All-male production in Nile tilapia (Oreochromis niloticus) through discrete anastrozole immersion: hormonal and histological insights.

Generating all-male cohorts of Nile tilapia (Oreochromis niloticus) is commercially essential in modern aquaculture, as males exhibit substantially faster growth rates than females. Although oral delivery of the synthetic androgen 17α-methyltestosterone (MT) has long been the prevailing industry method for sex reversal, a range of practical and environmental shortcomings have fuelled the search for alternative strategies. The present study evaluated whether brief, repeated bath immersions in anastrozole, a third-generation, non-steroidal aromatase inhibitor, could reliably induce male sex reversal in Nile tilapia fry. Using a completely randomized design with five treatment levels (0, 1000, 1500, 2000, and 2500 µg/l anastrozole), each replicated three times, 3-day-old post-hatch fry were exposed to 3-h immersion sessions on days 3, 5, and 8 post-hatching, followed by a 120-day grow-out period. A clear and potent dose-dependent masculinizing effect was recorded; the 1500- and 2500-µg/l concentrations each yielded fully male populations (100%), as verified through phenotypic assessment, plasma hormone profiling, and gonadal histology. Aromatase inhibition was confirmed by marked reductions in circulating estradiol alongside significant increases in testosterone. Cortisol showed a transient, dose-dependent rise after the treatment window, yet post-treatment survival throughout the grow-out phase remained excellent, exceeding 98% and showing no statistically significant variation across groups. Histological evaluation revealed characteristic testicular architecture and atretic ovarian regression in all treated fish, in sharp contrast to the controls. The data collectively demonstrate that discrete anastrozole immersion represents an effective, operationally practical, and environmentally preferable alternative to conventional dietary hormone administration for all-male tilapia production.