This examination was conducted using a methodology based on two standardized challenge tests aimed at assessing individual incipient lethal oxygen saturation (ILOS) and incipient upper lethal temperature (IULT). Results to these tests were then cross-correlated with swim tests during which individual critical swimming speed and standard and active metabolic rates were measured. Measurements of permeabilized ventricular myofibers oxygen consumption were also conducted, as well as various organ-to-body-mass ratios. Two strains of rainbow trout (O. mykiss) were used for this experiment, a fast growing strain and a slow growing strain. Inter-strain comparison aimed at examining the influence of body size and growth on environmental adaptation performance, whereas intra-strain contrasting allowed the deciphering of size-independent sources of interindividual variation. Moreover, the relevance of elements of scaling in understanding trade-offs between hypoxia tolerance and heat tolerance was examined.