Aquaculture, 61 (1987) 317-321 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 317 zyxwvutsr Technical Paper zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED Effect of Feeding High Levels of Androgens on the Sex Ratio of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED Oreochromis aureus ROBERT B. McGEACHIN*, EDWIN H. ROBINSON and WILLIAM H. NEILL Department of W ildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843 (U.S.A.) *Present address: 4101 Aspen Street, Bryan, TX 77801 (U.S.A.) (Accepted 24 September 1986) ABSTRACT McGeachin, R.B., Robinson, E.H. and Neill, W.H., 1987. Effect of feeding high levels of androgens on the sex ratio of Oreochromis aureus. Aquaculture, 61: 317-321. An experiment in which Oreochromis aureus fry were fed ethynyltestosterone (ET) at levels of 60,90,120, and 240 mg/kg of feed for 22 days at 27 to 32°C produced 100% male fish. Feeding methyltestosterone (MT) at levels of 60, 90, and 120 mg/kg of feed produced 96-99% males. Accidental overdosing of 0. aureus fry with ET or MT (to the extent of 300% of recommended doses) should not hamper practical-scale production of monosex males. INTRODUCTION Some commercial producers of tilapia feed the fry a diet containing male hormone prior to and during the period of sexual differentiation in order to convert genetic females into phenotypic males ( Rothbard et al., 1983 ) . Use of the resulting monosex male stocks eliminates unwanted reproduction in pro- duction ponds and takes advantage of the faster growth rate of males. Two forms of testosterone are in common use for “sex-reversing” tilapia fry; these are 17-alpha-ethynyltestosterone (ET) and 17-alpha-methyltestoster- one (MT). Each form has a recommended dosage level that produces nearly all male zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Oreochromis aureus: 60 mg/kg of feed for ET and 30 mg/kg of feed for MT ( Guerrero, 1975; Shelton et al., 1981) . Feeding ET at 60 mg/kg feed for 3 weeks has consistently produced stocks of 0. aureus comprised of 98% or more males ( Stickney, 1979). Deviation from optimal dosage rates can lessen the proportion of males pro- duced. Guerrero (1975) reported that, for 0. aureus, levels of ET lower than 60 mg/kg produced significantly lower percentages of males and that levels of MT both lower and higher than 30 mg/kgproduced fewer males than 30 mg/kg. 0044-8486/87/$03.50 0 1987 Elsevier Science Publishers B.V.