Sex control and manipulation in fish

Abstract

The basic nature of the greatly diversified sexualities in fish can be considered from the aspects of the control mechanisms, physiological and genetic sexes. The potencies of sex steroids to induce sex reversal are different for each steroid. Functional sex reversal from female to male is carried out by using 17α-methyltestosterone, 19-norethynyltesto-sterone or methylandrosterone. 11-ketotestosterone or androsterone have also been used but the dosage required was higher than those of synthetic androgens. The effective range of methyltestosterone required to induce masculinization in medaka, goldfish and tilapia is 20–30 mg/kg diet and rather wide, in the range 1–100 mg/kg diet, in zebrafish. In rainbow trout, the optimum concentration is quite low, in the range 0.1–3 mg/kg diet. In this species, treatment with more than 10 mg/kg diet induces sterile or hermaphroditic gonads and rarely induces sex reversal. Sex differentiation commences after hatching, either before or after the initiation of feeding. In viviparous fish such as guppy, the female is differentiated prior to parturition, and so the administration of methyltesto-sterone is started before parturition and continued until birth. In rainbow trout the first 8 weeks of the oral treatment are important. Treatment for longer than 60 days may be meaningless. Phenotypical feminization is induced successfully by using estradiol-17β. Estrone, diethylstilbestrol or ethynylestradiol are also used. Treatment with steroids is usually carried out by feeding. In salmon, where male sex differentiation is initiated before feeding commences, other procedures are used, such as immersion of alevins. Chromosome manipulation also has great possibilities for sex control. Artificial gynogenesis in species having female homogamety produces all female fry. Triploidy produces sterile fish in rainbow trout. This can be achieved easily by applying hydrostatic pressure or temperature shock. Particularly fruitful techniques for control of sex in aquacultured fish will involve the combination of steroid treatment with chromosome manipulation.