HORMONAL SEX REVERSAL OF TILAPIAS - IMPLICATIONS OF HORMONE-TREATMENTAPPLICATION IN CLOSED WATER-SYSTEMS

The phenomenon of incidental sex reversal was observed in experiments originally designed to determine the importance of feeding duration an d feed quality in the optimization of masculinization and feminization treatments of Oreochromis niloticus (L.) fry by oral application of h ormones, Four separate experiments were conducted: (1) masculinization and (2) feminization using three feeding durations and (3) masculiniz ation and (4) feminization using three alternative feed mixtures. Masc ulinization experiments incorporating 17 alpha-methyltestosterone (MT) into the feed at 40 mg kg(-1) were applied to putative monosex male p rogeny whilst feminization treatments were applied to putative all fem ale diethylstilboestrol (DES) at 1000 mg kg(-1). Treatments were appli ed outdoors in small cages suspended in 900-l circular concrete tanks with a now through rate of 1-l min(-1). Replicate cages for all treatm ents and the controls in each experiment were reared together in the s ame tank. Only the first experiment produced significant differences i n sex ratio, with the control treatment producing a slightly lower pro portion of males than the three MT treatments at different durations. The most significant finding was that in all experiments the sex ratio of the control differed significantly from expectations and in all bu t experiment 1, did not differ from those of the treated fish in which sex reversal had clearly been effected. It appears likely that incide ntal sex reversal occurred in the control fish as a result of the accu mulation of active metabolites from the treated fish and/or leachates from uneaten food given to fish in the same water body. The results ar e discussed in the context of improving the efficiency and safety of h ormonal sex reversal treatments.