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 17alpha-methyltestosterone (MT) thr ough the diet were investigated. The tested factors were temperature ( 22-38-degrees-C), photoperiod (darkness: light, 0 h:24 h to 24 h:0 h), stocking density (100-2,000 fry/m2), feeding rate (10 or 25% body wei ght/d), supplementation of the medium with a growth promotor (thyroxin e), and storage conditions of MT and MT-treated feed. Survival, growth , and sex-reversing potency of MT were significantly influenced direct ly or indirectly by temperature, feeding rate, photoperiod, and storag e conditions of both MT and MT-treated feed. Stocking density and supp lementation of thyroxine in the rearing medium influenced survival and growth, without modifying the sex-reversing potency of MT, when the t reated fry were fed ad libitum. Photoperiod and low rearing temperatur e did not directly alter MT-induced sex reversal but did affect sex re versal indirectly by inhibiting MT-treated feed intake. Among tested f actors, effect of high temperature (38-degrees-C) was paradoxical: hig h temperature induced feminization instead of masculinization. Feminiz ation may result from aromatization of excess androgen to estrogen or the combined effects of aromatization and inhibition of in vivo androg en synthesis. Methyltestosterone was stable and active when it was sto red in a darkened desiccator at room temperature. All fry fed MT-treat ed feed stored in a darkened desiccator at 4-degrees-C developed into males. It is suggested that all of the above factors, as well as genet ic factors, purity and dosage of hormone, solubility of hormone in sol vent, commencement and duration of treatment, and salinity of the rear ing water, must be taken into account for successful production and cu lture of all-male tilapias.