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

Abstract

Visual censuses were used to study the distribution and abundance of herbivorous grazing fishes (Acanthundae, Scandae, Sigan~dae) 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 sianificantlv fewer numbers of species and individuals of acanthunds and scands nshoro A major cumponeni i^f t h i varidbiliiy in lhr-data I , withm-n PI [holwof-n-/one, coriiporn~iii. The wnhin-rwf conip<.ncnt of variiihil~ly ussoci~itcd largely wit" diftc-ronces in the ds'semhldgfs 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 to 90 % of the variability in abundance For most species much of this variability is due to an absence or low abundance on inshore reefs MO