Temperature-driven body size variation is contingent on trophic position for global fishes

Consumer energy demand increases with body size (BS) and temperature, while energy availability in food webs decreases with increasing trophic positions (TP). This suggests that thermal energetic constraints may impose more severe limits on consumer BS at high TP. Consequently, it is expected that consumers at high TP would be smaller in warmer climates because of the more severe energy limitations. We tested this hypothesis using a global dataset of 1110 marine and freshwater ray-finned fish species, while controlling for the effects of species' phylogenetic history. As predicted, fish at higher TP (predators) had significantly smaller BS in tropical aquatic ecosystems compared to temperate ones. However, contrary to our expectations, we found that tropical fish occupying lower TPs-herbivores, detritivores, and omnivores-were larger than their temperate ecological equivalents, revealing a potential thermal-trophic paradox. This contrasting pattern suggests the effect of temperature on fish BS varies with species' TP-reversing the relative BS patterns between tropical and temperate fishes along the food web. Our results were consistent across freshwater and marine fishes. These findings support the idea that larger tropical predatory fish are constrained by stronger energetic limitations leading to smaller BS, while lower-TP tropical fish may benefit from enhanced efficiency in processing plant-based diets in warmer environments. Our results are important given the role of BS as a key functional trait in structuring aquatic food webs and in understanding how global warming may influence macroecological patterns of fish BS in global aquatic ecosystems.