An experiment was carried out to in two steps (the first was hormonal treatments and the other the larvae grow up phase) to determine the best ontogenic periods for larvae of the Nile tilapia (Oreochromis niloticus) regarding masculinization hormonal treatments with 17 alpha-methyltestosterone (MT) by immersion baths in 2.0 mg MT.L-1 solution for 36 hours. One thousand and two hundred larvae of Nile tilapia, from the same reproduction bunch, were allotted to 24 plastic containers with 0.5 L in a completely randomized experimental design composed of six treatments and four replicates. An experimental unit consisted of a 0.5 L plastic container with hormonal solution and 50 larvae. Treatments consisted of immersion of larvae at different ontogenic phases, 133.7 UTA (days - degrees) or 5 DAE (days after eclosion); 188.2 UTA or 7 DAE; 242.3 UTA or 9 DAE; 296.8 UTA or 11 DAE; 352.6 UTA or 13 DAE; and 408.2 UTA or 15 DAE. After the treatments the larvae were cultivated till they reached a standard length of 25 mm. Masculinization results showed 15 DAE or 408.2 UTA as the best period, with 85.19% males. Results also suggested a positive linear relationship between the ontogenic period and masculinization rates. There was no difference in treatments with regard to average final length and weight, condition factor and mortality.
androgyny; immersion baths; ontogenesis; Oreochromis niloticus; sexual reversion; tilapia
