Depth gradients in abundance and functional roles suggest limited depth refuges for herbivorous fishes

As some types of disturbance to coral reefs are attenuated with depth, the resilience of herbivorous fish species utilizing shallow areas (< 30 m) is likely enhanced if their population footprint extends into adjacent deeper reef areas. Using field surveys from an isolated coral reef, off northwest Australia, we created data-driven models relating herbivorous fish communities to habitat across an extended depth range (4–76 m). Models assessed variations in functional redundancy across depth to test if the ecological functions provided by herbivores at shallow reefs can be replenished by deep water populations. Eighty percent of herbivorous species (1967 fishes, 48 species total) were associated with depths < 30 m, where hard coral was the dominant benthos (peak diversity ~ 20 m). Some species were restricted to shallow reef (< 30 m), while others were ubiquitous (4–70 m), or most abundant in deeper reef (30–70 m). Functional diversity and redundancy were highest at < 30 m, almost 2 × higher than for deeper (> 30 m) reef areas. Scraper herbivores were associated with the reef crest (< 10 m), grazers with the reef slope (< 30 m), and browsers were evenly distributed across depth (4–70 m). Impacts to herbivores in shallow reefs (e.g. ocean warming, storms, fishing) may be ameliorated if species are widely distributed across depth, or have the ability to opportunistically relocate to adjacent undisturbed areas. Deeper habitat appears to support a subset of the herbivore community, by providing habitat and resources for the browser functional group (only herbivores capable of ingesting large macroalgae) and species with generalist depth distributions and known trophic flexibility (e.g. genus Scarus). Additional management of depth zones where functional group distributions overlap may maximize chances of sustaining herbivore diversity and functional redundancy, and provide enhanced protection across depth for important fished species such as large parrotfishes and unicornfishes.