aquatic vegetation such asHydrilla, Najas, Ceratophylum, duck weeds, etc. which can be kept in special enclosures in corners of the pond. Feeding preferably twice-a-day is advocated @ 5% of the initial biomass of stocking material for first month and then gradually reducing it.
Aeration may be done mechanically to increase the concentration of dissolved oxygen in ponds, by paddle wheel aerators, aspirator aerators and submersible pond aerators. It is also necessary to replace certain amount of water at regular intervals.
Harvesting of fishes is usually done after a culture period of 10 months to one year. However, fishes attaining the marketable size can be harvested periodically depending on several factors, which also reduces the pressure of density in the ponds and thereby providing sufficient space for the growth of fishes.
INDUCED REPRODUCTION IN FISH
Inducedfish farming has allowed farmers to breed and raise species that do not naturally reproduce in captivity, to manipulate the timing of reproduction to suit production cycles, getting fish to spawn on a predetermined date and fertilise and incubate eggs under hatchery conditions. There are two main strategies used to induce reproduction. The first is to provide an environment similar to that in which spawning occurs in nature. Catfish, for example, likes to spawn in enclosed spaces and goldfish in vegetation and at high temperatures.
The second strategy is to inject the fish with one or more naturally occurring reproductive hormones or their synthetic analogs to manipulate maturation of gonads and ovulation. The hormones injected include, Gonadotropin Releasing Hormone(GnRH)analogs, dopamine antagonists and gonadotropins.Leutinizing Hormone Releasing Hormone (LHRH) is a mammalian hormone that has been employed successfully to induce reproduction in fishes. Dopamine inhibits the action of LHRH and hence a dopamine antagonists are given for induced breeding.
Two types of gonadotropin extracts have been used to induce ovulation in fishes, namely, Human Chorionic Gonadotropin (HCG) and fish pituitary extract. Pituitary extracts are made by removing the pituitary from a fish and extracting the hormones, which may then be injected into another fish. HCG offers three major advantages over the pituitary extract, namely,1) it is much less expensive, 2) it is more stable and 3) it comes in a purified form. An intraperitonial injection is given through the ventral part of the fish behind the pelvic or pectoral fin. Intramuscular injections are commonly given on the dorsal part of the fish above the lateral line and below the dorsal fin.Two doses with a time gap of 12 to 24 hours are given.
OBTAINING PITUITARY EXTRACT (HYPOPHYSATION)
Pituitary gland contains gonadotropin hormone, which stimulates the production of sex steroids in the gonads and induce maturation of gametes. Gonadotropin is composed of follicle stimulating hormone (FSH) and luteinizing hormone (LH), which are responsible for the egg development and egg ovulation.The skullof the fish from which,hypophysis is to be collected, is cut open with a knife to remove the bone and meninges. The pituitary gland can be seen after the mid-brain has been folded back by using forceps.The gland is taken out and ground in the homogenizer and then distilled water is added and the gland is again ground. The solution is taken in a syringe for injection. The female can be injected with two doses with the time interval of 6 hours. The intramuscular injection is given in the area between the base of the dorsal fin and lateral line.
OVULATION & FERTILISATION
Stripping the fish is done by holding the female around the caudal fin with one hand, while applying slight pressure to the abdomen with the other hand. A stream of eggs will eject through the genital opening. The abdomen should be messaged from front to back to strip out all the eggs.Ovulation occurs about ten hours after the second injection of hormone. The eggs are collected in a dry plastic container. At the same time, the milt from a male fish is made to drip on the eggs by pressing the testes with fingers and pouring the water through the fine mesh cloth. Eggs and sperm are mixed and stirred gently. After about two minutes, water is added two or three times to cleanse the fertilized eggs and then they are transferred to the hatching happas, where most of the fertilized eggs hatch out within 24 hours. The
The yolk sac is absorbed in the body of fry in about 2 days, and then the larvae are transferred from happas to the nursery tanks, where the fries feed and grow. Food has to be given during the first 3 weeks and when the fries reach the size of 2–3 cm, they are distributed in the rearing tanks where they are cultured further to adult marketable stage.