The Thai Journal of Veterinary Medicine


The effects of an exogenous androgenic hormone on sex differentiation were examined in Nile tilapia (Oreochromis niloticus) and red tilapia (Oreochromis spp.) fry. Tilapia aquaculture in Thailand commonly applies methyltestosterone at the dose of 80 mg/kg diet on the first feeding of fish fry for 21-28 days to achieve male-monosex crop. In this study, male sex reversal was induced with an alternative androgenic hormone, 17α- methyldihydrotestosterone (MDHT), at a dose of 80 mg/kg diet for shorter periods; 5, 10, 15, or 20 days. Microscopic examination of fish gonad stained with aceto-carmine was used to determine male and female fish fry at 60 days post hatching (dph). Feeding supplementation with MDHT for up to 15 days yielded 100% male (n=100), while treatment for 5 and 10 days presented 87% and 90% male in Nile tilapia, and 82% and 88% in red tilapia, respectively. Histological examinations of the hormone-treated fish revealed normal microscopic morphology of tilapia gonadal tissues. In addition, analysis of total weight gain and body length among the fish fry showed no significant difference between the treatment groups and the control fish fed on basal diet. The present study demonstrated male sex reversal in tilapia fry with a decreased hormonal feeding to a 15-day period, while maintaining successful male-monosex and having no adverse effect on general fish growth.



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