Use of sex steroids for sex
In certain situations and species, it will be advantageous to restrict
fertility. A well-known example is the cichlid tilapia, which attains maturity
at an early age and breeds repeatedly at short intervals, overpopulating ponds
and other rearing facilities. This results in stunted populations, as energy is
expended for reproduction rather than growth. Among the techniques that can be
employed for restricting fertility is the application of hormones to produce
monosex populations. Androgenic and oestrogenic steroids are used for
masculinization of genotypic females and feminization of genotypic males
(Jalabert et al., 1974; Guerrero,
1975, 1979; Shelton et al., 1978).
Genotypic female fry of the species of Sarotherodon
(= Tilapia), when fed on
methyltestosterone and ethinyltestosterone have become males.
Similarly, monosex female tilapia have been produced by treatment with oestrone, ethinyloestradiol and stilboesterol. While the feasibility of sex reversal by steroid administration has been demonstrated, the percentage of fish that underwent sex change in any treated group varied greatly. Since the presence of even a small percentage of the opposite sex in a population is sufficient to initiate uncontrolled breeding, the value of the results achieved so far becomes less significant. Similar experiments to produce monosex fish have been conducted with salmonids and other species. Sex inversion of the protogynous species of Epinephelus (E. tauvina) has been acceleratedto produce male brood stock from three-year-old females, by oral administration of methyl-testosterone. Production of all-female eggs is now a common practice in a number of rainbow trout hatcheries. The initial functional males required for fertilizing ova from normal female brood stock are obtained by sex reversal, by treating with 17 methyl-testosterone through immersion or incorporation in starter feed in the fry stage.