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.

The knowledge of critical sites of interaction between pollutants and fish is a major means of investigation in order toExposure of fish to pollutants may occur at different levels of the reproductive cycle. The biosynthesis of steroid hormones provides enzymatic targets for xenobiotics, particularly the steps catalyzed by cytochrome P450-dependent enzymes. Although xenobiotic-metabolizing cytochrome P450s and steroid-metabolizing cytochrome P450s correspond to quite different members in the P450 super-family, it may be hypothesized 153 Marine Environ. Res. 0141-1136/92/$05.00

Ovarian early postovulatory development and oestrogen production were studied in rainbow trout from a spring-spawning strain, 5 to 50 days postovulation (PO). Early postovulatory development was assessed by the ovarian development index, corresponding to the relative volume occupied in the ovary by oocytes in an advanced stage of differentiation (containing cortical alveoli). This index was shown to be related to the maximal oocyte diameter and to the time since ovulation, as well as to metabolic criteria. In contrast, gonadosomatic index was not correlated with any of these, confirming its unreliability for assessing the gonadal development, at least at this stage. Size of follicles and presence of vitellus (lipoprotein) in the oocytes of several animals indicate that ovarian development was more advanced in our females than in other studies. It is suggested that young follicles start developing before ovulation of the preceding batch of oocytes. Oestradiol (E2) plasma levels were low but detectable, and showed, in some females, peak values (max 5.5 ng/ml) within the first month PO. Besides, in vitro metabolism was studied using two isotopes of androstenedione as precursors. No sharp discrepancies were observed between the two types of incubation. Conjugation of the precursor or its metabolites was high but decreased with time postovulation. This suggests that it proceeds from postovulatory follicles. E2 production potential was demonstrated from the shortest time postovulation. Moreover, it increased with ovarian development, suggesting that E2 is synthesized in developing follicles.