Effect of maintenance at elevated temperatures on ovulation and luteinizing hormone releasing hormone analogue responsiveness of female Atlantic salmon ( Salmo salar) in Tasmania

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To examine the effect of maintenance at elevated water temperatures on the responsiveness of female Atlantic salmon ( Salmo salar) to luteinizing hormone releasing hormone analogues (LHRHa), sexually maturing 2-year-old female Atlantic salmon were maintained at water temperatures of 6, 11 or 16 °C from late in vitellogenesis (early April in Tasmania) throughout the periovulatory period (late April to early June). In early May, fish were treated with LHRHa (25 μg kg −1 body weight) by injection or in a cholesterol pellet. Controls received saline injections and blank pellets. Fish were anaesthetised and blood sampled at treatment, and at 48-h intervals for up to 8 days post injection. Thereafter, regular ovulation checks were conducted until the end of the experiment. Plasma levels of the gonadal steroids testosterone (T), 17β-estradiol (E 2) and 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) were measured and egg fertility and survival to the eyed stage were assessed. In controls, maintenance at 11 °C delayed ovulation relative to fish held at 6 °C whereas, in fish held at 16 °C, ovulation was inhibited until holding temperature was reduced to 8 °C on day 35 post injection. Treatment with LHRHa advanced ovulation in fish held at both 6 and 11 °C but had no effect on ovulation in fish held at 16 °C. Elevated plasma 17,20βP in controls was only evident in those fish held at 6 °C (>100 ng ml −1), whereas LHRHa treatment was associated with high 17,20βP levels in fish held at both 6 and 11 °C (>60 ng ml −1). In contrast, there was little production of 17,20βP in fish held at 16 °C irrespective of treatment (<25 ng ml −1). In controls, prior maintenance at 16 °C was associated with significant reductions in the fertility and survival of ova (84.0% and 17.3%, respectively) relative to 6 °C (97.9% and 75.6%, respectively) and 11 °C (95.3% and 44.4%, respectively). The fertility and survival of ova from LHRHa-treated fish held at 6 and 11 °C did not differ significantly from that of controls but LHRHa-treated fish held at 16 °C either produced nonviable ova or died prior to ovulation. These observations indicate endocrine dysfunction and confirm a lack of maturational competence in Atlantic salmon maintained at elevated temperatures, and suggest that both impaired pituitary responsiveness and limited 20β-HSD activity may contribute to the observed lack of 17,20βP production in fish held at 16 °C.