A non-paradoxical dose response to 17α-methyltestosterone by Nile tilapia Oreochromis niloticus (L.): effects on the sex ratio, growth and gonadal development

Commercial tilapia production is dependent on monosex culture, commonly obtained through the inclusion of an androgen in the diet for a brief period soon after hatch. To determine a minimum effective dose and identify the problems associated with over-dosing, Nile tilapia Oreochromis niloticus fry were fed diets containing methyltestosterone (MT) at rates up to 1200 mg kg⁻¹ of diet for 28 days. The minimum effective dose for ≥95% males was 14 mg MT kg⁻¹ diet. Percent phenotypic males increased as the rate increased from 3.75 (80%) to 30 mg kg⁻¹ MT (99%). Methyltestosterone given at rates of 120 mg kg⁻¹ or more reduced efficacy but did not result in a reduced frequency of males relative to that of non-treated fish. The term ‘paradoxical feminization' does not adequately describe the observed sex ratios, where no fish were feminized but the efficacy of MT at high doses to masculinize females was reduced. At 1200 mg MT kg⁻¹, the frequency of females (48 ± 1%) was not different from that in the non-treated population. The mechanism for the reduced efficacy is not clear and is not adequately explained as an aromatization of androgen to oestrogen response.