Abstract
The endogenous circannual rhythm of maturation in the female rainbow trout and associated changes in serum oestradiol-17β and vitellogenin (as calcium) can be entrained by abrupt changes in the photoperiod. Groups of virgin fish (which naturally spawn in December) maintained under LD 18∶6 from mid-January (Year 1), and then subjected to a reduction to LD 6∶18 either on 1st March (Group A), 1st April (Group B), 1st May (Group C) or 1st June (Group D), commenced spawning on 31st July, 13th and 30th August, and the 16th September, respectively. Fish maintained on LD 18∶6 throughout the experiment (Group E) commenced spawning on 10th October. These advances in the timing of spawning can be described in the form of a phase-response curve analogous to the entrainment behaviour of circadian oscillators. It is concluded that under natural conditions the annual change in photoperiod serves to continuously entrain the circannual clock thus ensuring that maturation and spawning occur at the optimal season for the survival of the species.
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