Thermal performance in fishes varies systematically across latitude, habitat, and biological organization
Thermal performance curves are widely used to understand how ectotherms respond to anthropogenic climate change, yet comparable data remain fragmented for fishes despite their predicted sensitivity to warming. Here we introduce FishTherm, a compilation of 457 thermal responses from 107 wild fish species, spanning diverse performance metrics and temperatures. With this dataset, we estimated thermal optima, performance breadth, tolerance breadth, and activation energies, and tested how these thermal traits vary geographically, across latitude and habitat, and by type and functional context of performance. Thermal optima increase toward lower latitudes and warmer waters, consistent with established biogeographic patterns. We also found that responses measured at lower levels of biological organization exhibit higher thermal optima than population-level responses, supporting complexity constraints on thermal performance curves. Activation energies broadly matched predictions from the Metabolic Theory of Ecology, and we further found systematic variation of lower values in negatively-motivated responses, and associated with narrower thermal performance breadths, in line with theory. Our results provide supporting evidence for several unifying principles that advance our understanding of constraints and variation in thermal performance. FishTherm is a new resource for comparative thermal ecology and for improving our understanding of fish vulnerability under climate warming.