Effect of short-term temperature reduction on ovulation and LHRHa responsiveness in female Atlantic salmon (Salmo salar) maintained at elevated water temperatures

To examine the value of short-term temperature reduction and luteinizing hormone releasing hormone analogue (LHRHa) treatment as methods to promote ovulation in female Atlantic salmon (Salmo salar) maintained at elevated water temperatures, groups of sexually maturing 2-year-old female Atlantic salmon were maintained at 11°C (1 group) or 16°C (2 groups) from early April (late in vitellogenesis). In early May, the temperature of one of the 16°C groups was reduced to 11°C, and at that time, fish from each group were injected with saline, LHRHa, and/or 17-hydroxyprogesterone (17P). Fish were anaesthetised and blood sampled repeatedly at 48- to 72-h intervals for up to 10 days postinjection. Ovulation checks were conducted at similar intervals until 20 days postinjection. In controls, maintenance at 16°C followed by temperature reduction was associated with delayed ovulation relative to fish held at 11°C, whereas ovulation was inhibited in fish held at 16°C throughout. Treatment with LHRHa and 17P alone or in combination advanced ovulation both in fish held at 11°C and in those which underwent temperature reduction but had no effect on ovulation in fish held at 16°C. In controls, plasma 17,20βP was elevated to ~30 ng ml-1 in fish held at 11°C and in those which underwent temperature reduction, but in fish held at 16°C, 17,20βP levels remained near assay detection limits (~0.3 ng ml-1). Treatment with LHRHa and/or 17P resulted in plasma levels of 17,20βP in excess of 40 ng ml-1 in fish held at 11°C or exposed to temperature reduction. In fish held at 16°C throughout, 17,20βP levels were also increased (>40 ng ml-1) in response to treatment with 17P, with or without LHRHa, but only a modest increase in 17,20βP occurred in response to LHRHa alone (<25 ng ml-1). Maintenance at 16°C with subsequent temperature reduction was associated with a significant reduction in the fertility of ova from control fish (32.0%) relative to 11°C (>84%), and this was partially restored by treatment with 17P (~58%) and fully restored by treatment with LHRHa with or without 17P (>84%). These observations confirm earlier work showing that maintenance at elevated temperatures impairs maturation, ovulation, and subsequent fertility through the disruption of endocrine processes. In relation to commercial salmon culture, the present results also show that the combination of short-term temperature reduction and hormone therapy provides a means to recover egg production by restoring the ovulatory competence and fertility of thermally challenged broodstock.