Seasonal changes in plasma levels of gonadal steroids of sea bass, Dicentrarchus labrax L.

Levels of plasma testosterone (T) and 11-ketotestosterone (11-KT) in males and plasma 17 beta-estradiol (E2), 17 alpha-20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOH-P), and T in females were assayed by radioimmunoassay at monthly intervals throughout the sexual cycle of sea bass (Dicentrarchus labrax L.). 17 alpha,20 beta-DiOH-P was maintained at low levels (below 1 ng/ml) throughout the year, even during the spawning period (January-March). A bimodal seasonal pattern of plasma testosterone was observed. Plasma T and E2 levels became significantly increased in December (advanced gametogenesis period) and then showed further increases during January and February (first half of the spawning period) in parallel with the growth of the vitellogenic oocytes. Multiple spawnings of individual females were also observed during the spawning period affecting the relative fecundity of the eggs. A possible role of E2 on this behavior is discussed. In males, both plasma T and 11-KT initially increased in November and then showed further increasings during the rest of the period of gametogenesis (December) to reach their peak levels in the first half of the spawning period (end of January). These increased and sustained higher levels of plasma steroids coincided with the presence of spermiating males. A second peak of plasma testosterone appeared at the end of the postspawning period-beginning of the pregametogenesis period (May-June) both in males and females and their possible role with the preparation of the gonad for the next reproductive cycle is discussed.