ABSTRACT:

Coral reef environments are currently experiencing a global crisis. For restoration and conservation of such environments, substratum suitable for coral recruitment is important. Herbivorous fishes are likely to facilitate the survival and growth of coral reefs by removing sediment and epilithic algae on corals in the course of their feeding behavior. However, this facilitative function can be greatly impaired by anthropogenic stressors such as overfishing, which decreases the biomass of fishes, and by excessive sediment deposition due to land development, which might decrease the frequency of feeding. To examine these hypotheses, we conducted observations and experiments at 4 reefs with different degrees of anthropogenic stress in subtropical Japan. Our findings were as follows: (1) Underwater visual censuses, sediment trap surveys, and an analysis of fishery statistics revealed that biomass, population density, and species richness of herbivorous fishes were generally lower in reefs with high densities of fishermen and high sedimentation rates. (2) Caging experiments showed that, compared to control plots, caged plots inaccessible to herbivorous fishes had greater accumulation of sediment and epilithic algae. (3) Sediment removal experiments found that, compared to control plots, experimental plots with the sediment removed were more heavily grazed by some functional groups of herbivorous fishes (grazers). These results suggest that excessive deposition of sediment and overfishing depress the facilitative function of herbivorous fishes, which can further exacerbate the deposition of sediment.