An extensive literature exists on how environmental conditions, especially temperature, impact animal body sizes. However, there remains considerable discrepancies, and misunderstanding, in the key definitions and concepts of body size used to describe observed impacts across studies. Size can be measured using continuous growth metrics, including von Bertalanffy growth coefficients, or static 'size' metrics, such as population-averaged length or mass, average size-at-(arbitrary)-age, size-at-maturity, adult size, asymptotic size, or the maximum observed size. Critically, these concepts of size are not equivalent, and temperature is likely to affect each in different ways. The use of these disparate size and growth metrics as response variables estimated across different biological scales (individual, population, or community) and empirical contexts (laboratory, field) has led to unnecessary confusion and apparent contradictions among practitioners. Here, we review nine common confusions associated with the measurement of 'size' in fish and other water-breathing ectotherms. We then highlight outstanding knowledge gaps on how temperature and global warming might affect different size metrics. Clarifying concepts, definitions, and applications of body size measures is important as it can help reconcile divergent findings, target future research, and improve our predictions about the warming impacts on wild populations.

Aquatic ectotherms often attain smaller body sizes at higher temperatures. By analysing ~15,000 coastal-reef fish surveys across a 15oC spatial sea surface temperature (SST) gradient, we found that the mean length of fish length in communities decreases by ~5% for each 1oC temperature increase across space. This equated to a 50% decrease in mean length from 14 to 29oC mean annual SST. We found that trophic guild composition shifts from domination by herbivores and planktivores in the tropics, to invertivores and piscivores in cooler waters. By investigating the contribution of trophic composition to community-level mean length, we show ~25% of temperature-related changes could be attributed to trophic composition at the warmest sites, but <1% at colder temperatures. Our findings suggest that small changes in temperature will lead to large changes in fish community body sizes, driven both by community trophic composition in warm waters and mean sizes within trophic guilds.

Aquatic ectotherms often reach smaller body sizes at higher temperatures. By analysing ~15,000 coastal-reef fish surveys across a 15oC spatial sea surface temperature gradient, we show mean community-level fish length decreased by ~5% for each 1oC warming, equating to a decrease in mean community-level body-length by 50%, or mean weight by 90% from 14-29oC. We further show dominant trophic guild composition shifts from invertivores and piscivores, to herbivores and planktivores, as water temperature increases. By investigating the contribution of trophic-composition to overall community-mean-length, we show ~25% of temperature-related changes could be attributed to trophic-composition at the warmest sites, but close to zero at colder temperatures. Our findings suggest that small changes in temperature will lead to large changes in fish community body sizes, driven both by changes in community trophic composition in warm waters, and by changes to the average body sizes of fish within trophic guilds across all temperatures.

Climate change and fisheries exploitation are dramatically changing the species composition, abundances, and size spectra of fish communities. We explore whether variation in abundance-size spectra, a widely studied ecosystem feature, is influenced by a critical parameter thought to govern the shape of size-structured ecosystems—the relationship between the sizes of predators and their prey (predator-prey mass ratios, or PPMRs). PPMR estimates are lacking for vast numbers of fish species, including at the broader trophic guild scale. Using measurements of 8,128 prey items in gut contents of 97 reef fish species, we established PPMRs for four major trophic guilds (piscivores, invertivores, planktivores and herbivores) using linear mixed effects models. To assess theoretical predictions that higher mean community-level PPMR leads to shallower size spectrum slopes, we compared observations of mean community-level PPMR with size spectrum slopes for coastal reef sites distributed around Australia. PPMRs of individual fishes were remarkably high (median ~71,000), with significant variation between different trophic guilds (~890 for piscivores; ~83,000 for planktivores), and ~8,700 for whole communities. Community-level PPMRs were positively related to size spectrum slopes, broadly consistent with theory, however, this pattern was also influenced by the latitudinal temperature gradient. Tropical reefs showed a stronger relationship between community-level PPMRs and community size spectrum slopes than temperate reefs. The extent that these patterns apply outside Australia, and consequences for community structure and dynamics, are key areas for future investigation.