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

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 (E2) 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. © 2004 Elsevier B.V. All rights reserved.