Variation in Metabolic Scaling Reveals Aerobic Constraints With Temperature in Fishes

Aim Climate‐driven declines in fish body size suggest direct negative impacts on larger individuals and indirect impacts on species interactions, population dynamics and ultimately ecosystem functions and services. This study uses metabolic scaling as a mechanistic framework to identify aerobic limitations in fish across size to provide a foundation for a better understanding of the ecological consequences of size‐specific species vulnerability in a warming world. Location Global. Time Period Present. Major Taxa Studied Fish. Methods We synthesized over 12,000 individual maximum and resting metabolic rates (MMR, RMR) across optimal and supra‐optimal warm temperatures to determine how they scale with mass and calculated factorial aerobic scope (FAS = MMR/RMR) to assess a potential aerobic constraint in fish taxa globally and across species groups with shared ecologies and morphologies. Additionally, we identified interspecific and intraspecific variation in mass‐independent metabolic rates across diverse fish species. Results Across fish taxa globally, metabolic scaling slopes differed between MMR and RMR and across optimal and warm temperature conditions, but within both temperature categories, aerobic scope scaling slopes decreased as temperature increased. Metabolic scaling relationships varied among diverse species groups and across life stages. Generally, MMR and RMR scaling slopes were higher under warm compared to optimal temperatures, while the scaling slopes of FAS were more negative under warm conditions. Main Conclusions Factorial aerobic scope was more negative under warm conditions, suggesting that larger fish face greater metabolic constraints under warming. The synthesis highlights variation in scaling relationships across species groups and a general pattern in which larger fish experience aerobic limitations. Evidence for size‐specific vulnerability will improve by incorporating currently underrepresented data, such as larger‐bodied and freshwater fish, especially under warming.