Seasonal variation in thermal tolerance and hypoxia tolerance of a threatened minnow and a non-imperilled congener: a cautionary tale for surrogate species in conservation.

Freshwater organisms face multiple threats to their ecosystems, including warming associated with climate change and low dissolved oxygen (environmental hypoxia), which are both increasing in frequency and extent in freshwater systems. Understanding tolerance thresholds for these environmental stressors as well as the plasticity of responses is the key for informing the conservation of imperilled species. Direct measurement of imperilled species can be difficult, and the use of surrogate (non-imperilled but closely related) species has been proposed as a remedy, but the degree to which surrogate data are representative of the imperilled species has not been widely validated. In this study, we measured physiological performance of two species: one federally listed as Threatened in Canada (Pugnose Shiner, Miniellus anogenus) and a non-imperilled congener (Blackchin Shiner, Miniellus heterodon). Hypoxia tolerance (critical oxygen tension and loss of equilibrium) and upper thermal tolerance (CT_max) were measured streamside over a period of 5 months. We found that the Threatened Pugnose Shiner had lower tolerance to both elevated temperature and hypoxia than the non-imperilled Blackchin Shiner. The species also differed in their responses to environmental dissolved oxygen (DO). CT_max of Pugnose Shiner had a positive relationship with DO such that CT_max was lowered when environmental DO was low, whereas there was no effect of DO on CT_max of Blackchin Shiner. Blackchin Shiner also showed plasticity of hypoxia tolerance in response to changes in environmental DO, while Pugnose Shiner showed little plasticity. We conclude that Pugnose Shiner may be more sensitive to heat waves and hypoxia associated with climate change. We also assert that researchers should be cautious when using surrogate species to inform tolerance limits of imperilled species and highlight the value of measuring imperilled species directly when possible.