SUSTAINED PRODUCTION OF MILT IN RABBITFISH, SIGANUS-GUTTATUS BLOCH, BY WEEKLY INJECTION OF LUTEINIZING-HORMONE-RELEASING HORMONE ANALOG (LHRHA)

Mature male rabbitfish (Siganus guttatus Bloch) received weekly injections of 200 mug of luteinizing hormone-releasing hormone analogue (D-Ala6, Pro9-LHRH-ethylamide) per kg body weight for 5 consecutive weeks. Mean spermatocrit, or packed sperm volume (27-51%), and mean sperm density (3.2-9.6 X 10(6) spermatozoa per kg body weight) decreased significantly 24 h after each injection. The amount of expressible milt (mean: 5.8-11.7 ml per kg) in response to weekly injections of LHRHa increased significantly relative to saline-injected fish (1.0-2.9 ml per kg), but only during the initial 4 weeks of regular hormone treatment. Three weekly injections of LHRHa likewise augmented mean sperm production (29.2-112.5 X 10(9) spermatozoa per kg) in rabbitfish. However, no significant enhancement in sperm production by LHRHa-injected fish was observed over the last 2 weeks of hormone injection. These results demonstrate that weekly injection of LHRHa can sustain milt production in mature rabbitfish, although their capacity to produce spermatozoa is limited to only 3 consecutive weeks of regular hormone treatment.