Ornamental Fish Production and Management (1+1)

11.1.5 Hormonal induction of sex reversal

Sexuality in fish has a great role to play in aquaculture as there are differences in growth rate, body form and external characters between males and females in several species. The sex determining mechanism of some of the ornamental fishes is given below:

I. Hermaphrodites – Guppy, sword-tail, fighter fish

II. Polygenic determination of sex – sword-tail

III. Sex chromosomal sex determination

a) XX female – XY male – Goldfish

b) ZZ male – WZ female – mosquito fish

c) WXY – XY, YY – males

XX, WX – females – platy

The process of sex differentiation in teleost is protracted and labile rendering the hormonal induction of sex reversal possible in gonochoristic and hermaphroditic species. The induction involves administration of optimum dose of a sex steroid (e.g. 17α-methyl testo-sterone) during the labile period, which reverses the phenotypic expression of a genetic female into a male but the genetic male remains a male. The process of endocrine sex reversal comprises the entire sequence of differentiation, gametogenesis, ovulation/spermiation, courtship behaviour, secondary sexual characters and sex pheromone production. In most teleosts males grow faster than female (e.g. cichlidae); males of most ornamental fish are more colourful than females and thus have a higher commercial value. Ferminization has been attempted in 30 species to understand the process of sex differentiation and for the development of broodstock for all male or all female population. For example, hormonal sex reversal was induced to produce XY females in Poecilia reticulata which when crossed with normal males (XY), sired 25% homogametic (YY) males.

As many as 31 natural and synthetic steroids have been tested for sex reversal in economically important species. In general, the optimum dose required to induce complete sex reversal is species specific and in some species, strain specific. Among androgens, 17 α-methyltestosterone and estrogens, estradiol 17 α B was widely used for brooding (viviparous) poecilids higher dose of 300-500 mg per kg diet is required.

In the direct hormone sex reversal the hormones are administered to the fish in their feed. Almost any feed can be used as the hormone carrier provided it has a sufficient nutritional value for fry and is readily consumed. Simple pellet feeds made locally are suitable, provided that they have a high enough protein content (30-40%). It is not the concentration in the feed that is the determining factor, but rather the actual quantity of hormone consumed; the later is dependent on the feeding rate and how well the fry consume the hormone feed mixture. The treatment is easy to manage and carry out in farms. Generally, the hormones are administered for only a 40-60 day period, rarely 100 days. The hormones are reduced to unmeasurable quantities within 5 days after their withdrawal from the feed. These hormones are also very similar or identical to the naturally occurring hormones. If used at the correct levels they will cause no adverse changes in the fish under treatment.

In recent years, biotechnological research has led to the development of three additional breeding programmes that can be used to increase yields. One of the most common breeding programmes in aquaculture is the production of sex-reversed brood stock to produce mono sex populations for grow-out. This is done either because one sex is superior or more desirable or to prevent reproduction during grow out.

The production of sex-reversed broodstock is usually accomplished by feeding either estrogens or androgens (sex hormones) to sexually undifferentiated fry to sex-reversed them. Sex-reversed fish are individuals that are one sex phenotypically but the other genetically (XX O) is as example phenotypically male genitically female. If sex reversal is done properly, sex-reversed fish are capable of producing monosex populations for grow-out. The type of hormone used estrogens to produce sex reverse females or androgens to produce sex-reversed males depends on the sex determining system of the species and whether you want to produce an all male or an all female population.