The growth performance of three experimental groups consisting of mixed sex fish (control), hormone-treated fish and progeny of YY male tilapia, all originated from the genetically improved farmed tilapia (GIFT) strain was evaluated. Masculinization of sexually undifferentiated fry was achieved by providing a supplement of 5 mg of 17-α-methyltestosterone per kg of feed over a period of 21 days (after sac absorption). Both mixed sex and progeny of YY male groups were fed a standard commercial ration. Mixed sex fish did not deviate significantly (P>0.05) from the 1:1 male to female ratio. Percentages of male averaged 75% in hormone-treated fish and 95% in YY male group over the sampling periods and at final harvest. The effect of sex on weight and length was statistically significant (P less than 0.001). The model used to analyse weight and length included experimental group and sex in each culture period as the fixed effects, and replicate cages as the random effect. Over the culture period of 141 days, there were no statistical differences (P>0.05) in body weight and length between mixed sex, hormonally treated and progeny of YY males. There were also no significant differences in level of variability in harvest weight between three groups of fish when the data were classified into five categories (= less than100, 100 to less than150, 150 to less than200, 200 to less than250 and ≥250 g). It is concluded that monosex culture of all male tilapia would be of no advantage over mixed sex culture for the GIFT strain under conditions of cages suspended in earthen ponds
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