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.

Global climate change is driving the redistribution of marine species and thereby potentially restructuring endemic communities. Understanding whether conservation measures can confer resilience to ecosystems is therefore a management priority. Here, we utilise a time-series of benthic imagery to examine the resilience of a no-take marine reserve (NTR) to the establishment of barrens habitat formed by a range extending urchin species. We find evidence for a strong reserve effect, with a doubling in the odds of presence of barrens outside the NTR. However, we find a concerning trajectory of increasing barrens cover in both the reference areas and the NTR over time. Thus, whereas the reserve is demonstrating resistance to the establishment of barrens, there is currently no evidence of recovery. Continuation of the current monitoring program is required to assess whether the NTR can provide long term protection from a catastrophic phase shift and to inform adaptive management.