The most commonly observed reproductive dysfunctions in cultured fish are the unpredictability of final oocyte maturation (FOM) in females, and the diminished volume and quality of sperm in males. Gonadotropin-releasing hormone agonists (GnRHa) have been used extensively in order to stimulate the release of pituitary luteinizing hormone (LH) required to induce FOM, ovulation and spermiation. Because multiple hormonal treatments are often necessary for a successful response, fish must be monitored and handled extensively, which is labor intensive, stressful to the fish and can often result in broodstock mortalities. To ameliorate this problem, sustained-release delivery systems for GnRHa have been developed during the last two decades and have been increasingly applied in controlling reproduction of a variety of cultured fish. Solid implants of cholesterol or poly[ethylene-vinyl acetate], and biodegradable microspheres of poly[lactide-glycolide] or poly[fatty acid dimer-sebasic acid] release GnRHa for a period of time (from a few days to many weeks.) GnRHa-delivery systems do not cause desensitization of the pituitary gonadotrophs in fish, and by stimulating a sustained elevation of plasma LH they induce the natural progression of plasma steroid increases associated with FOM and spermiation. This method has been used with very encouraging results in females of more than 40 cultured species and has been effective in inducing FOM, ovulation or spawning in fish with synchronous, group-synchronous and asynchronous ovarian development. In males, GnRHa-delivery systems have been tested in more than 20 species, producing significant increases in milt production for up to 5 weeks. Future research should focus on the optimization of this technology in terms of (a) using the most potent GnRHa, (b) identifying the most appropriate GnRHa release kinetics according to the reproductive biology of different species, and (c) determining minimum effective doses. Developments in these areas will greatly enhance the effectiveness and efficiency of GnRHa-delivery systems, while at the same time reducing their cost thus making them more affordable to the aquaculture industry.
