Additive effects of advanced temperature and photoperiod regimes and LHRHa injection on ovulation in Atlantic salmon (Salmo salar)

In order to evaluate manipulation of spawning time as a potential means to extend 0+ Atlantic salmon (Salmo salar) smolt production in Tasmania, Australia, female salmon were exposed to a natural/simulated natural (42 S) photoperiod or an advanced (L:D 9:15) photoperiod from the austral summer solstice (20 December) under natural or advanced (similar to 6 degrees C below natural temperature) temperature conditions. In late summer (26 February) injections of a commercial LHRHa preparation or vehicle (propylene glycol) commenced. Regular ovulation checks were conducted and ova were fertilised using milt from LHRHa-injected males held under matching photothermal conditions. Plasma levels of 17 beta-estradiol (E-2) and testosterone (T) were monitored and reproductive success (cumulative ovulation, % fertilisation and % survival to the eyed-egg stage) was recorded. Ovulations commenced first (09 March) in LHRHa-treated fish that experienced advanced photoperiod and thermal regimes whereas sham-treated fish exposed to natural photoperiod and temperature conditions where the last to ovulate (22 May-08 June). Treatment-related sequential changes in the timing of ovulations were reflected by sequential advances in the timing of peaks in plasma levels of E-2 and T. The fertilisation of ova from LHRHa-treated fish that experienced advanced photoperiod and thermal regimes was significantly reduced (similar to 52%) relative to all other treatments (> 80%) but there were no significant treatment-related differences in the survivals of eggs to the eyed stage (similar to 50-90%). Consequently, a maximum advance in the timing of median ovulation of 71 days and commercially acceptable eyed-egg yields were generated, demonstrating that combinations of photoperiod, thermal and hormone treatments may be employed to significantly extend spawning and thereafter increase the availability of 0+ smolts for grow-out. (c) 2007 Elsevier B.V: All rights reserved.