Effects of constant short and long photoperiod regimes on the spawning performance and sex steroid levels of female and male sea bass

Under constant short photoperiod, the spawning time of 2-year-old sea bass Dicentrarchus labrax was advanced as compared to controls, whereas spawning s were delayed under constant long photoperiod. High plasma levels of 17 be ta-oestradiol (E-2) and testosterone (T) in females were coincident with th e appearance of vitellogenic oocytes in the ovary, while high levels of 11- ketotestosterone (11-KT) and T in males were coincident with the presence o f spermiating males. Although plasma levels of E-2 in females and 11-KT in males were low during the remainder of the cycle, levels of T were always > 1 ng ml(-1) in both sexes, suggesting that T could play an important role d uring the initial stages of gonadal development. The profiles of E-2 and T in females and 11-KT and T in males exposed to constant short days were sim ilar to those in the control group, but fish which were maintained under co nstant long photoperiods showed a bimodal pattern of these steroids. The re sults obtained from fish exposed to constant photoperiod regimes provide fu rther evidence that an endogenous process could be operating to control the reproduction of sea bass. (C) 1999 The Fisheries Society of the British Is les.