The following points highlight the five main hormones used in induced breeding. The hormones are: 1. Human Chorionic Gonadotropin (HCG) 2. Luteinizing Hormone — Releasing Hormone 3. Progesterone 4. Antiestrogens 5. Ovaprim.
Induced Breeding: Hormone # 1. Human Chorionic Gonadotropin (HCG):
Among all the mammalian gonadotropins, HCG has given the best result, HCG is widely used as it has an edge over pituitary gland (PG) mainly because of its easy to acquire, relatively cheap, long shelf-life and of known and consistent potency. HCG alone or in combination with fish pituitary (HCG 70% + PG 30%) has successfully been used to induce breed Indian major carps.
Drawback:
Fishes often become unresponsive to HCG injections because of immunological reactions. It has been reported that following HCG treatment regressive ovarian changes take place possibly due to overstimulation.
Induced Breeding: Hormone # 2. Luteinizing Hormone — Releasing Hormone (LH-RH) and its Analogues:
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LH-RH is a synthetic decapeptide. It and its superactive analogues (LHRH- A) serve as potential ovulating or spawning agents that induce gonadotropin secretion in fishes. Carps when injected with LHRH-A show greater success (78.5%) as against fish pituitary extract (75%).
From the experimental data available it has been observed that the naturally occurring releasing hormones are less effective than the synthetic analogues. This is probably due to the releasing hormones being more rapidly metabolised or removed from the fish. In general, the synthetic analogues like LHRH-A are up to 1000-fold more potent.
Induced Breeding: Hormone # 3. Progesterone:
Derivatives of progesterone such as 17-α-hydroxy-progesterone (17- α -Prog.) and 17- α -hydroxy-20-β-dehydroprogesterone (17- α -20- β-Prog.) are steroids that have been found in significant quantities in the blood plasma of maturing fishes. Injection with these steroids can bring about final maturation and ovulation in fishes. Therefore, progesterone can partially replace gonadotropin.
Induced Breeding: Hormone # 4. Antiestrogens:
These are synthetic, nonsteroidal compounds (such as Clomiphene citrate and Tamoxifen) that are capable of competing with estrogen for binding sites on estrogen receptors. They are used for inducing ovulation. Antiestrogens inhibit gonadotropin secretion, when used in excessive dosage. It may work better in those fishes where vitellogenesis is complete and blood estrogens are high.
Induced Breeding: Hormone # 5. Ovaprim (LINPE Method):
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The Chinese researchers have developed a technique, called the ‘Linpe’ method, where the female fish is induced to ovulate by injecting them with a combination of LHRH-A and the drug domperidone. The drug inhibits the action of dopamine (a substance produced by the fish that inhibits ovulation). Therefore, stimulating the sex organs of the fish.
Syndel International, a Canadian company, developed a single product, known commercially as Ovaprim, that contains both the hormone and the dopamine antagonist. Ovaprim has been marketed in India by ‘Glaxo India’ since 1990. Ovaprim has been tested successfully on a wide range of fresh, salt and brackish water species. It has been found to be very fruitful in the aquarium fish industry in India.
Advantages of Ovaprim:
(1) Although ovaprim is costly (almost double the cost of pituitary), yet it has the advantage of known potency and assured breeding response. Therefore, it reduces the loss incurred due to spawning failure resulting from poor quality of glands.
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(2) Ovaprim being in ready-to-use form, appears to be more convenient and can be easily adopted in carp seed production.
(3) Rates of spawning, fertilization and hatching and survival has been reported to be much higher than pituitary extract.
(4) Refrigeration of the solution is not required and it has a long shelf life.