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¹ Georg-August University, Department für Agrarökonomie und Rurale Entwicklung, Göttingen, Germany DOI : 10.4194/trjfas.2010.0404 Viewed : 4584 - Downloaded : 3465 This study investigated the effects of stocking density and photoperiod manipulation in relation to plasma estradiol-17β (E2) profile to enhance spawning activity in female Nile tilapia using F1 clonal crosses. The fish were divided into experimental and control groups and subjected to a combination of stocking density and photoperiod treatments (40 kg/m3; 6L:18 D, 40 kg/m³; 12 L:12 D and transferred into single compartments at 12 L:12 D; 14 kg/m³; 12L:12D, respectively). Blood samples were taken by caudal puncture from both experimental and control fish for estradiol profile analysis. Results showed that experimental fish exhibited significantly higher number of spawns per day, total and relative fecundity, hatching and swim-up rates (P<0.05). Regression analyses revealed a significant positive relationship between fish size (body weight), and total and relative fecundity (P<0.001). It was also revealed that E₂ levels demonstrated a pattern based on completed reproductive cycle in the experimental fish. The study therefore established that a combination of stocking density and photoperiod treatments can be adopted to manipulate the timing of spawning activity in female Nile tilapia without having adverse effect on other reproductive parameters such as egg qualities and fecundity. Findings further suggested that the effects of exogenous factors on manipulation of spawning activities of female Nile tilapia may be achieved as a result of hormonal changes including E₂ levels. Keywords : Stocking density, Photoperiod, Spawning, Estradiol levels, Nile tilapia