Aquaculture and Fisheries Management 1990, 21, 391-397 Effect of oral administration of different levels of 17a- methyltestosterone on the sex reversal, growth and food conversion efficiency of the tilapia Oreochromis spilurus (Giinther) in brackish water M. T. RIDHA & K. P. LONE Mariculture and Fisheries Department, Food Resources Division, Kuwait Institute for Scientific Research, Salmiya, Kuwait Abstract. The effect of oral administration of 17a-methyltestosterone (17-MT) to Oreochromis spilurus (Giinther) in brackish water (3-5 ppt) was studied in terms of masculinization, growth, food conversion efficiency (FCE), specific growth rate (SGR) and survival rate. A 38-day hormone treatment followed by 205 days of normal diet yielded a higher percentage of males, differing significantly from the normal 1:1 ratio. The highest proportion of males (90-3%) was in the replicates given 70 mg/kg of 17-MT. After the hormone treatment phase, no effects were observed on growth, FCE, SGR and survival rate. At the end of the experiment (243 days), the treated groups showed significant increases in growth, SGR and FCE. The 50 mg/kg replicates showed the highest growth and SGR, and the 70 mg/kg groups the highest FCE. The maximum survival rate was in the controls, and the minimum in the 50 mg/kg groups. It can be concluded that the androgen-induced sex reversal in O. spilurus is comparable to that in other tilapia species. However, further studies should be carried out to determine the optimum dose and duration to achieve 100% masculinization. Introduction Since tilapia males grow faster than females, the culture of monosex male populations is preferable. The use of male sex steroids to induce sex inversion of genotypic females into phenotypic males has proven to be one of the most successful methods to produce a monosex population (Hunter & Donaldson 1983). The androgen, 17a-methyltestosterone (17-MT) is the most widely used hormone for sex inversion in tilapia. Sex inversion studies using 17-MT have mainly been restricted to three tilapia species: Oreochromis mossambicus (Peters) (Clemens & Inslee 1968; Nakamura 1975; Guerrero 1976; Billy & Liley 1985; Macintosh, Varghese & Satyanarayana 1985; Pandian & Varadaraj 1987; McAndrew & Majumdar 1989), O. aureus (Steindachner) (Eckestein & Spira 1965; Guerrero 1975; McAndrew & Majumdar 1989) and O. niloticus (L.) (Tayamen & Shelton 1978; Owusu-Frimpong & Nijjhar 1981; Nakamura & Iwahashi 1982; McAndrew & Majumdar 1989). 17-MT has also been used to change the sex of O. hornorum (Obi & Shelton 1983) and Tilapia zillii (Gervais) (Yoshikawa & Oguri 1978). Other androgens used for inducing sex inversion in tilapia are 17-ethynyltestosterone (Guerrero 1975; Tayamen & Shelton 1978; Anderson & Smitherman 1978; Shelton, Rodriguez-Guerrero & Lopez-Macias, 1981; Rothbard, Solnik, Shabbath, Correspondence: Dr K.P. Lone, Mariculture and Fisheries Department, Food Resources Division, Kuwait Institute for Scientific Research, PO Box 1638 — 22017 Salmiya, Kuwait. 391