Pellets designed for continuous, long-term, oligopeptide, hormone delivery were implanted into the adult landlocked Atlantic salmon (Salmo salar) to investigate the effects of LHRH agonist analogue (LHRH-A) upon the reproductive system. Chronic elevations in plasma gonadotropin were observed in experimental fish at a variety of stages of reproductive development. Synthesis of pituitary gonadotropin was also evoked in sexually regressed fish and fish undergoing gonadal recrudescence, but the already elevated level of pituitary GtH observed in prespawning and spawning salmon was not augmented. In prespawning fish, the events of ovulation and spermiation were accelerated by LHRH-A treatment. The LHRH-A hormone treatment also increased milt volume and the duration of spermiation in ripe males. In sexually regressed males LHRH-A treatment did not induce the advanced phases of spermatogenesis but some testicular stimulation was indicated by the presence of increased numbers of B-type spermatogonia and enlargement of the lumen of lobules. The gonadal response to LHRH-A treatment during the rapid phase of gonad recrudescence differed in males and females. Based upon the changes in GSI, the LHRH analogue accelerated vitellogenic development of the ovaries in females; in males, however, the GSI was significantly reduced following LHRH-A administration. Whereas, it seems clear that a long acting preparation containing LHRH-A can be successfully used in prespawning fishes to advance the final events of gonadal maturation and initiate spawning, further studies are necessary to evaluate the potential of LHRH agonists to either stimulate or inhibit the reproductive development of fish at other stages of the seasonal reproductive cycle.Key words: fish, teleosts, landlocked salmon, LHRH analogues, gonadotropic hormone releasing hormones, seasonal reproductive cycle, gonad development, spawning
