Spatial variability in reef fish distribution, abundance, size and biomass: a multi-scale analysis

This study used underwater visual census techniques to quantify the distrib ution, abundance, fork lengths and biomass of scarid fishes on multiple ree fs across the continental shelf of the northern Great Barrier Reef. Spatial patterns in fish distribution were examined over a cross shelf environment al gradient using a hierarchical sampling design that covered a spectrum of scales ranging from metres to tens of kilometres. The design included repl icate reefs within mid and outer continental shelf positions and replicate sheltered and exposed sites within each reef. Most of the 21 species survey ed were found to be widely distributed across mid and outer shelf reefs and the number of species did not change significantly between reefs, exposure s or across the shelf. Despite a similar complement of species, scarid asse mblages differed markedly in the relative and absolute abundance of taxa be tween exposed and sheltered habitats on mid and outer shelf reefs. Principa l components and cluster analyses indicated that changes in the numerical a bundance of taxa resulted in consistently different assemblages from 9 shel tered mid shelf sites and 9 exposed outer shelf sites at opposite ends of t he surveyed environmental gradient. Variance components calculated for scar id abundance and biomass across 4 spatial scales revealed differences in th e scales at which significant variability occurred between sheltered and ex posed reef habitats. On exposed reef crests significant variation occurred at both scales of tens of kilometres (shelf position) and hundreds of metre s (sites), while on sheltered back reefs significant variation was attribut able only to the site scale. Mean scarid abundance (+/- SE) for exposed out er shelf sites (3060 +/- 460 ha(-1)) was on average 4 times higher than in all other habitat zones surveyed, while mean biomass (+/- SE) was only 3 ti mes higher (920 +/- 190 kg ha(-1)). The discrepancy between abundance and b iomass estimates is explained by a 30 % average decline in parrotfish fork lengths on exposed outer shelf reef crests relative to other habitats. Log- linear analysis indicated that at least 8 species of scarids displayed sign ificant changes in body size according to their location across the contine ntal shelf and/or the prevailing exposure regime. The observed patterns of variation in density and length frequency suggest density-dependent process es and that changes in mortality or growth rates may exist between habitats across the continental shelf.