Scale and Benthic Composition Effects on Biomass and Trophic Group Distribution of Reef Fishes in American Samoa.

Abstract

We determined spatial patterns in distribution and biomass of 163 fish species in nearshore waters around Tutuila Island, American Samoa. Visual surveys of reef fishes along 30 by 5 m belt transects were conducted using a hierarchical nested design at five spatial scales from individual transects to tens of kilometers, allowing assessment of broad geographic patterns. Benthic cover data were derived from video transect surveys to test the relationship between habitat and distributions of reef fishes. We found that fish biomass, density, and numerical abundance in American Samoa are dominated by herbivores from relatively few species in the families Acanthuridae and Scaridae. Subsets of carnivore species covaried positively with live coral, algae, and coralline algae cover. Herbivores, in contrast, covaried positively with filamentous algae and coralline algae (i.e., their foods). Biomass of fishes at different trophic categories was associated with higher abundance of food material and habitat availability. Significantly higher biomass occurred along the south shore of Tutuila and at reefs with greater exposure to wave energy, such as topographic points, despite the occurrence of lower live coral cover. Significant variations in fish biomass occurred at large spatial scales, specifically at habitat and exposure levels. Variations at these scales were apparently driven by association of the most dominant trophic group with its food source and the extent but not the quality of habitat.