MEPS 214:237-251 (2001)  -  doi:10.3354/meps214237

ABSTRACT: This study used underwater visual census techniques to quantify the distribution, abundance, fork lengths and biomass of scarid fishes on multiple reefs across the continental shelf of the northern Great Barrier Reef. Spatial patterns in fish distribution were examined over a cross shelf environmental gradient using a hierarchical sampling design that covered a spectrum of scales ranging from metres to tens of kilometres. The design included replicate reefs within mid and outer continental shelf positions and replicate sheltered and exposed sites within each reef. Most of the 21 species surveyed were found to be widely distributed across mid and outer shelf reefs and the number of species did not change significantly between reefs, exposures or across the shelf. Despite a similar complement of species, scarid assemblages differed markedly in the relative and absolute abundance of taxa between exposed and sheltered habitats on mid and outer shelf reefs. Principal components and cluster analyses indicated that changes in the numerical abundance of taxa resulted in consistently different assemblages from 9 sheltered mid shelf sites and 9 exposed outer shelf sites at opposite ends of the surveyed environmental gradient. Variance components calculated for scarid abundance and biomass across 4 spatial scales revealed differences in the scales at which significant variability occurred between sheltered and exposed reef habitats. On exposed reef crests significant variation occurred at both scales of tens of kilometres (shelf position) and hundreds of metres (sites), while on sheltered back reefs significant variation was attributable only to the site scale. Mean scarid abundance (±SE) for exposed outer 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 times higher (920 ± 190 kg ha^-1). The discrepancy between abundance and biomass 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 significant changes in body size according to their location across the continental shelf and/or the prevailing exposure regime. The observed patterns of variation in density and length frequency suggest density-dependent processes and that changes in mortality or growth rates may exist between habitats across the continental shelf.

KEY WORDS: Spatial scales · Exposure · Biomass · Environmental gradient · Body size · Tropical reef fish