Endocrine sex control strategies for the feminization of teleost fish

In many species of cultured finfish, females exhibit higher growth rates th an males and attain larger sizes. In addition, in some species, males matur e before reaching marketable size. Together, this results in a larger dispe rsion of sizes and an overall reduction in production. Therefore, there is great interest from the private sector to produce all-female stocks. This r eview 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 wort h applying. The biological basis on which hormonal sex manipulation rests, the process of sex differentiation-which, compared to that of other vertebr ates, is quite labile in teleost fish-is described in order to understand t he effects of treatments. Sex control is typically achieved by exposing sex ually undifferentiated fish to exogenous steroids in order to direct the pr ocess of sex differentiation towards the desired sex. These treatments fini sh 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 ex plained, comparing their respective advantages and disadvantages. Feminizin g 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 masculin ization. So far, oestrogens have been applied to at least 56 different spec ies, using 12 different oestrogenic substances (three natural and nine synt hetic). Special attention is given to the method of administration, includi ng immersion and dietary treatment, and to the variables of the hormonal tr eatment itself: steroids used, dose, timing and duration of treatments. The importance of correct treatment timing in relation to the degree of gonada l development is emphasized and the outcome of the treatment evaluated in t erms of survival, gonadal morphology and sex ratios, growth performance and deformities. Next. the current methods to produce all-female or essentiall y all-female stocks are presented for 35 different species, including eels, salmonids, cyprinids, poecilids, cichlids, gouramies and flatfishes. A sec tion on regulatory issues discusses the advantages of using the indirect me thod of feminization, when feasible, and emphasizes the convenience of usin g the natural oestrogen estradiol-17 beta rather than synthetic oestrogens. A guideline fur the development of monosex technology in new species is pr esented. The overall goal is to emphasize the use of the indirect method, w hich 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 proto col, to achieve maximum treatment efficiency with minimum exposure to stero ids. (C) 2001 Elsevier Science B.V. All rights reserved.