Differences in diet have implications for potential ecosystem roles of two key rocky reef fish

Abstract The influence of fishes on temperate rocky reef ecosystems is globally recognised, particularly where large predators may structure ecosystems through predation. In South-East Australia, the common Wrasse Achoerodus viridis (Blue Groper) and Sparid Chrysophrys auratus (Snapper) are sympatric rocky reef predators. While their diets are thought to be well understood, this is generally based on gut contents undertaken over limited spatial scales. Stable isotope approaches can provide a time-integrated dietary history. We used Bayesian mixing models (δ¹³C and δ¹⁵N) to examine diets of 45 Blue Groper and 73 Snapper collected across > 1,250 km of coastline. We found that both predators consume filter-feeders likely including mussels, oysters or serpulid polychaete worms based on previous gut contents, shifting to consuming omnivores such as crabs or the problematic sea urchin Centrostephanus rodgersii , and carnivores such as cephalopods, predatory fishes or nereidid polychaetes when large (> 400 mm TL). However, ontogenetic shifts differed, with Blue Groper relying primarily on filter-feeders at all sizes and carnivores/omnivores becoming equally important when large. By contrast, small Snapper relied both on filter-feeders and omnivores/carnivores, relying on omnivores/carnivores when large. Trends varied spatially for Blue Groper more than Snapper, with Blue Groper relying on carnivores/omnivores in the north and filter-feeders in the south, while Snapper relied primarily on carnivores/omnivores throughout. These results highlight species-specific trophic pathways, indicating that these predators ecological role differs markedly throughout their ontogeny. Key to ecological structuring processes affecting temperate rocky reefs, our results suggest large Snapper may impact C. rodgersii populations more than Blue Groper.