FRY PRODUCTION, GONADAL SEX DIFFERENTIATION AND SEX REVERSAL IN TILAPIA HORNORUM TREWAVAS

Gonadal sex differentiation studies of Tilapia hornorum at ambient water temperature (21-31(DEGREES)C) indicated that ovarian differentiation occurred in 15 to 24 mm (16 to 25-day-old fry) while testicular differentiation was not observed until the fry were 18 to 28 mm (19 to 28 days old). No testis-ova were observed.; Two synthetic androgens, ethynyltestosterone (ET) and methyltestosterone (MT), and one synthetic estrogen, ethynylestradiol (EE), in combination with methallibure (ME), a chemical antigonadotropic agent, were administered through the diet to gonadally undifferentiated fry. The hormones were administered at the following rates: ET at 30 and 60 mg/kg of diet was fed for 21 and 28 days; MT at the same concentrations was fed for 18, 21 and 28 days; and EE and ME diets at different combinations was fed for 35 and 49 days. Gonadal examination showed that all androgen treatments yielded over 92% males. All-male populations were produced only by MT-30 at 21 and 28 days, while a few females were found at its higher dosages or reduced treatment durations. Only one estrogen treatment, EE-50 + ME-150 for 49 days, significantly altered the sex ratio. Urogenital papillae were more easily feminized but were not effective indicators of successful estrogen sex reversal.; Gonadal examination of progeny from pair spawns and random matings indicated that although the individual sex ratio of the Auburn stock of T. hornorum may differ statistically from 1:1, the population sex ratio approximates 1:1 but with a tendency toward a slightly greater number of males. Male homogamety was suggested by 44% of the spawns from progeny tests but others gave sex ratios that would not support this hypothesis.; Fry production tests showed an inverse relationship between broodstock stocking density and the number of fry/female; but a direct relationship between broodstock stocking density and number of fry/m('2).