SEX REVERSAL OF TILAPIA Ronald P. Phelps Department of Fisheries and Allied Aquacultures Auburn University, Auburn, Alabama 36849 United States Thomas J. Popma Department of Fisheries and Allied Aquacultures Auburn University, Auburn, Alabama 36849 United States Phelps, R.P. and T.J. Popma. 2000. Sex reversal of tilapia. Pages 34–59 in B.A. Costa-Pierce and J.E. Rakocy, eds. Tilapia Aquacul- ture in the Americas, Vol. 2. The World Aquaculture Society, Baton Rouge, Louisiana, United States. ABSTRACT is compensated for the high survival of fry as the result of the large size at hatching with a large yolk Early maturation and frequent spawning are reserve and the mouth brooding maternal care given management challenges when working with tilapia. until the hatchlings are 10 mm or larger. The low Male tilapia are preferred for culture because of their fecundity is also compensated for by frequent faster growth. Of the various techniques that have spawning of these asynchronous species where the been developed to provide male tilapia for culture, low fecundity per spawn may be parlayed into a sex reversal is the most commonly used procedure. yearly quantity of eggs/kg equal to many group syn- Recently hatched tilapia fry do not have developed chronous spawning species. gonads. It is possible to intervene at this early point in the life history and direct gonadal development Ideally a fish species used in aquaculture will to produce monosex populations. Exogenous ste- not reproduce in the culture environment before roids given during the gonadal development period reaching market size. From this perspective tilapia can control the phenotype overriding the expression present some challenges to the fish culturist. Most of the genotypically determined sex. This process species of tilapia under favorable growth conditions is commonly referred to as sex reversal. Androgens will reach maturity within 6–8 mo of hatching at a direct the development to males and estrogens to size often less than 100 g. Under favorable condi- females. Methyltestosterone is the most commonly tions they will continue to reproduce, the offspring used androgen to direct the sex of tilapia. Various competing with the initial stock for food, resulting protocols regarding dose rate and treatment dura- in stunted growth and unmarketable fish. Swingle tion have been evaluated. All depend on hormonal (1960) found that in 169–196 d culture cycles of treatment with sexually undifferentiated fry. Fry may mixed sex Mosambique tilapia, Oreochromis be obtained by partial or complete harvests of spawn- mossambicus, production exceeded 3,000 kg/ha but ing containers. Containers used for tilapia spawn- >90% of the harvest was composed of fish <100g ing include indoor and outdoor tanks, earthen ponds Verani et al. (1983) produced 4,944 kg/ha of mixed and fine mesh net enclosures (hapas). When fish are sex Nile tilapia (O. niloticus) in 11 mo but the aver- treated from the beginning to end of the gonadal age weight harvested was <100g. High yields and differentiation period with a proper dose of andro- efficient nutrient utilizations are meaningless unless gen the resultant fish population will be highly a significant portion of the production is market- skewed to males. able. Tilapia have numerous advantages as an aquac- ulture species (Teichert-Coddington et al. 1997) but the ability to reproduce in the production setting has INTRODUCTION resulted in various techniques being developed to control unwanted reproduction. Tilapia are a paradox in reproduction. The rela- tive fecundity of the Oreochromis species is low, Different techniques including stock manipula- 6,000–13,000 eggs/kg/spawn (Siraj et al. 1983). This tion (Swingle 1960), polyculture of tilapia with 34 Phelps and Popma predatory fish (Lovshin 1975) and monosex culture of rainbow trout Salmo gairdneri, goldfish Carassius (Shell 1968) have been described to control tilapia auratus, Zebra fario (Yamazaki 1976), grass carps overpopulation. The use of a predator does not pre- Ctenopharengodon idella (Stanley 1976) and tila- vent reproduction but can prevent recruitment. Ti- pia (Clemens and Inslee 1968; Nakamura and lapia yields are often low, reduced by the slower Takahashi 1973). growing females, and often because the predator has lower tolerance to poor water quality than tilapia, Although commonly referred to as sex rever- forcing the producer to limit nutrient input in order sal, this term requires explanation. Hormone treat- to maintain adequate water quality for the predator ment does not alter the genotype of the fish but species. directs the expression of the phenotype. A treated population of fish may be phenotypically mono-sex All male culture of tilapia is preferred because but genetically will have remained the same as de- of their faster growth (Guerrero 1975; Shelton et al. termined at the moment of fertilization. As a result 1978). Several techniques have been adopted for of hormone treatment, it is possible to have pheno- production of monosex (all male) tilapia: manual typical male fish which are genetically female or sexing (Guerrero 1982); hybridization (Hickling phenotypically female fish that are genetically male. 1960); genetic manipulation (Pandian and Varadaraj For convenience in the rest of this chapter the alter- 1988); and sex reversal through sex hormone ad- ing of the phenotype by administration of sex hor- ministration (Yamamato 1951; Clemens and Inslee mones will be referred to as sex reversal. 1968; Shelton et al. 1978; Guerrero 1982). Production of all male population through ad- Monosex culture by either manually or mechani- ministration of androgen (17-" methyltestosterone) cally selecting males results in half of the potential is considered to be the most effective and economi- fish seed being rejected. Popma et al. (1984) devel- cally feasible method for obtaining all male tilapia oped an efficient system to produce hand-sexed fin- populations (Guerrero and Guerrero 1988). Follow- gerlings but 30% of a farm’s acreage had to be ing the techniques outlined by Popma and Green devoted to broodfish and fingerling production to (1990), less than 15% of a farm’s acreage would need support the remaining 70% in food fish production. to be devoted to broodstock and fingerling produc- Fingerlings were reared to 20–30 g for sexing, pro- tion to support the remaining acreage in food fish. ducing the equivalent of 4,000 kg/ha/y of females, most of which were discarded. Such efficiency and simplicity in production techniques has resulted in hormone sex reversal be- Selective crosses of tilapia using a homogametic coming the commercial procedure of choice to pro- male of one species crossed with a homogametic duce male tilapia fingerlings and has been a female of another has resulted in all male hybrids significant factor in the rapid growth of the tilapia (Lovshin 1982). This approach, first reported by industry. Hickling in 1960, became a common method to pro- duce males, but was largely replaced by the mid- 1980’s. Difficulties in maintaining 2 pure lines of MASCULINIZATION broodfish and keeping them separate, and the space required contributed to the decline. In addition ap- Male tilapia are the desired sex as they grow parent autosomal influences affected sex ratios of- faster and divert less energy into reproduction. A ten resulting in populations that were less than 100% number of types of chemicals have been used to males even if pure lines were maintained. control sexual development in tilapia. Steroids are a group of lipids with several unique properties af- The use of hormones to alter the sex ratios of fecting growth and development. Steroids are called fish was first demonstrated in species other than ti- androgens if they are able to induce male character- lapia. Yamamato (1951) concluded that sex hor- istics and estrogens if they induce female character- mones, in addition to modification of secondary sex istics. Androgens have 2 physiological actions: (1) characteristics, also affect the gonads. Androgen in- androgenic activity, promoting the development of duced masculinization and estrogen resulted in femi- male sex characteristics; and (2) anabolic activity, nization. He produced 100% female medaka (Oryzai stimulating protein biosynthesis. Androgens can be latipus) with an estrogen in 1951 and a nearly all classified into 2 groups: androstane derivatives, hav- male population with an androgen in 1954. The gen- ing both androgenic and anabolic properties and 19- eral technique has successfully altered the sex ratio nor-androstane derivatives, having anabolic 35 Sex Reversal of Tilapia properties but being only weakly androgenic effective in a number of different species of tilapia (Camerino and Sciaky 1975). From a sex reversal and under a variety of management scenarios perspective, and rostane derivatives are of more (Table 1). Other synthetic androgens have been in- value because of their potential to direct the sexual corporated into the diet of tilapia for sex reversal development of fish into males. When evaluating an are given in Table 2. androgen for sex reversal by oral administration of hormone 3 main criteria for selection should be con- None of these androgens can be considered the sidered: metabolic half-life, androgenic strength and best for tilapia sex reversal. The range of dose rates solubility in water. and treatment protocols make it difficult to compare compounds. One perspective is to evaluate these an- Testosterone is the principal androgen secreted drogens based on the local costs to produce a given by the testis and the main androgenic steroid in the number of fish. Phelps et al. (1992) discussed how plasma of human males (Murad and Haynes 1985). fluoxymesterone was more expensive than methylt- It is often used as the standard to evaluate the an- estosterone but the effective dose was lower, thus drogenic properties of a steroid. It is ineffective when compensating for the cost difference. Selection of given orally and has a short duration when given by an androgen to be incorporated into the feed should injection due to rapid hepatic metabolism.