Tilapia are native to Africa and their cold tolerance is a limiting factor for their aquaculture expansion in temperate areas including some parts of Iran. In this study, 207 individuals of Oreochromis niloticus, 0. aureus and 0. mossambicus were assayed for cold tolerance. The quantitative indicators of cold tolerance used were temperatures at which fish showed certain cold-induced behaviours including disequilibrium (TeD), Loss of equilibrium (TeL) and unconsciousness (TeU). There was intraspecific variation in each trait and phenotypic correlations were evident. Although the total variation within each of the three species was low (CV 7.3%) the relative variation was most in 0. mossambicus. 0. nioticus was detected as to be intermediate and 0. aureus showed the least variation. The interspecific variation was very highly significant (P<0.001) and the rank of cold tolerance in the species under investigation was determined as follows : 0. mossambicus. O. nioticus
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