Hierarchical and scale-dependent effects of fishing pressure and environment on the structure and size distribution of parrotfish communities

© 2014 The Authors. Parrotfishes are considered to have a major influence on coral reef ecosystems through grazing the benthic biota and are also primary fishery targets in the Indo-Pacific. Consequently, the impact of human exploitation on parrotfish communities is of prime interest. As anthropogenic and environmental factors interact across spatial scales, sampling programs designed to disentangle these are required by both ecologists and resource managers. We present a multi-scale examination of patterns in parrotfish assemblage structure, size distribution and diversity across eight oceanic islands of Micronesia. Results indicate that correlates of assemblage structure are scale-dependent; biogeographic distributions of species and island geomorphology hierarchically influenced community patterns across islands whereas biophysical features and anthropogenic pressure influenced community assemblage structure at the within-island scale. Species richness and phylogenetic diversity increased with greater broad-scale habitat diversity associated with different island geomorphologies. However, within-island patterns of abundance and biomass varied in response to biophysical factors and levels of human influence unique to particular islands. While the effect of fishing activities on community composition and phylogenetic diversity was obscured across island types, fishing pressure was the primary correlate of mean parrotfish length at all spatial scales. Despite widespread fishery-induced pressure on Pacific coral reefs, the structuring of parrotfish communities at broad spatial scales remains a story largely dependent on habitat. Thus, we propose better incorporation of scale-dependent habitat effects in future assessments of overexploitation on reef fish assemblages. However, strong community-level responses within islands necessitate an improved understanding of the phylogenetic and functional consequences of altering community structure.