Fish responses to challenge tests were expressed as time to loss of equilibrium, similar to time to death in survival studies, and were analyzed following procedures classically used for survival analysis. The relationship between the percentage of resisting individuals and time was estimated using the Kaplan– Meier procedure followed by a COX proportional hazards model to test for difference between strains (Cox F-test). The coefficient of variation (CV¼standard deviation/ mean) was used as an index of the extent of inter-individual variation. To generate mass independent data of MO2 (SMR and AMR), ILOS, IULT, Ucrit, and organ-to-body mass ratios, residuals were calculated from leastsquares linear regressions on body mass. If not stated otherwise, values are given as mean 7SEM, between strains comparisons were done using student t-test and statistical significance was set to po0.05. All statistical analyses were performed using Statistica-9 (Stat Soft).