SUMMARY

Studies of inter-individual variation in fish swimming performance may provide insight into how selection has influenced diversity in phenotypic traits. We investigated individual variation and short-term repeatability of individual swimming performance by wild European sea bass in a constant acceleration test (CAT). Fish were challenged with four consecutive CATs with 5 min rest between trials. We measured maximum anaerobic speed at exhaustion (U_CAT), gait transition speed from steady aerobic to unsteady anaerobic swimming (U_gt), routine metabolic rate (RMR), post-CAT maximum metabolic rate (MMR), aerobic scope and recovery time from the CATs. Fish achieved significantly higher speeds during the first CAT (U_CAT=170 cm s^–1), and had much more inter-individual variation in performance (coefficient of variation, CV=18.43%) than in the subsequent three tests (U_CAT=134 cm s^–1; CV=7.3%), which were very repeatable among individuals. The individual variation in U_CAT in the first trial could be accounted for almost exclusively by variation in anaerobic burst-and-coast performance beyond U_gt. The U_gt itself varied substantially between individuals (CV=11.4%), but was significantly repeatable across all four trials. Individual RMR and MMR varied considerably, but the rank order of post-CAT MMR was highly repeatable. Recovery rate from the four CATs was highly variable and correlated positively with the first U_CAT (longer recovery for higher speeds) but negatively with RMR and aerobic scope (shorter recovery for higher RMR and aerobic scope). This large variation in individual performance coupled with the strong correlations between some of the studied variables may reflect divergent selection favouring alternative strategies for foraging and avoiding predation.