Identity and behaviour of herbivorous fish influence large-scale spatial patterns of macroalgal herbivory in a coral reef

The functional role of macroalgae-consuming fishes is particularly important in coral reefs, as they can mediate the recovery of degraded systems when macroalgae become esta b lished. However, herbivory on coral reefs is often spatially and temporally variable, and the mechanisms that underpin variation in the consumption of algae are largely unknown. In this study, we examined local (within-reef, 100s of m) and broad-scale (regional, 100s of km) patterns of macroalgal herbivory along the spatial extent of Ningaloo Reef in northwestern Australia. Using underwater video cameras, we quantified the feeding rates of individual fish on a dominant macroalga Sargassum myriocystum, and we quantified social feeding behaviour to determine whether patterns in algal removal are influenced by whether fish feed as solitary individuals or as part of single-species or multi-species groups. While 23 fish species were observed biting macroalgae, spatial patterns in the intensity of herbivory were strongly driven by 3 species: Naso unicornis, Kyphosus vaigiensis and K. bigibbus. Feeding rates by individuals of these 3 species were always highest when individuals were part of monospecific groups, and we found a strong positive relationship between spatial patterns in the amount of macroalgae removed and the proportion of bites taken as part of groups, irrespective of fish size. This study highlights the importance of species-specific algae–herbivore interactions in coral reefs and shows that local variation in feeding behaviour is an important trait that contributes to explain spatial variability in macro algal herbivory.