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 (CTmax) and agitation temperature (Tag) 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 CTmax and Tag 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 CTmax across populations, and neither average DO nor average water temperature per site were significant predictors of CTmax and thermal safety margin (°C difference between CTmax and the maximum environmental temperature). Tag and agitation window (°C difference between Tag and CTmax) 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 CTmax 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.