Abstract
Adult yellowfin bream ( Acanthopagrus australis ) were captured from the wild and maintained in outdoor tanks until the onset of the natural spawning season (May ‐ June). Three replicate trials were conducted to examine the effects of gonadotropin releasing‐hormone analogue (GnRHa) and human chorionic gonadotropin (hCG) on milt characteristics and gonadal steroid levels. Spermiating males were injected intraperitoneally with either saline solution (control) or a single dose of GnRHa (25, 50, 75, 100 μg kg −1 ) or hCG (250, 500, 750, 1000 IU kg −1 ). Milt and blood samples were collected before hormone administration (0 h) to examine initial reproductive status, and at 6, 24, and 48 h post injection (p.i.) to assess the effects of hormone treatment. There was a progressive increase in the reproductive condition of fish held throughout the experimental period, with increases in milt volume and sperm concentration over time, and a corresponding fall in the duration of sperm activity. Treatment with saline solution or hCG did not significantly alter reproductive parameters. Treatment with GnRHa at a dose of 100 μg kg −1 significantly increased milt volume and reduced sperm concentration at 48 h p.i. The cumulative number of sperm collected over the 48 h sample period was also elevated. The duration of sperm activity was unaffected by hormone dose or type. All GnRHa treatment groups had elevated levels of 11‐ketotestosterone (11‐KT) at 24 and 48 hp.i. Plasma testosterone levels did not change with treatment. The increase in milt volume and cumulative sperm number in the 100 μg kg −1 group coupled with the elevated levels of 11‐KT throughout this period suggest gonadotropin‐dependent production of 11‐KT as a mediator of spermiogenesis, but not spermiation in yellowfin bream. The lack of response to hCG may be explained by low potency of the hormone in yellowfin bream during the early spawning season.