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 the feminization hormonal treatments by immersion baths during 36 hours with a 2.0 mg solution of estradiol valerate17 (VE).L-1. One thousand and two hundred larvae of Nile tilapia, from the same reproduction bunch were used. Larvae were allotted to 24 0.5 L plastic containers in a completely randomized experimental design with six treatments and four replicates. An experimental unit consisted of a 0.5L plastic container with hormonal solution and 50 larvae. Treatments comprised immersion of larvae at different ontogenic phases, or rather, 175.2 UTA (days - degrees) or 6.5 DAE (days after eclosion); 217.2 UTA or 8 DAE; 273.2. UTA or 10 DAE; 329.0 UTA or 12 DAE; 383.9 UTA or 14 DAE; and 438.1 UTA or 16 DAE. After treatments, the larvae were cultivated until they reached a standard length of 25 mm and thus their results could be estimated. Feminization results were best for younger larvae, with 6.5 DAE or 175.2 UTA, with 39.20% females. Results also suggested a negative linear relationship between the ontogenic period and feminization rates. There was no difference in treatments regarding indexes of average final length and weight, final condition factor and mortality. It was concluded that 175.2 UTA or 6.5 DAE is the best period for sexual reversion with EV, with 39.20% females.
immersion bath; estrogens; ontogenesis; Oreochromis niloticus; sexual reversion; tilapia
