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Among the techniques employed to restrict fertility is the application of hormones to produce monosex populations. Some success has been achieved in the development of such mono-sex cultures by the use of androgenic and estrogenic steroids for masculinization of genotypic females and feminization Of genotypic males, respectively (Jalabert et al., 1974; Guerrero, 1975, 1979; Shelton, Hopkins and Jensen, 1978).

Tilapia, a favoured fish for culture in native Africa and elsewhere, is known to attain sexual maturity at an early age and breed repeatedly at short intervals thereafter. This often results in stunted growth due to crowded conditions for energy is expended for reproduction rather than for growth, (Hickling, 1968). Masculinization of genotypic females of three species of tilapia, Sarotherodon (Tilapia) mossambicus, Sarotherodon (Tilapia) nilotica, Sarotherodon (Tilapia) zilli and Sarotherodon (Tilapia) aurea, has been achieved by feeding methyltestosterone and ethynyltestosterone in the diet to fry, and similarly monosex female tilapia have been produced by treatment with estrone, ethynylestradiol and stilbesterol (Hopkins, Shelton and Engle, 1979; Guerrero, 1979), While steroid administration is capable of reversing sex in tilapia, the percentage of fishes showing sex reversal is highly variable. Since the presence of only a few males in an all-female population or vice versa often jeopardizes the very purpose of monosex cultures, it is imperative that nature and amount of sex-reversing substances be reinvestigated to obviate chances of failure.

Similar experiments to produce monosex fishes have been conducted on salmonids (Johnstone, Simpson and Walker, 1979) and several other species (Stanley, 1979). A novel technique for producing sterility by inducing autoimmune gonad destruction in the Atlantic Salmon has been reported (Laird et al., 1978). Since some of the cultivable marine and brackishwater species such as Sparus auratus, Polynemus sexfilis and Lates calcarifer, are protandrous, while a few others like members of genus Epinephelus are protogynous, sex reversal is now receiving attention (see Reinboth, 1980). Chen et al. (1977) have accelerated sex inversion in Epinephelus tauvina to produce male broodstock from 3-year old females by oral administration of methyltestosterone. This technique has great potential in aqua-culture since natural sex inversion can take a Very long time (Moe, 1969; Dipper and Pullin, 1979).

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