The efficacy of exogenous hormones in stimulating changes in plasma steroids and ovulation in wild black bream Acanthopagrus butcheri is improved by treatment at capture

Abstract

Sexually mature female black bream were captured by rod and line and injected with saline, human chorionic gonadotropin (hCG) or luteinizing hormone releasing hormone analogue (LHRHa) at capture, or 24 h post capture (saline and LHRHa treatments only). All fish were bled and checked for ovulation for 5 days post injection. Plasma levels of oestradiol (E 2), testosterone (T), 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and cortisol were determined by radioimmunoassay. Saline-injected fish ovulated only on day 1, whereas treatment with LHRHa or hCG resulted in fish ovulating throughout the experiment. Treatment with LHRHa at capture resulted in a better ovulatory response than treatment with hCG at capture or LHRHa 24 h post capture. Plasma E 2 levels in saline-injected fish were high at capture and had significantly dropped 1 day after capture. Injection with hCG or LHRHa at capture resulted in plasma E 2 levels remaining significantly elevated for 2 days post injection. Injection of LHRHa 24 h post capture failed to significantly elevate plasma E 2 levels over controls. Plasma T levels were similar to E 2 profiles. Plasma levels of 17,20βP were not significantly different between any treatments, but showed a tendency to increase after capture. Plasma cortisol levels showed no treatment effects and were initially low at capture before becoming elevated between days 1 and 2 post capture. These results show that capture and handling stress reduce the responsiveness of fish to exogenous hormone treatment and that best results are obtained if hormonal treatment is administered at the time of capture.