Abstract.An investigation was undertaken to study the effect of 17α‐methyiteslosterone (17‐MT) on the sex reversal and growth of tilapia,Oreochromis spilurus(Günther), in brackish water (3–5 ppt) and sea water (40 ppt). Seven experiments were performed; four in brackish water and three in sea water. Yolk‐sac fry (1 or 3 day age) were immersed (7 or 24 h/day) in water containing hormone at a concentration of 0–10mg/l for 4–25 days. The swim‐up fry were subsequently fed either normal or hormone‐impregnated diets (30–70 mg/kg) for another 35 days starting from first feeding. The growth and SGR of fry were better in groups which received hormone both in water and food as compared with water or food alone. Immersion In 5–10 ppm (24 h/day) brackish water solution for 5 days did not bring any change in growth or sex ratio. However, immersion in sea water solution for 25 days (7 h/day) at a concentration of 1 ppm induced positive growth but no change in sex ratio. Higher doses either did not affect or reduced growth and SGR but induced higher percentages (77–83) of males. Complete masculinization (100) was observed only in brackish water when fish were immersed in 2·5 ppm solution for 4 days (24 h/day) and later fed 50 mg/kg in diets for 35 days. These results were further confirmed when the immersion schedule was modified to 10 days (7 h/day) and feeding 50–70 mg/kg hormone. It can be concluded from these studies that in brackish water complete masculinization ofO. spilurusis possible through immersion in 5·0 ppm for a minimum of 75 h followed by feeding with 50 mg/kg food for a minimum of 40 days
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