In conclusion, by comparing two genetically distinct strains of rainbow trout, we highlighted differences in environmental tolerance between the fast growing and the slow growing populations. Results also suggest a trade-off between thermal sensitivity and tolerance to hypoxia. Size is a key element of that trade-off but functional, suborganismal components are also involved. In the context of contemporary environmental trends, this sheds a new light on the possible evolutionary response of fish populations faced with a warmer and less oxygenated environment. However, discriminating genetic features from phenotypic plasticity in the observed patterns will be a mandatory next step.