Density of herbivorous fish and intensity of herbivory are influenced by proximity to coral reefs

Ecosystems can be profoundly influenced by consumers, sometimes to the extent that the entire appearance of the ecosystem is altered. We used remotely sensed images to identify distinctive halos around patch reefs in the lagoon of Ningaloo Reef, the world's largest fringing coral reef. Thirty-four halos were identified along the length of Ningaloo Reef. Five halos located within a 122 km tract of the reef were investigated. The halos extended > 90 m from each central patch reef and were found to be associated with a high biomass of, and intensive grazing by, herbivorous fish, especially the large-bodied Kyphosus sydneyanus. Large brown algae mainly of the genera Sargassum, Dictyopteris and Lobophora were the dominant macroalgae, but were almost absent immediately adjacent to the patch reefs. Other taxa of herbivorous fish were present near the patch reefs, including Naso spp., Siganus spp. and Scarus spp., but the biomass of each was low and none were significant contributors to grazing. The sizes of the halos reported here are tenfold larger than those previously reported in coral reef systems and are likely to be the result of intensive herbivory by K. sydneyanus.