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Description: Lake warming can negatively impact cool-water fishes through both temperature and oxygen stress. We modeled the joint dynamics of water column temperature and oxygen to quantify oxythermal habitat for yellow perch (Perca flavescens) in Fish Lake, Wisconsin, USA. To estimate annual oxythermal stress, we developed a novel metric (cumulative oxythermal stress dosage; COSD) that integrates both stress duration and stress magnitude. We find that COSD better predicts observed perch declines than the published TDO3 metric (temperature at depth where dissolved oxygen is 3 mg·L −1 ), which was developed for cold-water fish. Simulations show increases in COSD between 1911 and 2014, punctuated by a sharp rise since 1989. Extreme COSD years result from the intersection of high maximum daily dosage and prolonged duration exceeding the tolerance threshold. Temperature perturbation experiments to explore future climate scenarios reveal that COSD would increase greatly if the atmosphere warms by 3 °C. Applying the COSD metric broadly to temperate lakes could help direct management efforts toward the ecosystems most likely to serve as climate refugia for cold- and cool-water fishes in the future.