Constant photoperiod regimes and the entrainment of the annual cycle of reproduction in the female rainbow trout (Salmo gairdneri).

The effect of constant photoperiod regimes on the timing of maturation and the associated changes in serum oestradiol-17 beta (E2), testosterone (T), and total calcium (Ca2+, as an index of vitellogenin) were investigated in virgin female rainbow trout (Salmo gairdneri). From mid-January four groups of fish were maintained on constant long days (18L:6D) and one group on constant short days (6L:18D; group A). On May 8 the photoperiod of three of the groups on long days was abruptly reduced from 18L:6D to 6L:18D (group B), to 10L:14D (group C), or to 14L:10D (group D); Group E remained on 18L:6D throughout the experiment. The reduction in photoperiod per se appeared more important than the magnitude of the reduction in the advancement of the time of spawning with the first fish in groups B, C, and D spawning on September 3, 13, and 24, respectively; spawning of the remaining fish in each group was spread over 6-week periods. In contrast, spawning in group E began on October 24 and was spread over a period of 12 weeks. Spawning in group A was delayed until the following March. Although a long photoperiod earlier in the year was clearly necessary for the advancement of spawning, as evidenced by the delay in maturation under constant short days, no clear increases in E2, T, and Ca2+ were detected until after the reduction in photoperiod. Thereafter, the sequence of changes in serum E2, T, and Ca2+ levels was similar in all groups although the timings of these alterations were modified in relation to the time of spawning. Thus, peak levels of the three serum parameters were attained earliest in the year by fish in group B followed by those in groups C, D, E, and A, respectively. Collectively, these data support the hypothesis that photoperiod entrains an endogenous rhythm of maturation rather than having a direct, driving influence on the control of reproduction.