Fish Physiology and Biochemistry 28: 503–504, 2003. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 503 The effect of photoperiod on the reproductive performance of the Nile tilapia, Oreochromis niloticus A. Campos-Mendoza, N. Bromage and B.J. McAndrew Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland, UK (E-mail: ac7@stir.ac.uk) Key words: aquaculture, broodstock management, reproduction, seed production Abstract The reproductive performance of O. niloticus was evaluated in four different photoperiods. Results suggested that long days were the most suitable for egg production and reduction of inter-spawning interval. Introduction Low fecundity and asynchronous spawning behaviour are the most important constraints on tilapia culture (Coward and Bromage 2000). Photoperiodic manip- ulation is applied in several fish species in order to control their reproduction (Bromage et al. 2001). In tilapias the effect of photoperiod on reproduction is poorly understood, however, it has been reported that photoperiod could play an important role on tilapia reproduction (Ridha and Cruz 2000). The aim of the present work was to understand and develop pho- toperiodic manipulation of O. niloticus broodstock in order to improve egg production and inter-spawning interval. Materials and methods 32 sibling female O. niloticus were exposed to four trials (6L:18D, 12L:12D, 18L:6D and 24LL) for 180 days, eight fish per trial. Fish were fed twice daily. The reproductive performance was evaluated in terms of number of spawns, total fecundity and inter-spawning interval (ISI). Fish biometry was recorded and spe- cific growth rate (SGR) evaluated. This experiment was carried out in a closed recirculation system with mean light intensity of 530 lux and water temperature of 27 ± 1 ◦ C. Results No significant differences were found between the group mean weight and length at the beginning or end of the experiment. However, significant differences were found in the SGR (P < 0.05) between groups and the reproductive performance shown in Table 1. Discussion Our results showed that 18L:6D was the most cost effective way of enhancing egg production. This trial gave the highest fecundity and number of spawns, sug- gesting that these fish increased oogonial proliferation as a reproductive strategy, as well as a reduction in ISI. The SGR demonstrated that those fish reared un- der shorted photoperiods (6L:18D and 12L:12D) had the highest increase in weight but the lowest fecund- ity and number of spawns as well as the longer ISI. Results were similar to those reported by Ridha and Cruz (2000) who observed that seed production was enhanced more by extended photoperiods rather than