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 ( \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}${\dot{M}}_{{\mathsf{O}}_{\mathsf{2}}}$\end{document} ) 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.