MESTANOLONE INDUCED MALE SEX-REVERSED NILE TILAPIA (OREOCHROMIS NILOTICUS) FRY

Abstract

Tilapia (Oreochromis niloticus) is an important freshwater fish in Thailand yielding approximately 250,000 tons production per year. Along with intensive farming, androgenic hormones are applied during the farming process to produce monosex-male tilapia because of its better yield. A monosex population also allows for more effective management of a single crop. A synthetic androgenic steroid, mestanolone or 17α-methyldihydrotestosterone, is used in newly hatched tilapia fry for sex reversal, but its residual level in these fry has not been examined. The effects of exogenous androgenic hormone on sex differentiation was examined in Nile tilapia fry. Male sex reversal was mediated using oral administration of mestanolone at a dose of 80 mg/kg diet for 5, 10, 15, and 20 days. The microscopic examination of fish gonad stained with aceto-carmine was used to determine male and female fish fry at 60 days post hatching. Treatment with mestanolone yielded 100% male after feeding with hormonal diet for either 15 or 20 days, while treatment for 5 and 10 days presented 87% and 90% male, respectively. The results of histological examination revealed no differences in gonadal tissues of hormonal treated fish compare with normal fish. In addition, statistical analysis of the total weight gain among the fish fry revealed that there was no significant difference (P >0.05) between the treated groups and the control group. This study also investigated residual mestanolone after a course of oral administration to tilapia fry at a dose of 80 mg/kg feed for 15 (minimized dose) and 23 (practical dose) consecutive days. The analyses were performed at 1, 2, 3, 5, 7, 14 and 21 days after the last dose using liquid chromatography tandem mass spectrometry. The amounts of mestanolone detected in 15 and 23 days hormonal treated fry on days 1, 2, 3 and 5 after hormone withdrawal ranged from 0.28-3.20 ng/g and 0.29-3.22 ng/g, respectively. Mestanolone was not detectable in fry after hormonal withdrawal for 7 day (limit of quantitation, LOQ, 0.09 ng/g), which suggests that negligible levels of mestanolone will be present in tilapia during the growth stage of 6-8 months after an adequate withdrawal period following treatment of early-stage fry. In conclude, the present study successfully minimized the use of mestanolone in male sex-reversed tilapia to a 15-day period, while maintaining 100% masculinization and having no adverse effect on general fish growth. The residue analysis in this study is important for establishing consumer trust in food safety for hormonally treated tilapia.