The study was done to examine some aspects of the reproductive biology of Nile tilapia (Oreochromis niloticus) in Tekeze Reservoir a newly man made hydropower reservoir. A total number of 1826 specimens of Nile Tilapia (O. niloticus) were collected from the reservoir from July 2015 to June 2016. Size at first maturity, sex ratio, gonado-somatic index (GSI), breeding season and fecundity were studied. The overall sex-ratio female: Male (F: M) was 1.6:1 which is deviated from the expected 1:1 sex ratio (χ2 = 10.13; p<0.05). Length at first maturity was 14 and 15 cm for females and males respectively. The breeding period of females extended from January to September. Two annual breeding seasons were noticed. A minor breeding season which extended from January to March and a major breeding season from July to September and monthly Condition Factor (CF) (mean ± SD) value of O. niloticus in the reservoir ranged from 1.73±0.03 in July to 2.05±0.02 in November for males and 1.61±0.02 in July to 1.99±0.05 in April for females. Fecundity ranged between 399 to 2129 g. Fecundity correlated well with total weight (r = 0.086), total length (r = 0.77) and ovary weight (r = 0.72) than with gonadal weight (r = 0.86). Nile tilapia at Tekeze Reservoir showed isometric growth pattern (b=2.92), indicating that the reservoir was favorable for the fish growth. For proper management of the fish species in the reservoir, it is better for the fishermen not to capture the fish during the breeding season of the year in the reservoir.
Key words: Reproductive biology, Oreochromis niloticus, Gonado-somatic index, fecundity.
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