Reproductive biology and endocrinology of black bream Acanthopagrus butcheri

This study provides baseline information on the annual reproductive activity of wild black bream, the effect of capture and confinement on plasma steroid levels, suitable induced ovulation protocols and the effect of salinity on reproductive development, induced ovulation and egg fertility and development. The annual change in reproductive condition and plasma levels of sex steroids in black bream, was investigated by measuring changes in gonadosomatic index (GSI), hepatosomatic index (HSI), gonad stage and plasma concentrations of sex steroids. Black bream have an annual reproductive cycle with a 3 month spawning season in spring / early summer with daily cycles of gonadal maturation and plasma steroid levels. Elevated levels of plasma estradio1-1713 (E2), testosterone (T) and 11-ketotestosterone (11KT) were associated with gonadal recrudescence, and elevated plasma 17,2013-dihydroxy-4-pregnen- 3-one (17,201313) levels were associated with final oocyte maturation and spermiation in female and male fish respectively. The stress-induced changes in concentrations of plasma sex steroids in black bream were investigated by blood sampling at capture and in fish confined for 15, 30 minutes, 1, 3, 6, 12 or 24 hours. Confinement resulted in significantly elevated plasma cortisol levels, reduced plasma levels of E2 and T within 1 h in females, and suppressed plasma levels of T and 11KT after 30 min and 6 h respectively in males. Plasma levels of 17,2013P increased decreased or remained unchanged. This study indicates that stress exerts a rapid inhibitory effect on gonadal steroidogenesis in black bream. The effect of hormone therapy to induce ovulation was assessed by injecting mature female black bream with saline, human chorionic gonadotropin (hCG) or luteinizing hormone releasing hormone analogue (LHRHa) at capture, or 24 hrs post capture. Treatment with LHRHa or hCG resulted in fish ovulating throughout the experiment, with LHRHa treatment at capture resulting in the best ovulatory response. Injection with hCG or LERHa at capture resulted in the short term elevation of plasma E2 and T, whereas, injection of LHRHa 24 hrs post-capture did not elevate plasma E2 or T levels over controls. Plasma levels of cortisol and 17,2013P were unaffected by hormone treatment. These results show that capture and handling stress reduces the responsiveness of fish to exogenous hormone treatment and that best results are obtained if hormonal treatment is administered at the time of capture. The effects of salinity (5, 20 or 35%) on seasonal reproductive development, plasma steroid levels, the efficacy of LHRHa to stimulate ovulation, sperm motility, and egg fertility and development to hatching were investigated. Gonadal maturation and seasonal plasma steroid levels were essentially unaffected by salinity in both sexes. Hormone therapy resulted in the typical endocrine and ovulatory response in all three salinities, however, egg production was reduced in fish held at 5 %0. Both fertilisation and sperm motility were significantly reduced at 5%o. Egg development was best over a salinity range of 20-35%0.