Size-dependent variation in the functional role of the parrotfish Scarus rivulatus on the Great Barrier Reef, Australia

Biodiversity loss and fishing-induced changes in the size distributions of fishes can impact ecosystem function on coral reefs. These changes have led to an urgent need for studies on the particular roles of species to underpin effective coral reef management. The present study focuses on the feeding ecology of 6 size classes (from 2.5 to 30 cm total length) of Scarus rivulatus (family Labridae), one of the most abundant parrotfishes on the Great Barrier Reef, Australia. Individuals in all 6 size classes strongly selected the epilithic algal matrix for foraging and rejected other substratum types, including coral, macroalgae and sand. The 6 size classes also had similar feeding rates (bites min–1) and diel feeding patterns, with higher feeding activity during the afternoon. However, the size of grazing scars by S. rivualtus differed significantly among the 6 size classes, with small individuals scraping a greater substratum area per unit biomass and larger individuals taking a greater volume of material per unit biomass. Thus, biomass cannot be not be viewed as a proxy for ecosystem impact; different sized individuals of S. rivulatus, and probably other parrotfish species, have a markedly different impact on the reef substratum. Selective harvesting of large individuals will therefore change the functional role of this species. These results emphasize the importance of considering the size of individuals when evaluating the role of reef species in ecosystem process. These intraspecific functional differences will be critical when formulating reef management strategies and evaluating the impact of fishing activity on reef ecosystems.