Thermal and hypoxia tolerance of the endangered lake chubsucker Erimyzon sucetta: implications for conservation under climate change

Freshwater fishes face numerous threats to their persistence, including warming induced by climate change and low dissolved oxygen (environmental hypoxia), both of which are increasingly prevalent in freshwater ecosystems and which can co-occur. Assessing tolerance to environmental stressors as well as the plasticity of responses to them is essential for the conservation of imperilled species. In this study, we measured tolerance to warming and hypoxia in lake chubsucker Erimyzon sucetta , a species federally listed as endangered in Canada under the Species at Risk Act. We measured upper thermal tolerance (critical thermal maximum [CT max ] and agitation temperature [ T ag ]) and hypoxia tolerance (critical oxygen tension [ P crit ] and loss of equilibrium [LOE]) of juvenile lake chubsucker streamside across 2 yr. Lake chubsucker had a mean CT max of 35.04°C and T ag of 28.23°C, suggesting they are less tolerant to warming than previously determined by behavioural measurements in a population of this species from South Carolina, USA. Alternatively, we found fish were relatively tolerant to hypoxia, with a mean P crit of 20.35% air saturation and LOE at 3.62% air saturation. We found little evidence of seasonal acclimatization in thermal tolerance; however, this may be due to the narrow temperature scale during the measurement period (daily mean range: 19.26-24.71°C). Overall, this study highlights the need for direct assessments of at-risk populations within their imperilled ranges, as the study population appears to be experiencing temperatures that may disrupt their normal behaviour and levels of DO below P crit that may limit aerobic performance.