Residues analysis revealed no correlation between cMO2 and individual values of SMR, AMR and Ucrit. Additionally, no relationship was observed between SMR and any of the organ-to-bodymass ratios. The lack of relationship between cMO2 and complex performance traits such as SMR and AMR may not be surprising considering that the myocardium representso0.2% of one fish body mass (Fig. 5; Eliason et al., 2011) and that myocardium oxygen consumption amounts to approximately 1% of that of a whole fish (Ewart et al., 1988; Davie and Franklin, 1992). As discussed above, it is the diffusion of oxygen from the ventricular lumen into the thick-walled ventricle that determines heart working ability and not its aerobic metabolic performance per se (Farrell, 1991). This predominance of oxygen diffusion in determining cardiac performance is also to be linked with the absence of relationship between cMO2 and Ucrit.