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Abstract
Orechromis niloticus fry were effectively sex-reversed to males (>97%) when given a ration containing 60 mg methyltestosterone/kg for 28 days, independent of the length of time fry were confined to hapas (0, 7, 14, 28 days) before being released into outdoor 20-m2 static water tanks. Fish from all treatments were raised an additional 30 days in the tanks and were fed a non-hormone treated feed. After 28 days of hormone treatment, fry stocked directly into the tank at 150/m2 were the largest, with an average weight of 1.9 g. Those held in hapas for 28 days were significantly smaller (0.6 g) than those not held in hapas or held for a shorter period. After a total culture period of 58 days, mean individual weights and survival rates were similar among all treatments. The results of this study demonstrate that it is possible to sex reverse tilapia fry in open outdoor environments and reduce the steps in the nursery phase. Confinement in hapas is not necessary for successful sex reversal, and it limits growth.