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The propagation of fishes has been practised from time immemorial in different parts of the world. Aquaculturists, especially those involved in the rearing and propagation of fishes, have often the disadvantage of cultivating species, the seeds of which have largely to be captured from the wild. Techniques for production of adequate quantities of high quality fish seed from captive broodstock are necessary for large-scale expansion of aquaculture. The lack of such techniques has been an important constraint in successful culture of several cultivable species.

Paucity of basic information on the reproductive physiology of cultivated fishes has caused aquaculture to lag behind agriculture. Such information, whenever available, has played an important role in increasing fish yields and it is no coincidence that among the most successfully cultivated fishes are those on which the largest amount of basic information is available. In view of the importance and relevance of basic research on reproductive physiology of cultivated fishes for the expansion of aquaculture, work on many of these areas has been the subject of several reviews (Pickford and Atz, 1957; Dodd, 1960; Reinboth, 1972; Donaldson, 1973; de Vlaming, 1974; Fontaine, Y.A., 1975; Fontaine M., 1976; Harvey and Hoar, 1979; Peter and Crim, 1979).

The present review covers several aspects of endocrinology of reproduction in teleost fishes, principally those used for large-scale farming in Asia, Africa and Latin America (see Table 1), and which possess the greatest potential for aquaculture development in the countries of these regions.

A perusal of the. list of cultivated species (Bardach, Ryther and McLarney, 1972; Jhingran and Gopalakrishanan, 1974) reveals that among the world's fish species cultivated at present those inhabiting fresh water predominate. Of these, only a few species such as the rainbow trout, Salmo gairdneri; the common carp, Cyprinus carpio; the catfish, Heteropneustes fossilis; the grey mullet, Mugil cephalus; and tilapia, Sarotherodon (Tilapia) mossambicus, have been investigated in detail. Information available on many aspects of their reproductive physiology as well as that on other species is briefly described under appropriate headings.

This review has a dual purpose. First, to briefly discuss recent information on the reproductive physiology of fishes with emphasis on cultivated species and, secondly, to indicate areas of reproductive physiology where basic research needs to be carried out. Further, aspects of research that are directly or indirectly relevant to increasing our ability to breed cultivated fishes in captivity are highlighted.

The review begins with a consideration of teleost reproductive cycles highlighting the. role played by environmental stimuli in the regulation of these cycles. The appropriate sensory receptors located on the body of the fish convey information about environmental stimuli to the brain in the form of neural inputs. This neural information on reaching the hypothalamus causes the release of hypothalamic peptides, known as releasing hormones, which in turn stimulate the pituitary gland to release the gonadotropic hormone(s) which act on the gonads. The gonads in turn produce the sex steroid hormones which are responsible for the formation of gametes as well as for the regulation of secondary sexual characters, nuptial coloration and breeding behaviour.

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