Endocrine sex control strategies for the feminization of teleost fish

In many species of cultured finfish, females exhibit higher growth rates than males and attain larger sizes. In addition, in some species, males mature before reaching marketable size. Together, this results in a larger dispersion of sizes and an overall reduction in production. Therefore, there is great interest from the private sector to produce all-female stocks. This review concentrates on the use of oestrogens for sex control, discussing the advantages of producing monosex female stocks for finfish aquaculture, and pointing out those cases in which hormonal sex reversal technology is worth applying. The biological basis on which hormonal sex manipulation rests, the process of sex differentiation--which, compared to that of other vertebrates, is quite labile in teleost fish--is described in order to understand the effects of treatments. Sex control is typically achieved by exposing sexually undifferentiated fish to exogenous steroids in order to direct the process of sex differentiation towards the desired sex. These treatments finish months or years before marketing and steroid residues disappear in less than a month after the end of treatment. The currently available methods to produce monosex female stocks, the direct and the indirect methods, are explained, comparing their respective advantages and disadvantages. Feminizing treatments are also used to produce all-male stocks in some species. Thus, this review concentrates on the use of oestrogens for sex control, either in the direct method of feminization or in the indirect method of masculinization. So far, oestrogens have been applied to at least 56 different species, using 12 different oestrogenic substances (three natural and nine synthetic). Special attention is given to the method of administration, including immersion and dietary treatment, and to the variables of the hormonal treatment itself: steroids used, dose, timing and duration of treatments. The importance of correct treatment timing in relation to the degree of gonadal development is emphasized and the outcome of the treatment evaluated in terms of survival, gonadal morphology and sex ratios, growth performance and deformities. Next, the current methods to produce all-female or essentially all-female stocks are presented for 35 different species, including eels, salmonids, cyprinids, poecilids, cichlids, gouramies and flatfishes. A section on regulatory issues discusses the advantages of using the indirect method of feminization, when feasible, and emphasizes the convenience of using the natural oestrogen estradiol-17beta rather than synthetic oestrogens. A guideline for the development of monosex technology in new species is presented. The overall goal is to emphasize the use of the indirect method, which means that fish that reach the marketplace have never been exposed to steroids. If this method is not feasible, as it happens in many species, an alternative is the use of the direct method, applied in an optimized protocol, to achieve maximum treatment efficiency with minimum exposure to steroids.