Seasonal profiles of sex steroids in blood plasma and ovarian tissue of Clarias batrachus.

Sex steroids were measured in plasma and ovaries of the catfish, Clarias batrachus, mainly over the course of their annual reproductive cycle. Plasma testosterone levels increased in a stepwise fashion, the first significant increase occurring in the previtellogenic phase (March) and the second in the early vitellogenic phase (May). The levels remained elevated till the early postvitellogenic phase (July) and reached basal levels in the regressed phase (September). Estradiol 17 beta and estrone were at low levels in the previtellogenic phase, increased rapidly in the vitellogenic phase, and attained their peaks in the late vitellogenic phase. Ovarian testosterone levels were very low without any significant change from the late previtellogenic to the postvitellogenic phases (April to August). Estradiol-17 beta and estrone levels in the ovary increased significantly in the previtellogenic phase (April), reached a peak in the vitellogenic phase (June), and declined in the postvitellogenic period (July). Ovarian levels of sex steroids were always lower than plasma levels, indicating low storage and rapid secretion. Gonadosomatic index (GSI) was low in the regressed and previtellogenic phases and increased in the vitellogenic and postvitellogenic phases, with a peak in July. Plasma estradiol-17 beta levels could be correlated with GSI from May to July, and plasma estrone and testosterone levels with GSI in early June and July, respectively.