Intraspecific variation in hypoxia tolerance, aerobic and anaerobic swimming performance: contributions to ecological performance and mortality selection in cultured (European sea bass) Dicentrarchus labrax (Barcelona)

Studies of inter-individual variation in hypoxia tolerance and fish swimming performance may provide insight into how selection has influenced diversity in phenotypic traits that may be important for responding to climate change. We investigated individual variation and short-term repeatability of individual hypoxia tolerance and its relationship to swimming performance in European sea bass measured with a constant acceleration test (CAT). We measured maximum anaerobic speed at exhaustion (UCAT), gait transition speed from steady aerobic to unsteady anaerobic swimming (Ugt), routine metabolic rate (RMR), post-CAT maximum metabolic rate (MMR) and aerobic scope. Fish were subsequently released into artificial estuaries for five months at densities that ensured competition for natural forage. Individual variation in UCAT could be accounted for almost exclusively by variation in anaerobic burst-and-coast performance beyond Ugt. The Ugt itself varied substantially between individuals. Individual RMR and MMR varied considerably, but the rank order of metabolic rates was repeatable and inversely related to hypoxia tolerance. Survival in the estuaries was 40% and the relationship between mortality selection, hypoxia tolerance, metabolic rates and swimming performances will be discussed.