Differential changes in the taxonomic and functional composition of herbivorous fishes from shallow to mesophotic coral ecosystems at the Coral Sea Marine Park, Australia

Abstract

Mesophotic coral ecosystems (MCEs: 30–150 m) are characterized by environmental conditions and biological assemblages that are distinct from those of shallow reefs (< 30 m). Several studies have described changes in species richness, abundance and composition of reef fish assemblages, from shallow reefs to MCEs; however, few have considered how different functional groups change over the same gradient. Here, remotely operated vehicle (ROV) surveys were used to examine how depth (2–98 m) and slope orientation influence the abundance, biomass, richness and composition of all herbivorous fishes, and the abundance and biomass of five functional groups of herbivorous fishes (scrapers, excavators, algal croppers, browsers and detritivores) on oceanic reefs of the Coral Sea Marine Park, Australia. While the abundance, biomass and species richness of herbivorous fishes generally declined with depth, the rate of decline varied among groups. The rate of decline in abundance of excavating (7.0% m−1) and detritivorous (7.4% m−1) fishes was nearly double that of croppers (4.2% m−1) and scrapers (3.9% m−1), while rates of decline in biomass of detritivores (3.5% m−1) was ~ 50% higher than croppers (2.4% m−1). Further, gentle to moderate slope orientations (6–45°) generally supported higher richness, abundance and biomass of herbivorous fishes than both flatter (< 5°) and steeper (> 81°) orientations. These results show that the response of herbivorous fishes to depth is not homogenous and varies among functional groups, and may be related to changes in resource availability, slope orientation and predation from shallow reefs to MCEs.