Effects of accelerated photoperiod regimes on the reproductive cycle of the female rainbow trout .1. Seasonal variations of plasma lipids correlated with vitellogenesis

Female rainbow trout were exposed to a simulated natural photoperiod (control group N) and to two accelerated photoperiod regimes (S9 and S6 groups). Early spawning was achieved in both the S9 and S6 groups, coupled, however, with a reduction of mean egg size. To investigate this reduction of egg size, some morphometric parameters, as well as some plasma parameters such as lipids and vitellogenin levels, were regularly measured. Regardless of the photoperiod regime, a preferential utilization of lipids for energy purposes was observed during previtellogenesis and Type I-vitellogenesis, whereas during Type II-vitellogenesis, mobilized lipids were preferentially used for lipoproteins and notably for vitellogenin synthesis. Different patterns of the viscerosomatic index, triacylglycerols, and non-esterified fatty acids were observed during previtellogenesis and Type I vitellogenesis, whereas-similar patterns of all paremeters were observed during Type II-vitellogenesis. From these results it may be concluded that previtellogenesis and Type T-vitellogenesis are photosensitive periods, whereas Type II-vitellogenesis is probably under the control of an endogenous biological rhythm synchronized by the photoperiod. Our findings also demonstrate that the reduction of egg size could be explained neither by lower plasma TG levels nor by lower vitellogenin (Vtg) levels, since both were much higher during Type II-vitellogenesis. (C) 1997 Elsevier Science Inc.