Optimizing 17α-methyltestosterone dose and exposure time for masculinization of yellowtail tetra (Astyanax lacustris)

The fact that the yellowtail tetra ( Astyanax lacustris ) exhibits sexual dimorphism in growth, with females typically outperforming males, has led to increased interest in sex control strategies for monosex production. This study aimed to refine a dietary masculinization protocol using 17α-methyltestosterone (MT) to induce phenotypic masculinization. Fish were fed diets containing 0, 10, 15, or 20 mg/kg MT for either 30 or 60 days, followed by rearing on a hormone-free diet until 212 days post-hatch. Sex ratio, growth performance, and gonadal traits were then evaluated. Treatment duration significantly influenced all assessed parameters at equivalent MT doses, whereas dose-dependent effects within the same exposure period were limited to body depth and gonadal weight. The 10 mg/kg MT treatment administered for 30 days yielded the highest masculinization efficiency, producing 83.3% phenotypic males. These results demonstrate that lower MT doses combined with shorter exposure periods can efficiently induce phenotypic masculinization in A. lacustris . This protocol provides a practical foundation for future studies aimed at evaluating the feasibility of neomale-based strategies for all-female production in freshwater aquaculture.