Aquaculture and Fisheries Management 1989, 20, 35-47 Growth studies on juvenile tilapia using pure species, hormone-treated and nine interspecific hybrids B. J. McANDREW & K. C. MAJUMDAR* Institute of Aquaculture, University of Stirling, Stirling, Scotland Abstract. The comparative growth performance of the juveniles of six pure tilapia species, Oreochromis mossambicus, O. spilurus, O. macrochir, O. aureus, O. nilotictts and Sarothero- don galilaeus were assessed under standardized conditions in a recirculated water system. The effects of standardized treatments of 17oi methyltestosterone and 17p estradiol were assessed by comparison with untreated controls of O, niloticus, O. mossambicus and O, aureus and the growth performance of nine interspecific hybrids in comparison to their parental species were studied. The results showed significant differences in growth rate for the strains of the pure species under the conditions used. Standardized hormone treatment had significant effects on sex ratios and growth performance of all the species studied but did not change their overall performance ranking when compared to the pure species. No hybrid performed significantly better than the best parent for SGR although O, aureus X O. niloticus hybrid was significantly better than O. nitoticus when corrected weight gain was used. This work shows that species choice hormone treatment and hybridization have implications for tilapia culture. Introduction The group of fishes collectively termed tilapias belong to four genera in the tribe Tilapiini (Trewavas 1983) and total over 70 species. These fish are of great importance as food fish, from both wild and farmed sources in the tropics and subtropics, encompassing many of the areas suffering from a lack of animal protein. The majority of the aquaculturally important species are from the genus Oreochromis and are characterized by the maternal mouthbrood- ing behaviour of these fishes. Tilapia as a group display many favourable attributes as cultured species. These include general hardiness, resistance to disease, high yield potential, ability to grow on a wide range of natural and cheap artificial foods, ability to withstand low oxygen tensions, overcrowding, and a wide range of salinities and still produce a highly acceptable flesh. These advantages have been reviewed in a number of recent works (Balarin & Hatton 1979; Pullin & Lowe-McConnell 1982; Wohlfarth & Hulata 1983). Despite the many advantages of tilapia, the overall world production is still low (FAO 1981). This is a result of the widespread introduction of poor culture species (O. mossambicus) and the problems associated with excessive fecundity in mixed sex, poorly managed pond systems, resulting in overcrowding and stunting ofthe fish (Balarin & Hatton 1979). In recent years there has been a move towards more suitable culture species (O. niloticus) and improved husbandry techniques as well as the use of all-male or predominantly male fish populations produced by hormone sex-reversal or hybridization. These improve- ments have resulted in a dramatic increase in the quality and quantity of tilapia produced •Present address: Centre for Cellular and Molecular Biology, Hyderabad 500,007, India, Correspondence: Dr B,J, McAndrew, Institute of Aquaculture, University of Stirling, Stirling, FK9 4LA, Scotland, 35