Chronic Heat Tolerance Reveals Overestimated Thermal Safety Margins and Increased Vulnerability in Marine Fish Populations

Predicting vulnerability to global warming remains an elusive goal in thermal biology. In marine fishes, ongoing changes in distribution contrast with their apparent capacity to tolerate temperatures from 5°C up to 25°C higher than current conditions. Employing a data set of 786 upper critical temperatures across 213 species and recent theoretical developments, we provide conclusive evidence that these so-called thermal safety margins overestimate the resilience to warming and that most species inhabit thermal conditions approaching their physiological tolerance limit. This result holds across latitudes and based on historical records, several populations have encountered stressful temperatures in the recent past. While warming tolerance remains similar across geographic regions, behavioural responses are constrained at low latitudes as distribution shifts required to encounter cooler waters are disproportionally higher in the tropics. Overall, our results illustrate how thermotolerance measures can be extrapolated to the field and used to quantify vulnerability to warming.