This article will help you to differentiate between natural breeding and induced breeding.
Natural Breeding:
The Indian major carps do not breed in confined water. Natural spawning also has many hazards.
a. Factors required for breeding are beyond human control.
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b. Spawns are not of good quality and often mixed with those of unwanted fishes, particularly predators.
c. High mortality in transit from collection point to culture pond.
Induced Breeding:
Present (Jay wide spread hypophysation or induced breeding is the result of contributions of a number of workers. Pioneers in this field are: Houssay (1930) of Argentina suggested the idea; Brazil introduced it in 1934; in India, Khan (1937) made the first attempt; Chaudhuri (1955) was successful with Esomiis danricus; Chaudhuri and Ali Kunhi (1957) introduced large scale spawning in Labeo rohita, L. bata, Cirrhinus rnirgal, C. reba, Puntius sarana, etc.
In induced breeding, the potential breeders, i.e., sexually mature male and female are made or induced to spawn in confined water at desired time, of course with certain limitations, by injecting them with pituitary gland extract.
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It ensures:
a. Spawning more than once a year.
b. Production of quality seeds.
Technique adopted:
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1. Collection and preparation of gland extracts:
The pituitary gland is located on the ventral side of the diencephalon and lodged in a cavity in the base of the skull.
Collection of gland:
a. Intact glands are collected in May to July from fully ripe, fresh or ice-preserved (5-7 days maximum) carps of both sexes of same or closely related species, and stored in absolute methanol.
b. After 24 hrs. glands are transferred to fresh ethanol in another vial.
Preparation of gland extract:
a. The required amount of glands are taken out and placed on a filter paper for partial drying.
b. The glands are macerated in a tissue homogenizer with a small amount of distilled water or 0.3% saline solution.
c. The tissue debris is centrifuged off and the aliquot (supernatant) containing the hormones is drawn into a syringe for injection.
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2. Preservation of gland extract:
a. Distilled water is added to homogenized glands to make 1/3rd of the desired volume to be used and stored in a refrigerator for 24 hours.
b. Two volumes of glycerine (double the volume of the extract) is added and again stored in a refrigerator for another 24 hours.
c. The tissue debris in the mixture is centrifuged off and the aliquot transferred to ampules, sealed and stored for future use.
3. Selection of breeders for injection:
a. A mature male is distinguished by roughness of the dorsal surface of the pectoral fin, which is smooth in female.
b. Milt oozes out freely in ripe males with gentle pressure on the belly near the vent.
c. The belly of a mature female is soft, rounded and bulging and the genital opening is pinkish.
d. One set of breeders, one male and two females are released in a breeding hapa after recording their individual body weight.
4. Rearing of breeders prior to selection:
The potential breeders, each weighing above 1 kg, are stored 5 to 6 months prior to breeding season and are reared properly. Health and maturity status are periodically examined.
5. Determination of dose for injection:
a. It is calculated in terms of mg/pituitary/kg body weight of the recipient fish.
b. The female is given 2 injections. The first one at a dose of 2-3 mg/kg and the second one 5-8 mg/kg after 6 hours.
c. The male is given only one injection, 2-3 mg/kg, simultaneously, with the second injection to the female.
d. Very ripe breeders need small dose for spawning.
6. Site of injection:
a. Inter-muscular injection at 45 is given in the region of caudal peduncle or in the maximum fleshy portion of the body dorsal to the lateral line.
b. During injection the breeders should be held with sponge mattress.
7. Breeding:
a. Injected male and female breeders are released in a breeding hapa prepared at a corner of the pond.
b. Breeding starts 3-6 hours after the second injection.
c. Sexual activity is manifested by jumping and darting movements. The female spawns and the male sheds milt.
d. The released eggs are immediately fertilized in water.
e. Fertilized eggs are transparent, swell up to a diameter of 3.5 to 5.5 mm. Unfertilized eggs are whitish in colour.
f. A female catla yields about 0.15-0.2, a rohu 0.2-0.3 and a mrigal about 0.7 million eggs in a single spawn.
8. Hatching and management of spawns:
a. Eggs are transferred to hapa for hatching. During transfer, mortality is very high due to sudden change of temperature and other factors.
b. A hatching hapa is really two rectangular hap as—made of net—one placed inside the other. The inner hapa measures 1.8 x 0.9 x 0.9 m and its meshes are larger than that of the outer one.
c. The eggs are uniformly spread on the bottom of the inner hapa. At an optimum temperature 27°-31°C the eggs hatch out by 15 to 18 hours.
d. The hatchling escape to the outer hapa. Un-fertilised eggs and egg shells left are removed with the inner hapa.
e. On the third day, the spawns are collected and transferred to nursery ponds.
Abiotic factors conductive to spawning:
a. Rain and rain water,
b. Water current range,
c. Temperature range 27-31 °C,
d. pH of water and
e. Light—soft or shady.