Body Size and Thermal Sensitivity of a Facultative Air-Breathing Fish Amidst Environmental Change

Many freshwater fish populations are increasingly required to respond to warming waters and low dissolved oxygen concentrations as a result of land-use change and climate change. In addition, the average size of fish in harvested populations can be drastically reduced due to heavy fishing pressure. These factors may be more pronounced in African countries due to current high rates of forest conversion and a greater dependency on local fish catch. To investigate the relationship between fish body size and heat tolerance, we compared the upper thermal tolerance limits (estimated as the Critical Thermal Maximum, CT_max) of the smooth-head catfish (Clarias liocephalus), a facultative air-breathing fish, sampled from Lwamunda Swamp, Uganda. We included additional CT_max trials under low dissolved oxygen concentrations (0.20–0.50 mg/L) to test for potential interactive effects between body size and hypoxia on upper thermal tolerance. Body size did not affect C. liocephalus upper thermal limits, as small and large individuals had similar CT_max values. CT_max values decreased under hypoxic conditions but were similar across a range of body sizes. The agitation temperature (temperature during the CT_max trials when individuals appeared to become agitated, likely avoidance behaviour) was positively related to body size. This suggests that smaller individuals may be more thermally sensitive than larger fish in terms of their behavioural response to acute temperature increase.