The main objective of the present study was to investigate determinants of environmental adaptation ability in a fish population faced with a combination of reduced oxygen availability and increased water temperature. Body size and growth rate are obvious sources of inter individual variation. However, preliminary experiments had shown that, within an age cohort, variability in size was too limited to allow scrutinizing the interaction between growth rate and environmental tolerance with sufficient analytical power. To get around this difficulty, two lines of rainbow trout displaying marked differences in size at age were compared. Clearly, the main consequence of choosing this option was the confounding effects resulting from the difference in gene pool between the two strains. Thus, in the present study, inter-strain comparison was specifically aimed at examining the relationship between body size and environmental adaptation performance, whereas intrastrain contrasting targeted size-independent sources of interindividual variation.