MANIPULATIONS OF THE REPRODUCTIVE-SYSTEM OF FISHES BY MEANS OF EXOGENOUS CHEMICALS

Environmental control of reproductive activity of captive fish is feas ible (or potentially feasible) but, with few exceptions, is currently impractical for most species, Therefore, chemical methods of manipulat ing reproductive activity continue to be widely used in fish productio n operations worldwide. However, the control of fish reproduction in c aptivity cannot be exercised without regard to adequate environmental conditions, which can differ markedly for different species. This revi ew provides a synopsis of relevant aspects of fish reproductive physio logy and addresses current and promising future chemical methods of se x control, gonadal recrudescence, and spawning. Most research on the c ontrol of reproduction in fishes has focused on female physiology beca use ovarian development and maturation are easily disturbed by environ mental stressors. Control of sex ratios by steroid treatment has becom e a well-established technique for several fish species, but the techn ique continues to be problematic in some cases. Final gonadal growth a nd spawning usually can be achieved by implant treatment with gonadotr opin-releasing hormone analogs (GnRHa), which in some species have to be applied in combination with dopamine antagonists to enhance respons iveness to GnRHa. However, efforts to accelerate gonadal recrudescence and maturational competence by chemical means have yielded mixed resu lts, reflecting a relative lack of understanding of the basic physiolo gical and biochemical mechanisms controlling these processes. The pote ntial benefits of using reproductive pheromones to manipulate gonadal development and spawning has been demonstrated in a few species, but f urther research is needed to determine whether this technique is appli cable to fish culture. Because a reliable supply of young fish is crit ical for the expansion and diversification of fish culture operations, the use of chemicals in combination with adequate environmental condi tions to control gametogenesis and spawning in fishes will continue to be an important tool for the fish culturist.