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 ºC
up to 25 ºC higher than current conditions. Employing a dataset 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, behavioral 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.