Reproduction and Genetic Selection
As discussed, one of the major criteria in selecting a species for culture is the existence of either suitable techniques for controlled breeding or easy availability of spawn, larvae or juveniles from natural breeding grounds. Even when culture can be initiated using ‘wild seed’, it is essential to achieve controlled reproduction as early as possible, to ensure timely availability of young ones in adequate numbers for large-scale rearing. It is also a basic need in the domestication of the animal and for taking advantage of the benefits of genetic selection and hybridization that have contributed so much to terrestrial agriculture and animal husbandry.
Controlled breeding will obviously be possible only if there is adequate knowledge of the factors governing reproduction of the animal and its breeding behaviour. Lack of such knowledge has hampered the progress of aquaculture of several important species. The extensive culture of Chinese carps, Indian carps, mullets, milkfish, seabass, sea-bream, penaeid shrimps, oysters and mussels has been based until recently on ‘seed’ obtained from natural breeding. Despite advances made in techniques of controlled or semi-controlled breeding, the techniques have not been sufficiently perfected or adapted for large-scale production of seed, with the result that the aquaculturist has still to depend partially or entirely on natural seed resources. There are also species like the eels for which no propaga-tion technique has so far been developed, even though some progress has been made in maturing and spawning under laboratory conditions.
Among the aquaculture species, finfish as a group has received greater research attentionin controlled reproduction. The reproductive cycles of almost all fish are regulated by environmental stimuli. Appropriate sensory receptors convey the 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 characteristics, nuptial coloration and breeding behaviour. This pattern of reproductive mechanism provides the basis for methods of induced reproduction, namely the provision of appropriate environmental stimuli and the administration of hormones for maturation and release of gametes.