Circadian variation in circulating levels of melatonin, testosterone and estradiol-17β in a tropical freshwater catfish, Clarias batrachus, during different phases of the annual reproductive cycle

In the present investigation, daily variations in plasma levels of melatonin, testosterone and estradiol-17 beta were analysed in the tropical freshwater catfish, Clarias batrachus during early and late-preparatory, pre-spawning, late-spawning and postspawning phases of its annual reproductive cycle. Plasma melatonin levels exhibited a significant circadian rhythm during all the reproductive phases but peaks were invariably at night. The amplitude of melatonin was high during the early-preparatory phase and low in the pre-spawning phase. Testosterone also displayed significant rhythm during all the studied phases except post spawning phase. Estradiol-17 beta revealed diurnal rhythm only during preparatory and pre-spawning phases. The peak in the levels of plasma testosterone and estradiol-17 beta were recorded during the photophase. During the late-preparatory phase, the rhythm of testosterone coincided with the rhythm of estradiol-17 beta.