The techniques of artificial propagation/ induced spawning in fish also known as induced breeding of fish are manifold, all of which are aimed at producing plenty of spawn, fry and fingerlings for utilization in culture of for restocking water bodies or water courses.
Some kind of intervention by man in the natural propagation of cultivated or cultivable fishes may help to achieve a better survival of their offspring.
The demand for quality fish seeds is particularly great for modern intensive and super-intensive culture systems. The polyculture system has further increased the demand for the seeds of fishes of different feeding habits.
Breeding is the biological activity of individuals to produce offspring which is usually controlled by environmental stimuli (figure 1), but in aquaculture it implies to reproduce and raise individuals, especially for commercial purposes.
The breeding mechanism of a fish is a bio-chemical mechanism of an individual in which environmental stimuli forced that particular individual to show breeding behavior, and also help in gonad development.
A basic precondition of fish breeding is to have sexually mature male and female fish producing milt and eggs.
Breeders of fish species which cannot be cultured or kept domestically, are collected before spawning from rivers or lakes , their milt or eggs are stripped and then they are released again. These species are sturgeons (mullets) mugils (Mugilidae), and salmonids (Salmonidae).
The brood stock of cultured fish species are sorted out with aimed selection and cultured in fish ponds up to sexual maturity. The males and females of breeders are cultured in the same fish pond while they are young. Then, when sexually mature they are selected and cultured in separate fish ponds.
Induced Spawning In Fish
Induced spawning in fish embraces all the processes involved with inducing gonad maturation and consequent release of the gamete in fish.
In female, induced spawning is normally carried out at the end of vitellogenesis when the ova are ripped and it involves the following processes:
The induction of germinal (nucleus of an oocytes before formation of polar bodies) vesicles migration to the pepriphery. The movement of nucleus towards micropyle. Sometime induced spawning may start from this stage
Geminalvesilcle breakdown or final oocyte maturation, that is, resumption to meiosis.
Ovulational follicular rupture.
Oviposition or spawning i.e. the release of ova to the outside. This step may be replaced by stripping.
All these processes occur in a female. However, in a male the following processes are involved: In males, the processes involved are;
Spermiation which is the release of spermatozoa from sertolic cells into the lumen of the tubules.
Seminal hydration refers thinning of the seminal fluid of plasma.
Ejaculation is release of sperm to the outside. This can also be replaced by stripping.
Techniques in Induced Fish Breeding
There are two approaches to induction of spawning in fish, and these are: Hormonal approach, and Environmental approach.
After treatment, the fish in each approach may be stripped for artificial fertilization or left for natural spawning and fertilization. The fertilized eggs are then incubated and the hatched larvae raised to fry or fingerling stage – the hatchery.
The two approaches may and are often combined to obtain the best results in induction of spawning in fish.
Induced Spawning by Hormonal Approach
Induced spawning by hormonal approach is based on the understanding of neuroendocrine control of oocyte maturation, ovulation and spawning behavior in females and of spermiation and seminal hydration (males).
Generally, in fish hormonal treatment is known to:
Stimulate the release of gonadotropins (Gonadotrophic hormones – GTH) into the blood streams
When a certain level of GTH is reached the germinal vesicle migrate to the periphery and grannula cells of the follicle are stimulated to secrete a maturation-inducing steroid (MIS).
The MIS Induces germinal vesicle breakdown (GVBD) , that is, resumption of meiosis or final oocyte maturation and other associated events such as hydration of eggs, grouping and the condense of yolk granules and or lipid droplets in some species.
In the males, steroid induction and seminal hydration is not clear, in essence, how gonadotropins act on the testis to produce steroid which bring about spermation and seminal hydration is not properly understood. There are several method of hormonal inducement of spawning in fish;
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Hypophysation (Fish Pituitary Extracts)
The use of pituitary extracts to induce spawning in fish is known as hypophysation. Induced ovulation and spawning achieved through hypophysation amounts to a “short cut” of the natural process. In nature, ovulation in fish is regulated and brought about by its own gonadotropic hormone(s), produced and stored by the pituitary gland.
The stored hormone is released into the blood when all the requisite conditions become favourable. But in the hypophysation technique, gonadotropic hormone extracted from the pituitary of some other fish (donor) is injected into the breeder and this bring about the final ovulation.
Hypophysation is presently the most commonly used technique for the artificial propagation of fish. It is employed not only in propagation experiment, but also in the commercial production of millions of young fish.
Hypophysation is effective only when the eggs in the ovary have reached resting or vitellogenesis (stage iv). At this stage, the eggs are maternally ready for further development to be triggered by gonadotropins. Therefore, it is important to choose the right time and fish for collection of the pituitary glands.
Usually, the pituitary glands are obtained from sexually maturing or mature donor fish. Pituitary glands of donor fishes, collected fresh or preserved are used in hypophysation. It is necessary that these glands contain an adequate amount of stored gonadotropic hormones to bring about successful spawning.
The pituitary gland (hypophysis) acts as an intermediary between the brain and the gonads. Its cells produce and store gonadotropins, and release them only when the gland receives the necessary command.
The gonadotropins content of the pituitary gland varies during different season and during different stages in the life cycle of a fish. Immature fishes have only small quantity of gonadotropins in their pituitary glands while after natural spawning, spent fish are completely without gonadotropins in their pituitary.
On the other hand, gonadotropin is at the highest in pituitary of sexually ripped, matured fishes when their gonads have reached the resting phase or through-out the duration of the resting phase.
In summary, artificial propagation as practiced in different parts of the world may vary, depending on local conditions and facilities.
It may start with the collection and further rearing of naturally produced eggs, spawn or fry, or with the production of the egg itself through artificial inducement/induced breeding, followed by controlled fertilization, hatching and rearing of larvae and fry.
Artificial propagation, therefore, involves human intervention in the process of natural propagation and has the advantages of;
(a) Better rates of fertilization and hatching,
(b) Protection against enemies and unfavorable environmental conditions and
(c) Better conditions for growth and survival.
The techniques of artificial propagation/ induced breeding of fish are manifold, all of which are aimed at producing plenty of spawn, fry and fingerlings for utilization in culture of for restocking water bodies or water courses.
Induced spawning in fish embraces all the processes involved with inducing gonad maturation and consequent release of the gamete in fish.
There are two approaches to induction of spawning in fish, and these are: Hormonal approach and the Environmental approach.
The use of pituitary extracts to induce spawning in fish is known as hypophysation. Induced ovulation and spawning achieved through hypophysation amounts to a “short cut” of the natural process. In nature, ovulation in fish is regulated and brought about by its own gonadotropic hormone(s), produced and stored by the pituitary gland.
The stored hormone is released into the blood when all the requisite conditions become favorable. But in the hypophysation technique, gonadotropic hormone extracted from the pituitary of some other fish (donor) is injected into the breeder and this brings about the final ovulation.
Hypophysation is presently the most commonly used technique for the artificial propagation of fish. It is employed not only in propagation experiment, but also in the commercial production of millions of young fish.
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