Hypoxia tolerance and oxygen regulation in Atlantic salmon, Salmo salar from a Tasmanian population

For aquatic ectotherms, increasing water temperatures cause an exponential increase in metabolic rate and decreasing oxygen solubility. Fish species that regulate their metabolic rate to low dissolved oxygen concentrations are understood to be hypoxia tolerant whereas salmonid fish are considered to be classic metabolic conformers and their metabolic rate is dependent on the environmental oxygen concentration. This study examined Atlantic salmon, Salmo salar, undergoing a progressive hypoxia at optimal temperatures and at temperatures nearing the upper thermal tolerance limit for the species to determine if metabolic regulation occurred. Oxygen consumption was measured on individual Atlantic salmon (150.7 ± 40.8 g) in 66-L static respirometers; oxygen measurements were taken every 5 min until the fish lost equilibrium. Metabolic regulation was observed at all temperatures and occurred in 67, 50 and 50% of the fish at 14, 18 and 22 °C, respectively. The plateau metabolic rate (VO_2PL) was 293.4 ± 24.5 mg•kg•h^-1 at 22°C which was significantly higher than in the 14 and 18°C treatments (191.1 ± 24.5 and 203.9 ±12.6 mg•kg•h^-1, respectively). This difference was also reflected in the critical oxygen threshold (P_crit) where the value for the 22°C treatment (4.59 ± 0.32 mg•L^-1) was significantly higher than those of the 14 and 18°C treatments (3.46 ± 0.14 and 3.39 ± 0.26 mg•L^-1 respectively). These results indicate that some fish from the Tasmanian population of Atlantic salmon have the ability to regulate metabolic rate to low oxygen concentrations and therefore show a relatively high degree of hypoxia tolerance.