The yellowtail tetra (Astyanax lacustris) has recently shown significant production volume growth in Brazil. The variation in body size, which favors female production (Figure 1), has sparked interest in all-female production, which has achieved higher growth rates than traditional methods. All-female progeny can be obtained through hormone-induced sex reversal techniques, administered either through immersion or by diet during the period of histological sex differentiation. An indirect method of hormone sex reversal treatment can be used to produce neo-males (genotypically females-XX that are phenotypically males) to be crossed with normal females (XX). This study aimed to determine the optimal dose for masculinization of Astyanax lacustris using 17α-methyltestosterone.
For this purpose, two trials were carried out using different doses of 17α-methyltestosterone (0, 10, 15, and 20 mg/kg) administered in the feed for 30 days. At the adult stage (+ 4 months post-hatching), 30 individuals from each treatment group were euthanized by immersion bath with eugenol, to measure final body weight and total length. Additionally, their gonads were histologically evaluated to assess the efficacy of the hormone treatments. Total length and body weight did not show significant differences among treatments (p >0.05).
For the individuals from each treatment who had their gonads evaluated, it was possible to identify males, females, intersex, and sterile individuals (Figure 2). All hormone treatments led to a higher proportion of males compared to the control (no hormone) sex ratio. A dose of 10 mg/kg for 30 days was highly effective in masculinizing fish, yielding 83.3% of males. Lower doses may achieve 100% of male offspring. Ongoing research explores distinguishing natural males from neo-males and their reproductive viability