Abstract
Understanding intraspecific variation in upper thermal tolerance is essential for assessing climate change impacts on freshwater fish communities. We experimentally evaluated thermal tolerance in a cyprinid fish, Pseudaspius sachalinensis , using critical thermal maximum (CT max ) measurements. Fish were collected from two tributaries in Hokkaido, Japan, differing in thermal variability but with similar mean temperatures. The population experiencing greater thermal fluctuations exhibited significantly higher CT max , whereas individual‐level factors (body size and condition) had no detectable effect. These findings highlight the importance of population‐specific conservation strategies under climate change.