Control of algae on coral reefs by large herbivorous fishes
Coral reefs harbor a vast amount of global diversity relative to their size, and are an important economic resource to coastal communities. Over the past few decades, many coral reefs have undergone a phase shift from a substrate dominated by coral to one dominated by algae, largely due to anthropogenic stress. Herbivorous fishes play a major role in topdown control of algal growth and composition; however, depletion of biomass due to overfishing and habitat degradation has threatened the top-down control of fish herbivory on algal composition. This experiment compared endolithic turf algae (TA) composition on coralline rubble under complete fish exclusion, large fish exclusion (>13 cm), and no fish exclusion treatments. There was a significant increase in growth, richness, and percent cover of TA in response to reduced herbivory. The greatest compositional shift occurred in complete fish exclusion treatments. Crustose coralline algae (CCA), important in coral recruitment and growth, significantly increased in cover under every treatment except complete fish exclusion. This illustrates the importance of large-bodied herbivorous fish in controlling TA growth and maintaining bare substrate to facilitate coral recruitment and growth. This study provided insight into how Caribbean reefs go through initial stages of a phase shift from a coral dominated benthos to one dominated by taller, denser algae. Finally, it illustrates how Bonaire’s reef, currently regarded as one of the most intact in the Caribbean, could change in composition if large herbivorous fish are removed from the ecosystem.