Distribution and abundance of herbivorous grazing fishes in the central Great Barrier Reef. II. Patterns of zonation of mid-shelf and outhershelf reefs

Abstract

This study describes the patterns of species richness and abundance of herbivorous grazing fishes (Acanthuridae, Scandae, Siganidae) in 5 zones (reef slope, reef crest, reef flat, lagoon, back reef) of 3 reefs on the mid-continental shelf and of 3 reefs on the outer continental shelf in the central region of the Great Barrier Reef. Herbivorous fishes (all 3 families considered together) occur in assemblages which are characteristic of several major zones and this pattern of zonation is consistent on the 6 reefs. Assemblages in deep, outer-slope zones are distinctive from those in shallow zones, There are distinctive assemblages of herbivorous fishes on reef flats, reef crests and over sandy areas in both lagoon and back reef zones. Similarity in these patterns of zonation between reefs suggests that they will be maintained through time. Families display distinctive patterns of zonation also, but patterns differ between families. Acanthurids and scarids generally have higher numbers of species and individuals on reef crests and in lagoons than on reef flats or reef slopes; siganids have higher numbers of species and individuals in lagoons and back reefs than in the other 3 zones studied, Variation in species richness and abundance of individual families between zones was, however, not consistent among reefs at the same shelf location. Different guilds of herbivorous fishes were distributed differently between zones. Suckers feeding on fine sediments were most abundant near windward and leeward edges of reefs; suckers feeding over sand were most abundant in back reefs and lagoons; large and small croppers and scrapers were more abundant in shallow zones (reef crest, reef flat, lagoon) than in deep zones (reef slope, back reef).