Jessica Emilie Desforges , Kim Birnie-Gauvin, Fredrik Jutfelt , Kathleen M. Gilmour, Keri E. Martin, Erika J. Eliason, Terra L. Dressler, David J. McKenzie, Amanda E. Bates, Michael J. Lawrence, Nann Fangue, Steven J. Cooke

Critical thermal maxima methodology (CTM) has been used to infer acute upper thermal tolerance in fishes since the 1950s, yet its ecological relevance remains debated. Here, we synthesize evidence to identify methodological concerns and common misconceptions that have limited the interpretation of CTmax (value for an individual fish during one trial) in ecological and evolutionary studies of fishes. We identify limitations of and opportunities for using CTmax as a metric in experiments, focusing on rates of thermal ramping, acclimation regimes, thermal safety margins, methodological endpoints, links to various performance traits such as swimming ability, and repeatability. Care must be taken when interpreting CTM in ecological contexts, since the protocol was originally designed for ecotoxicological research with standardized methods to facilitate comparisons within study individuals, across species and contexts. CTM can, however, be used in ecological contexts to predict impacts of environmental warming, but only if parameters influencing thermal limits, such as acclimation temperature or rate of thermal ramping, are taken into account. Applications can include mitigating the effects of climate change, informing infrastructure planning or modeling species distribution, adaptation and/or performance in response to climate related temperature change. Our synthesis points to several key directions for future research that will further aid the application and interpretation of CTM data in ecological contexts.