Cold tolerance of tilapia species and hybrids

Several experiments were conducted,involving Oreochromis mossambicus, O. aureusand their F₁ and F₂ hybrids to studythe genetic basis of cold tolerance in tilapiinefishes. Groups of fish, of similar age and grown underidentical conditions, were housed in replicated netenclosures in a controlled cooling water system.Survival time through a regime of controlledtemperature reduction was the observed parameter forcold tolerance. Intra-population variation anddifferences among species, hybrids and repeated spawnswithin a species were examined. There was nocorrelation between cold tolerance and fish size(within the range of 23–105mm standard length), andthe distribution for the trait was not normal. O. mossambicuswas the most cold-sensitive group,followed by the F₂, and the F₁ which wassimilar to O. aureus. Genetic variation in coldtolerance seems to have a large dominance component,based on the similarity of the F₁ hybrid to the O. aureus parent.