The fish used for this study were an F2 hybrid family obtained by mating of F1 hybrids
derived from crossing O. mossambicus with O. aureus. The F2 family consisted of 60 fullsib
fish, with an average weight of 10.4F5.9 g.
The fish shared the same tank throughoutthe experiment. Each fish was weighed, and its cold tolerance was measured under a regime of controlled temperature reduction and calculated as ‘‘Cooling-Degree Days’’ (CDD) to death, as described by Cnaani et al. (2000).