Distribution and abundance of herbivorous grazing fishes in the central Great Barrier Reef. I. Levels of variability across the entire continental shelf

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Abstract
Visual censuses were used to study the distribution and abundance of herbivorous grazing fishes (Acanthuridae, Scaridae, Siganidae) on reefs in the central region of the Great Barrier Reef. Three locations on the continental shelf (inshore, mid-shelf, outershelf), 3 reefs within each of these locations and, for the mid- and outershelf reefs, 5 zones within each of these reefs were examined. The assemblages of herbivorous fishes on inshore reefs were distinctive from those on mid- and outershelf reefs, with significantly fewer numbers of species and individuals of acanthurids and scarids inshore. A major component of the variability in the data is a within-reef (between-zone) component. The within-reef component of variability, associated largely with differences in the assemblages with depth, is greater than that between mid- and outershelf locations and between reefs within these locations. When zones are examined individually and compared with reefs inshore the majority of species display significant cross-shelf change in abundance irrespective of the zone examined, with cross-shelf change accounting for often 80-90% of the variability in abundance. For most species much of this variability is due to an absence or low abundance on inshore reefs.