Season-dependent effect of thermopulse on gonadal recrudescence in the female catfish, Clarias batrachus.

In the present study, the female Clarias batrachus, held under long photoperiod (13L:11D), were exposed to high water temperature either constantly (24 h) and/or in form of thermopulse of 6 h and 12 h durations, separately, at different times of the day/night cycle for six weeks during the early post-spawning and late post-spawning phases of its reproductive cycle. The effects of high water temperature (30 +/- 1 degrees C) on gonadosomatic index (GSI), plasma levels of testosterone (T) and oestradiol-17beta (E2) were observed. During the late post-spawning phase, thermopulse of 12 h duration given in the morning hour increased all the studied parameters most effectively as compared to that given at evening hour of the day/night cycle or even in comparison to the fish exposed to constant high temperature. Thermopulse of 6 h duration given in the morning or noon also raised these parameters compared to the controls, but the magnitudes of stimulation were moderate. However, exposures of the catfish to such photothermal regimes during the early post-spawning phase completely failed to bring any change in the studied parameters. These findings, thus, clearly indicate that treatment with high temperature under long photoperiod may stimulate gonadal activity in C. batrachus, provided given at appropriate season of the year. A diurnal basis of response to high temperature and the existence of a rigid gonado-refractory phase (perhaps just after the spawning) are also evident in the reproductive cycle of C. batrachus.