Extreme temperature combined with hypoxia, affects swimming performance in brown trout (Salmo trutta)

Climate change is predicted to impact freshwater aquatic environments through changes to water temperature (T(water)), river flow and eutrophication. Riverine habitats contain many economically and ecologically important fishes. One such group is the migratory salmonids, which are sensitive to warm T(water) and low O(2) (hypoxia). While several studies have investigated the independent effects of T(water) and hypoxia on fish physiology, the combined effects of these stressors is less well known. Furthermore, no study has investigated the effects of T(water) and O(2) saturation levels within the range currently experienced by a salmonid species. Thus, the aim of this study was to investigate the simultaneous effects of T(water) and O(2) saturation level on the energetics and kinematics of steady-state swimming in brown trout, Salmo trutta. No effect of O(2) saturation level (70 and 100% air saturation) on tail-beat kinematics was detected. Conversely, T(water) (10, 14, 18 and 22°C) did affect tail-beat kinematics, but a trade-off between frequency (f(tail)) and amplitude (A, maximum tail excursion) maintained the Strouhal number (St = f(tail)• A/U, where U is swimming speed) within the theoretically most mechanically efficient range. Swimming oxygen consumption rate ([Formula: see text]) and cost of transport increased with both U and T(water). The only effect of O(2) saturation level was observed at the highest T(water) (22°C) and fastest swimming speed (two speeds were used—0.6 and 0.8 m s(−1)). As the extremes of this study are consistent with current summer conditions in parts of UK waterways, our findings may indicate that S. trutta will be negatively impacted by the increased T(water) and reduced O(2) levels likely presented by anthropogenic climate change.