Depth gradients in diversity, distribution and habitat specialisation in coral reef fishes: Implications for the depth-refuge hypothesis

Abstract

Studies assessing the structure of coral reef fish assemblages have focused on shallow reefs and the importance of coral cover. However, the ecology of reef fish communities varies with depth, although trends in diversity, community structure and reliance on corals have seldom been described. Deeper reef habitats may provide refuge from shallow water disturbances, depending on the depth distributions of species and patterns of habitat specialisation with depth. We examined fish communities down to the bottoms of reefs at 20 m. Communities comprised species with preferences for shallow water and others with broader depth ranges, which were more abundant at greater depths. Diversity exhibited linear declines with depth. Species that were common around the mid-point of the depth gradient had the greatest depth ranges, whereas depth ranges were more restricted at the shallow and deep extremes. Niche breadth decreased with increasing mean depth of occurrence, suggesting that deeper species were more specialised. Unexpectedly, there was a higher association with branching corals in the deepest strata, suggesting a greater reliance on coral habitat at the patchy reef edge. Clearly, there are dramatic changes in the ecology of reef fishes and their habitat between 0 and 20 m, and a variety of physical and biological factors that are likely to be important. Although coral-associated species found deeper may occupy a refuge from shallow water disturbances, the narrow distributions of species at deeper depths and the high reliance on corals are unlikely to contribute to long-term resilience in relation to widespread reef degradation.