A major problem in tilapia aquaculture is the frequent reproduction of female fish, leading to increased competition for
supplemented food and stunted somatic growth. The feasibility of using photoperiod manipulation to arrest the reproductive
performance of tilapia Oreochromis niloticus was therefore examined. Newly hatched O. niloticus were reared in the laboratory
under 12L:12D photoperiod at 28 8C. Fish (230–340 g) were maintained under 6L:6D, 12L:12D and 14L:10D photoperiod at
28 8C during the course of this study. Effect of photoperiod manipulation on reproductive parameters of fecundity, gamete
quality, offspring viability and overall reproductive success were evaluated. Steroid levels (estradiol-17h, E2; testosterone, T)
during the spawning cycles of fish were analyzed on days 1 and 3 postspawning and at 3-day intervals thereafter. A total of 72
female fish from each photoperiod treatment were investigated the changing pattern of E2, T and GSI with fish that have
spawned once, twice and three times. Fish exposed to 12L:12D and 14L:10D photoperiod spawned successfully throughout the
study. Whereas the spawning of fish exposed to 6L:6D photoperiod was arrested after three to four spawning cycles. The
arrestment of spawning in fish exposed to 6L:6D photoperiod was paralleled by a significant decrease in plasma levels of E2
(Pb0.05). By contrast, there was no major difference in T levels among the treatments. These findings suggest that photoperiod
manipulation can be used to arrest the spawning in tilapia O. niloticus. The utility of this in controlling the problem of
overcrowding due to excess offspring in tilapia aquaculture is applicable.