EXPERIMENTAL-STUDY ON THE EFFECTS OF GRAZING BY PARROTFISHES AND ROLE OF REFUGES IN BENTHIC COMMUNITY STRUCTURE

Fishes in densities of 0.6-1.5 parrotfish [probably Scarus taeniurus] per m2 or 9-17 g wet wt of fish/m2 of feeding surface had an optimum effect, resulting in the greatest benthic species richness and biomass on 2-dimensional surfaces. The presence of refuges (3-dimensional habitats) has a greater impact on benthic community structure (number of species and biomass) than does just the density of parrotfish in such an experimental system. Coral recruitment is enhanced by the presence of refuges and, like coralline algae, is more successful under increased grazing pressure. These optimum densities of parrotfishes relate well to observed field densities where, in a collection from a Hawaiian [USA] patch reef, there were 1.1 fish or 10.8 g wt wt of parrotfish/m2 of collection area. The success of coralline algae and corals under high grazing pressure may have important consequences for the stability and structure of modern coral reefs.