39 THE EFFECTS OF α 17 -METHYLTESTOSTERONE ON SEX REVERSAL OF RED TILAPIA (OREOCHROMIS NILOTICUS X OREOCHROMIS MOSSAMBICUS) IN THE FREE-SWIMMING STAGE *Supamas Sriwongpuk 1 1 Faculty of Agricultural Technology, Rajamangala University of Technology Thanyaburi, Thailand *Corresponding Author, Received: 14 Dec. 2018, Revised: 30 Dec. 2018, Accepted: 10 Jan. 2019 ABSTRACT: The effects of 17α-Methyltestosterone (MT) on sex reversal of Red tilapia (Oreochromis niloticus x Oreochromis mossambicus) in the free-swimming stage by immersion and feeding techniques were studied at the Department of Fisheries, Faculty of Agricultural Technology, Rajamangala University of Technology Thanyaburi, Pathumthani Province from April to July 2017. The experimental design was a factorial experiment in CRD which studied two factors: 1) sex reversal by immersion treatment of 17α-MT (0, 150 and 300 μg/l) for 3 hours (T 1 , T 2, and T 3 ) and 2) sex reversal by feeding treatment of 17α-MT (40, 60 and 80 mg/kg) for 28 days (F 1 , F 2 and F 3 ). Results from these studies indicated that during the free-swimming stage, the immersion treatment of 17α-MT in 300 μg/l for 3 hours (T 3 ) had the highest growth rate in terms of weight, total length (28 days), ADG, FCR and survival rate (0.731 g, 3.340 cm, 0.024 g/d, 0.627 and 95.443 % respectively). The immersion treatment of 17α-MT in 150 μg/l for 3 hours (T 2 ) had a medial growth rate among the other factors. The immersion treatment of 17α-MT in 0 μg/l for 3 hours (T 1 ) had a minimal growth rate but had the highest sex ratio of males (8 3 . 8 8 9 %). The effects on sex reversal by feeding treatment of 17α-MT in 60 mg/kg for 28 days (F 2 ) had the highest growth rate in terms of weight, ADG and FCR (0.644 g, 0.022 g/d and 0.714 respectively). The effects on sex reversal by the feeding treatment of 17α-MT in 80 mg/kg for 28 days (F 3 ) had a medial growth rate and the highest survival rate (95. 037 %) and sex ratio of males (85.667 %33T). The effects on sex reversal by feeding treatment of 17α-MT in 40 mg/kg for 28 days (FR 1 R) had a good growth rate but the other factors were minimal 33T. Keywords: 17α-Methyltestosterone, Sex reversal, Red tilapia (Oreochromis niloticus x Oreochromis mossambicus), Free-swimming stage 1. INTRODUCTION Tilapia are freshwater fish belonging to the family Cichlidae. They are native to Africa but were introduced into many tropical, subtropical and temperate regions of the world during the second half of the 20th century [1]. Tilapia is a worldwide fish of great commercial importance and it is recognized as one of the most important aquaculture species of the 21 st century. Tilapia is currently ranked second only to carps in global production [2]. The world’s total tilapia production in 2012 was 4.2 million tons. Tilapia aquaculture is rapidly expanding with global production of about 2.8 million metric tons in 2008 [3] and estimated to increase to 8.89 million metric tons by the year 2020 [4]. This rapid global production of tilapia is due to the introduction of improved strains of Nile tilapia (Oreochromis niloticus) (Fig.1) which is the major farmed tilapia species. The important aquaculture species are the nile tilapia (O. niloticus), blue tilapia (O. aureus), Java tilapia (O. mossambicus) and Zanzibar tilapia (O. hornorum). The red tilapia (Oreochromis hybrids) (Fig.2) developed from crosses of the main Oreochromis culture species also has become popular in recent years. Other tilapia species are cultured only by small-scale farmers in Africa [5]. Nile tilapia was then introduced in different countries during the 1970s but problems including inbreeding, insufficient fish seed supply, stagnant production and poor fish growth hindered small-scale aquaculture production. Large-scale genetic improvement programs have been established for O. niloticus in Asia [6] and genetic methodologies to control sex have now resulted in the reliable production of all male fry to help overcome the problem associated with excessive fry production in culture ponds [7]. The Genetic Improvement in Farmed Tilapia (GIFT) project in the Philippines created strains of O. niloticus that grew up to 60% faster than their relatives [8]. The benefits of the GIFT strain include significantly faster growth rates than other farmed strains, improved survival in polluted waters and they can be raised in extensive systems without the need for commercial feeds. The development of hormonal sex-reversal techniques in the 1970s International Journal of GEOMATE, April 2019, Vol.16, Issue 56, pp. 39 - 45 ISSN: 2186-2982 (P), 2186-2990 (O), Japan, DOI: https://doi.org/10.21660/2019.56.4554 Special Issue on Science, Engineering & Environment