Cnaani, A., Department of Aquaculture, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Gall, G.A.E., Department of Animal Science, University of California-Davis, Davis, CA 95616, United States
Hulata, G., Department of Aquaculture, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel

Several experiments were conducted, involving Oreochromis mossambicus, O. aureus and their F1 and F2 hybrids to study the genetic basis of cold tolerance in tilapiine fishes. Groups of fish, of similar age and grown under identical conditions, were housed in replicated net enclosures in a controlled cooling water system. Survival time through a regime of controlled temperature reduction was the observed parameter for cold tolerance. Intra-population variation and differences among species, hybrids and repeated spawns within a species were examined. There was no correlation between cold tolerance and fish size (within the range of 23-105mm standard length), and the distribution for the trait was not normal. O. mossambicus was the most cold-sensitive group, followed by the F2, and the F1 which was similar to O. aureus. Genetic variation in cold tolerance seems to have a large dominance component, based on the similarity of the F1 hybrid to the O. aureus parent.