Thermal tolerance of an East African fish across a natural dissolved oxygen gradient

Climate warming can affect ectotherms, such as fish, through increased metabolic demands and oxygen consumption rates. There is growing interest in exploring the relationship between hypoxia tolerance and thermal tolerance, as adaptations to hypoxia can improve fish oxygen uptake and delivery capacities. Therefore, we asked whether fish from hypoxic habitats differ in their thermal tolerance from conspecifics inhabiting well-oxygenated waters. To do so, we quantified the critical thermal maximum (CT_max) and agitation temperature (T_ag) of multiple populations of the cyprinid Enteromius neumayeri found in low-oxygen swamps and high-oxygen streams of Kibale National Park, Western Uganda. We also measured CT_max and T_ag under acute hypoxia exposure. Fish were collected from eight sites with similar thermal conditions, but with a strong natural dissolved oxygen (DO) gradient. There was little variation in average CT_max across populations, and neither average DO nor average water temperature per site were significant predictors of CT_max and thermal safety margin (°C difference between CT_max and the maximum environmental temperature). T_ag and agitation window (°C difference between T_ag and CT_max) were not predicted by average water temperature per site; however, there was a weak relationship between both parameters and average DO per site. Fish from low-oxygen sites tended to agitate at lower temperatures and have larger agitation windows. Finally, fish from high- and low-oxygen sites exposed to acute hypoxia had significantly lower CT_max and agitated at lower temperatures, which could affect this species' persistence during dry seasons when hypoxic events become more frequent in stream and swamp habitats.