Effects of acclimation to elevated water temperature and hypoxia on thermal tolerance of the threatened pugnose shiner (Notropis anogenus)

For freshwater fishes, elevated water temperatures associated with climate warming and hypoxia can co-occur and are likely to interact as both affect oxidative metabolism. We quantified the effects of acclimation to elevated temperature and hypoxia on the thermal tolerance of pugnose shiner (Notropis anogenus), a cyprinid fish threatened in its Canadian range. In one experiment, pugnose shiner underwent two-week sequential acclimations to six increasing temperatures. Fish acclimated to warmer waters increased their critical thermal maximum (CTmax), while the agitation temperature (T-ag) was 3.4 degrees C lower than the CTmax. In another experiment, fish were acclimated to three dissolved oxygen treatments (>95%, similar to 56%, or similar to 40% air saturation) for two weeks and tested under >95% and 40% conditions. CTmax was lower when measured under 40% for normoxia-acclimated fish but not for hypoxia-acclimated fish. Hypoxia-acclimated fish had higher Tag and smaller agitation windows than normoxia-acclimated fish, suggesting that hypoxia acclimation improves aspects of thermal tolerance. We examine the plasticity of thermal tolerance of pugnose shiner, showing that they may be more vulnerable to high temperatures compared with other non-imperiled Notropis species.