Sex reversal in the genus Oreochromis: optimization of feminization protocol

Abstract. High rate of feminization was achieved by oral administration of diethylstilbestrol (DES) and ethynyloestradiol (EES) to gonadally undifferentiated Oreochromis mossambicus (L.) and O. mossambicus × O. urolepis hornorum Trewavas all-male hybrid. All-female groups were produced at doses > 10mg/kg DES and >50mg/kg EES, when fry immediately after yolk-sac absorption were fed hormone-treated feed ad libitum. The critical period for feminization was about one week. Treatments longer than 10 days did not result in a higher proportion of females, and treatments starting later than one week after yolk-sac absorption were not effective. Optimal derived protocol was applied to O. aureus (Steindachner), resulting also in complete feminization.