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, Twater (10, 14, 18 and 22 degrees 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)center dot A/U, where U is swimming speed) within the theoretically most mechanically efficient range. Swimming oxygen consumption rate ((M)over dot O-2) and cost of transport increased with both U and Twater. The only effect of O-2 saturation level was observed at the highest T-water (22 degrees 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.