ABSTRACT
The present study attempted for the first time to explore the importance of photoperiod in the regulation of seasonal ovarian functions in any subtropical major carp. Adult Indian major carp Catla catla were transferred to a long photoperiod (LP; LD 16:8) or a short photoperiod (SP; LD 8:16) for 30 days on 4 dates corresponding to the beginnings of 4 reproductive phases in an annual cycle, and responsiveness of the ovary was evaluated by comparison with the gonadal weight (I(G)), relative number of developing oocytes, serum levels of vitellogenin, and the activity of 2 important steroidogenic enzymes, that is, Delta(5)3beta-hydroxysteroid dehydrogenase and 17.beta-hydroxysteroid dehydrogenase, in the ovary of fish in a natural photoperiod. Exposure of fish to LP during the preparatory phase (February-March) resulted in a significant (p < 0.001) increase in the values of vitellogenin and in the activity of both the steroidogenic enzymes but not in the ovarian weight and in the relative number of different stages of oocytes. A more stimulatory influence of LP was noted during the prespawning phase (April-May), when precocious maturation of ovary was evident from a significant (p < 0.001) rise in the values of each studied features of ovarian functions. However, no ovarian response was found when the fish were transferred to LP during the spawning (July-August) and the postspawning (September-October) phases. On the other hand, the SP was found to have an inhibitory influence on ovarian growth and maturation during the prespawning and the spawning phases or to have no influences on ovarian functions during the preparatory and the postspawning phases of an annual cycle. The results of our study provide the first evidence that photoperiod per se plays an important role in the seasonal maturation of ovary in a subtropical freshwater major carp.
