Cold tolerance of tilapia species and hybrids

Several experiments were conducted, involving Oreochromis mossambicus, O. a ureus and their F-1 and F-2 hybrids to study the genetic basis of cold tole rance in tilapiine fishes. Groups of fish, of similar age and grown under i dentical conditions, were housed in replicated net enclosures in a controll ed cooling water system. Survival time through a regime of controlled tempe rature reduction was the observed parameter for cold tolerance. Intra-popul ation variation and differences among species, hybrids and repeated spawns within a species were examined. There was no correlation between cold toler ance 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 F-2, and the F-1 which was similar to O. aureus. Genetic variation in cold tolerance seems to have a large dominance component, based on the similarity of the F-1 hybrid to the O. aureus pare nt.