Comparison of conditions for hormonal sex reversal of Mozambique tilapias

Roles of selected factors on sex reversal of fry of Mozambique tilapia (Tilapia mossambica) administered 17∝-methyltestosterone (MT) through the diet were investigated. The tested factors were temperature (22-38°C), photoperiod (darkness: light, 0 h:24 h to 24 h:0 h), stocking density (100-2,000 fry/m<SUP>2</SUP>), feeding rate (10 or 25% body weight!d), supplementation of the medium with a growth promotor (thyroxine), and storage conditions of MT and MT-treated feed. Survival, growth, and sex-reversing potency of MT were significantly influenced directly or indirectly by temperature, feeding rate, photoperiod, and storage conditions of both MT and MT-treated feed. Stocking density and supplementation of thyroxine in the rearing medium influenced survival and growth. without modifying the sex-reversing potency of MT, when the treated fry were fed ad libitum. Photoperiod and low rearing temperature did not directly alter MT-induced sex reversal but did affect sex reversal indirectly by inhibiting MT-treated feed intake. Among tested factors, effect of high temperature (38°C) was paradoxical: high temperature induced feminization instead of masculinization. Feminization may result from aromatization of excess androgen to estrogen or the combined effects of aromatization and inhibition of in vivo androgen synthesis. Methyltestosterone was stable and active when it was stored in a darkened desiccator at room temperature. All fry fed MT-treated feed stored in a darkened desiccator at 4°C developed into males. It is suggested that all of the above factors, as well as genetic factors, purity and dosage of hormone, solubility of hormone in solvent, commencement and duration of treatment, and salinity of the rearing water, must be taken into account for successful production and culture of all-male tilapias