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 O₂ (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 O₂ 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 O₂ saturation level on the energetics and kinematics of steady-state swimming in brown trout, Salmo trutta. No effect of O₂ 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 O₂ 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 O₂ levels likely presented by anthropogenic climate change.