Dr Amy Rose Coghlan1, Professor Julia L. Blanchard1,2, Dr. Simon Wotherspoon3, Associate Professor Rick Stuart-Smith1, Professor Graham J. Edgar1, Dr. Asta Audzijonyte1,2
1Institute for Marine and Antarctic Studies, Hobart, Australia, 2Centre for Marine Socioecology, Hobart, Australia, 3Australian Antarctic Division, Kingston, Australia
A decline in body size with warming is universally predicted for aquatic ectotherms, on the basis of physiological processes. Nevertheless, the interactions of ecological and physiological processes in natural environments operate at varying scales, and are difficult to predict from models or fundamental principles. We evaluated the net outcomes of these interacting drivers of fish body size using ~15,000 underwater visual survey census data from coastal rocky and coral reef fish communities around Australia. Surveys spanned 10 years, a mean sea surface temperature (SST) range of ~14⁰C to 29⁰C, and included individual body sizes of across all conspicuous species and trophic groups. Mean individual fish body size decreased strongly with increasing temperature. Mean fish size ranged from ~14.5 cm at 15⁰C mean SST to ~8.9 cm at 25⁰C. This equated to an ~5% decrease in mean fish body length per 1⁰C incease in SST increase across space. When converted to mass, these results suggest a fourfold decrease (from 30 g to 7 g) in mean surveyed fish body mass from 15⁰C to 25⁰C SST. A clear trend was also evident in trophic guild composition across the temperature gradient in space, with herbivores accounting for a greater proportion of community abundance and biomass at warmer temperatures, and the opposite trend observed for invertivores. Mean individual body size within herbivores, planktivores, and invertivores followed a similar decrease trend in warmer waters to the overall mean trends across species, but no relationship was found between piscivore mean individual body size and SST. We show that compositional changes may also arise in a way that changes trophic structure, providing novel evidence of an ecological driver of changes in fish body size with temperature. If spatial trends in community function translate to temporal trends, global warming could lead to considerable reorganisation of marine ecosystems.
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