Reproduction:
Fish farmers need fry or “seed” predictably and in very large amounts. When the number of wild caught fry falls below what farmers need to stock their ponds or cages, producers must induce adult fish to reproduce in captivity.
Although it is possible to control fry production by using natural reproduction and environmental management, hormonal stimulated spawning is widely employed.
With such a method, the better the reproductive physiology of fish is understood, the better are the chances of success. The nervous and endocrine systems of fish act together to coordinate reproduction. Neural stimulation starts the chain of events, and latter links are hormonal.
Stimuli from the environment (photoperiod, temperature, food etc.) are processed by sensory receptors; the resulting neural signal, when reaching the hypothalamus, induces the production of chemical messengers known as releasing hormones (gonadotropin-releasing hormone GnRH or luteinizing-hormone releasing hormone LHRH, which is a certain kind of GnRH). GnRH is small peptides (10-amino acid) and they are identical or only slightly different in most fish.
They initiate the release of gonadotropin hormones by the pituitary gland (also named hypophysis). They are glycoproteins and they influence the production of sex steroids in the gonad itself. Sex steroids are responsible for gamete maturation and, if the appropriate environmental and social signals are present, ovulation (or spermiation) and spawning follow.
The gonadotropin release is inhibited by many biological systems, particularly hormonal ones. Although, there is some evidence for inhibitory action of a brain hormone, the neuropeptide Y (NPY), the strongest evidence for gonadotropin inhibition in fish is for dopamine.
Fish culturist wanting to induce breeding with hormones will first have to assess the maturity of breeders, as the success of the technology depend on accurate information about the state of the gonad.
Fish readiness can be judged by considering the external appearance (large soft abdomen and a swollen gonad papilla for female, release of milt when abdomen is squeezed for males) or using more complex and time-consuming methods based on gonad biopsy and egg analysis (egg diameter and size distribution, morphology of oocyte, and localization of nuclei in cells).
Fish manipulations must be carried out in a way that minimizes the stress of reproducers: use of anesthetics, avoid of overcrowding, moist of hands and all cloth nets or holding slings before handling fish, cover of the fish eyes, minimization of noise etc.).
Spawning hormone is generally administered intramuscularly or intraperitoneally, but some attempts were made to deliver them orally or by mixing them with a binding material (implantation). The last method permits a very slow release of hormone, over weeks or months.
The choice of hormone depends on many factors including species, cost and availability, egg incubation or larval-rearing facilities, and training. Fish farmers will use either a gonadotropin or a GnRH analogue, with or without a dopamine antagonist.
Hypophysation, the injection of crude fish pituitary extracts, has been used since the 1930s. It has many advantages and many drawbacks, as extracts are highly impure and contain accessory hormones and other components that may stimulate some fish and inhibit others.
But, most experienced fish breeders agree that if good pituitaries are available, hypophysation is an excellent method of spawning.
A typical modern hypophysation technique for freshwater fish involves two injections into females: a small dose stimulates germinal vesicle migration and is followed about 12 hours later by a larger one that induces germinal vesicle breakdown, ovulation and spawning.
use to research. HCG, the human chorionic gonadotropin can also be employed but the molecule is so different from fish gonadotropin that high dose must be used for many species and some may not respond at all. Moreover, the availability of gonadotropin hormones like HCG is low, the stability of hormonal products is weak and these compounds induce a prejudicial immunity response in several fish species.
On the contrary, many releasing hormones are available and they have three great advantages:
they act early in the hormonal chain and cause the fish to produce its own gonadotropin, thereby eliminating all the problems caused by using a gonadotropin from another species. Second, the GnRH molecule itself is not highly species-specific. Third, they are simple, easily manufactured, stable molecules whose biological activity does not vary from lot to lot and, because they are active at such low concentrations, their use is economical.
GnRH analogues can be used solely or together with a dopamine antagonist (pimozide or domperidone). Injecting a GnRH analogue followed by, or in combination with, a dopamine antagonist has been called the Linpe method, after Lin and Peter the researchers who started it. Effective doses of hormones vary widely but the trend is toward single injections of 5-20 µg.kg- 1 GnRH analogue doses, although 1-100 µg.kg- 1 have shown to be effective.
Domperidone is usually effective at doses of 1-5 mg.kg- 1. Table 7 shows a comparison of Linpe method and traditional hypophysation in induced reproduction of Indian carps.
Table 7. A comparison of induced reproduction results in female Indian carps, using carps hypophyses (CH) or Ovaprim® (commercial product containing 10 mg.l-1 domperidone and 20 µg.l-1 salmon GnRH analogue, by Syndel, Vancouver, Canada).
* The differences between these two lines come from unachieved ovulations (From Billard, 1995).
Fish that have been induced to ovulate and spermiate with hormones are often strip-spawned the gametes are removed by gently compressing the abdomen and then mixed manually. Artificial mixing of eggs and spermatozoa can produce very high fertility, over 90%, but techniques vary from fish to fish. The dry method is the best basis for fertilization.
To make the most out of the very short period of sperm motility, mixing with eggs is done before any water is added.
Quickly after, just enough of the natural spawning water or diluting solution is added to wet them thoroughly and after several minutes, fertilized eggs are washed with larger volumes of water, before being transferred to incubators (Zoug bottles).
Fish culturists have deadlines and production quotas, and persuading administrators and owners to put more effort into achieving natural reproduction, when hormone-induced spawning may be at least partially successful, will be difficult.
But some of the best evidence that the natural method works, comes from the world of tropical aquarium fish where most of the breeding is done by natural means and successes are legion.
In the case of the Asian arowana (Scleropages formosus), young fish are desired for display but can no longer be captured legally and, as a consequence, they command a very high price. In spite of this fact, the strategy that worked for reproducing it was simply to provide undisturbed space, water of the correct hardness and high quality diet.
In fact, reproduction induced by management of environmental factors can be applied to many other species: carps (through a thermal stimulation, or by breeding fish in weedy ponds or ponds with artificial sub-strata/s), and osteoglossids (by breeding them in weedy pond, or by simulating flooding) etc. The development of new management methods for a better-controlled natural reproduction might be a very efficient tool for future.
Authors:
Lionel Dabbadie and Jerome Lazard
