Conditions for effective androgen sex reversal in Oreochromis niloticus (L.)

Abstract

An initial study evaluated the effect of dietary administration of the androgen 17 α-methyltestosterone 40 mg·kg −1) on the growth and survival of Oreochromis niloticus fry, held at high density (1000 m −2), in tanks and in nylon, fine-mesh hapas. Treated and control fry were nursed and on-grown in ponds to determine their subsequent growth, survival, sex ratio and recruitment. A second study investigated the effect of increasing stocking density (1000–5000·m −2) on the growth, survival and sex ratio of fry, hormone-treated in hapas, with a higher dose of androgen (60 mg·kg −1). In the first study, the androgen had no significant effect on growth and survival of fry during the treatment period and produced mean sex ratios of 98.4% male in tanks and 95.4% in hapas. Growth was slowest for fry held in tanks, but both these and those held in hapas had significantly lower ( P<0.01) growth rates than the control fish held at low density (250·m −2) in hapas. Compensatory growth, during 14-day post-treatment nursing and early grow-out in ponds, negated these significant differences. Subsequent grow-out in ponds for 112 days yielded some significant ( P<0.05) differences in growth rates, with hormone-treated fish having faster growth than untreated controls. Recruitment was significantly higher in the ponds stocked with the untreated control groups. Contrary to expectation, hormone-treated populations had significantly ( P<0.05) more heterogeneous individual weight distributions at harvest than untreated populations. Gonadosomatic indices although lower in hormone-treated fish, were not significantly different, from those of the controls. In the second study, the higher stocking density (2000–5000·m −2) of fry, hormone-treated in hapas, increased the efficacy of sex-reversal up to 99.4% male but resulted in decreased growth and survival.