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Photoperiod-induced phase-shifts of the endogenous clock controlling reproduction in the rainbow trout: a circannual phase-response curve.

Different groups of winter-spawning female rainbow trout that had been maintained under seasonally changing daylength and temperature were exposed to 2 months of continuous light at different times of the year. The same photoperiod produced advances in the time of spawning of up to 232 days and delays of up to 80 days, depending upon the timing of exposure in relation to the phase of the reproductive cycle. The proportion of fish spawning in each group varied from 18% to 100%, again dependent on the timing of exposure to continuous light. The photoperiod-induced changes in spawning time can be interpreted as phase-dependent phase-shifts of an endogenous circannual clock controlling maturation. It is proposed that long days, occurring earlier or later than they would under a natural photoperiod, were perceived as indications that the clock was running slow or fast, thus initiating corrective forward adjustments (advance phase-shifts) or backward adjustments (delay phase-shifts), respectively. Collectively, these responses can be described in the form of a circannual phase-response curve.