The successful development of the technique of induced breeding through hypophysation, evolved at CIFRI, ensures breeding of both Indian and Chinese major carps in captivity. The technique involves the injection of fish pituitary gland extract to sexually mature fishes of the prime condition under favourable water and climatic conditions during monsoon season. The conducive water temperature range is 24 – 33°C with cool and drizzling weather. Rainwater is helpful in spawning and also ensures a high rate of fertilization. Water temperature between 27 – 31°C results in better hatching.
The normal breeding season of carps is generally during the south-west monsoon period from April to September, depending on the rainy season in different parts of the country. With the advent of monsoon, breeding generally starts in April/ May in Assam and during June/July in rest of the country.The package of practices of induced breeding of Indian and Chinese carps is described below
Maintenance of brood stock
Raising of properly maintained broodstock of both sexes to the prime mature condition is a pre-requisite for successful induced breeding. Potential brood fishes of over 2 years age, need be stocked in ponds @ 1000-2000 kg/ha by November in Assam and January in other parts of the country and reared for a period of about 6 months. For a target production of one crore spawn (60 lakh of Indian major carps and 40 lakh of Chinese carps), during a season 750 kg of broodstock (300 kg of Indian major carps+450 kg of Chinese carps) comprising males and females in an approximate ratio of 1 : 1 by weight and 2 : 1 by number would be required.
The management of the pond includes maintenance of sanitary condition in the predator free pond devoid of weeds, enriching the nutrient status by application of manures and fertilizers as in stock ponds (CIFRI Aquaculture Extension Manual, New Series No.2) and providing supplementary feed mixture of groundnut/mustard oilcake and rice bran in 1 : 1 ratio by weight @ 1 – 2% (generally 1%) of total body weight of fish per day.
Collection of the fish pituitary gland
The hormone, excercising a decisive control over maturation of gonads and breeding in fishes is secreted by the pituitary gland. In fish, the pituitary gland is situated ventrally to the brain immediately behind the optic chiasma in a concavity on the floor of the cranium.
Pituitary glands are collected from fresh as well as properly preserved fishes (weighing atleast 2 kg) of both the sexes of same (homoplastic) or allied species (heteroplastic). Glands from induced bred fishes, soon after spawning, are also potent. Since mature specimens of common carp are available during most part of the year in the farms/ponds, they also serve as good source of gland material .. The glands are conveniently collected during the pre-monsoon months: April – June (March in Assam). The glands are preserved in absolute alcohol and preferably kept in refrigeration. The collection methods are as follows:
- Using a sharp butcher’s knife a portion of the scalp is removed and the brain is exposed. The entire brain is lifted with a pair of forceps. The pituitary gland is then seen covered by a membrane which is also removed by forceps. The gland, then, is taken out very carefully avoiding rupture.
- In fish markets where severed heads of fish are usually separately sold, pituitary glands can be collected by adopting a simple technique. The posterior part of the brain case is cut by using a bone cutter and a bigger opening is made in the region of the foramen magnum thereby exposing the posterior region of the brain. Through the opening thus made, the gland can easily be removed.
Selection of mature fish for breeding
At the advent of monsoon brood fishes of 2 kg and above are netted out and selected for induced breeding. A male is easily distinguished by roughness on the dorsal surface of the petcoral fin. When it is ripe milt oozes out freely on gentle pressing at the belly near the vent. The females possess soft, round, bulging belly and swollen, pinkish genital opening
While the Indian major carps and males of Chinese carps are easy to select with the help of the above symptoms alone, a catheter may be useful in confirming the stage of maturity of female grass and silver carps. Inserting the catheter through the genital opening some oocytes are taken out and examined by keeping them in a petridish. Silver carp eggs of uniform size, pale blue in colour and grass carp with brownish or copper coloured eggs of uniform size indicate proper maturity. Females with such eggs are selected for induced breeding, individually weighed and kept in hapas.
Dosage of injection
The dosage of injection is calculated in terms of a milligram of pituitary gland per kilogram of body weight of the recipient fish. Indian major carp females are given two injections at an interval of 6 hours. Depending on the maturity of fish and climatic conditions etc., the two split up doses vary from 2 – 4 mgjkg body weight for the first injection and 5 – 10 mgjkg body weight for the second injection. The males are given a single injection of 2 – 4 mg kg body weight at the time of the second injection to the female.
Chinese grass and silver carp females receive @ 3 – 4 mgjkg body weight at first injection and @ 8 – 10 mgjkg body weight as the second dose. The males receive the only injection @ 3 – 4 mg kg body weight at the time of the second injection to the female.
Recent research has indicated that gonadal hydration is a prerequisite for successful spawning of carps. Pituitary extract injection induces the hydration process thereby increasing the body weight of the spawners. A 3% difference in body weight of female brood fishes between the two injections, thus, indicates better breeding success
Preparation of injection material
After deciding the dosage, the quantity of glands required for injecting the spawners is calculated. The required quantity of the glands is taken out, the excess alcohol is allowed to evaporate in about a minute and weighed. The glands are then macerated in a tissue homogenizer with a small quantity of distilled water or 0.3% saline solution and further diluted by the same liquid to the desired volume.
The extract is thereafter centrifuged and only the supernatant solution is utilized for injection. The extract can also be prepared in bulk, preserved in glycerine (1 part of extract: 2 parts of glycerine) and kept in refrigerator or ice for later use. It can even be ampouled and transported to other places.
Injection to brood fishes
Usually, a breeding set consists of one female and two males, the combined weight of males being equal to that of the female. Keeping the spawner inside a hand net and placing it on a soft cushion, intramuscular injection is given in-the caudal peduncle region, avoiding the lateral line . The volline,gland extract normally injected in one dose varies from 0.5 – 2.0 mljfish depending on the size of the spawner. The first injection is preferably administered in the late afternoon hours.
The injected brood fish sets are then released in indoor breeding tanks of metal, cement or plastic pools with cooling tower or air conditioner to control the water temperature. A feeble flow of current is preferable. In absence of such a facility, each set of brood fishes may be released in a closed breeding hapa fixed in ponds, marginal waters of canals, lakes and reservoirs. The water sheet need be free from algal bloom, common carp, tilapia and crabs etc. A breeding hapa is a rectangular cloth container (2.5 X 1.25 X 1.0 m), stitched with close meshed cloth, having an opening on one side with tying arrangements, through which spawners are introduced and taken out.
Spawning and hatching
Spawning takes place generally inside the breeding tank or hapa usually within 4 – 6 hours after the second injection. While the Indian major carps generally breed naturally inside the hapa, the Chinese grass and silver carp are preferably stripped for better fertilization. Dry method of stripping is generally adopted where the females, wrapped in a towel, are stripped to collect eggs in a dry enamel/plastic basin and are fertilized by milt stripped from males .
Ovulated eggs, swell up in water. Fertilized eggs are transparent while the unfertilized ones appear opaque and whitish Eggs are usually collected from the breeding tankfhapa 4 – 6 hours after spawning with a mug of known capacity and kept for hatching.
A glass jar hatchery with circulating water system, developed by CIFRI, enssures about 95% hatching success. Water hardened eggs are incubated in vertical hatchery jars. In a jar of 6.35 litres capacity, 50,000 eggs can be kept for hatching.
The flow of water in the jars is so regulated during incubation that the eggs are stirred without being spilt. Normally the rate of flow of water maintained is 600 – 800 ml/ minute for Indian major carps and 800 – 1000 mlfminute for Chinese carps. It normally takes 12 – 15 hours for the developing eggs to hatch out in the hatchery. Various modifications of this hatchery system are now available in the country. Based on the same principle, the CIFRI has also recently developed a low-cost earthen hatchery for marginal and small farmers.
In absence of modern hatching facilities, fertilised eggs can also be hatched in double-walled hatching hapas fixed in ponds free from algal bloom, common carp, tilapia and crabs. About 50,000 to 1,00,000 eggs are spread in the inner portion of the double-layered hatching hapa. While outer hapa (1.8 X 1.0 x 1.0 m) is made of thick meshed markin cloth/nylon, the inner part (1.5 xO.8 xO.5 m) is made of round meshed mosquito netting cloth. The eggs hatch out in 15 – 18 hours at a temperature range of 26 – 31°C. The hatchlings pass through the meshes of inner hapa to the outer. After ascertaining complete hatching, the inner hapa with egg shells etc. is removed. The hatchlings are left undisturbed in the outer hapa till the third day when the spawn are ready for stocking in prepared nursery ponds
Expenditure and return
The economics of producing one crore of spawn is based on a moderate assumption that 80% of the brood stock will be available in prime condition for spawning with 60% success in Indian major carps and 40% in Chinese carps. Fecundity has been taken as 1.5 lakhs and 1.0 lakh eggs/kg body weight of females for Indian major carps and Chinese carps respectively. The fertilization and hatching success have been taken as 80% and 70% respectively. It has also been presumed that cloth hapas are used for both breeding and hatching purposes. The generalised economics has been calculated on the basis of present market rates.
- Central Inland Fisheries Research Institute, Barrackpore