The structure of fish communities on the outer reef slopes of one inshore, one mid-shelf and one outer shelf reef in the central region of the Great Barrier Reef was examined using small, quantitative explosive charges. Ten collections, each covering approximately 150 m2 of reef surface, were made on each reef. A total of 323 species belonging to 46 families and comprising 28,537 individuals were collected. Species composition of the communities differed greatly among the 3 reefs. Most species were collected on only 1 or 2 of the 3 reefs. Species richness (number of species per unit area) was greatest on the mid-shelf, lowest inshore and of an intermediate value on the outer shelf reef. The evenness component of species diversity increased across the shelf from inshore to mid-shelf to outer shelf reefs. The biomass of different trophic categories and overall trophic structure differed considerably among the reefs The most notable differences were the lower biomass of algal grazers on the inshore reef than elsewhere and the considerably greater biomass of planktivores on the mid-shelf reef than elsewhere. Within these major trophic categories and their guilds, there was considerable of major taxa among the 3 reefs A number of possible mechanisms causing variation in the distrbution and maintaining the differences in community structure across the continental shelf are discussed.
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