Non-consumptive predator effects constrain herbivorous fish distribution and abundance at upper mesophotic depths (30-50 m).

Abstract

Herbivorous fishes play integral roles in preventing macroalgae from displacing corals, yet appear to decline with depth despite the presence of coral-dominated habitats to >80 m. We investigated the relationship between herbivorous fish and a suite of bottom-up and top-down parameters (algal turf, macroalgal and coral cover, nutritional quality and palatability, temperature, light, and predator density) along a depth gradient from 3-50 m. Fishes were not food-limited as upper mesophotic algae had similar nutritional content, assemblages, and appeared highly palatable from algal choice experiments. In-situ temperature recordings suggest temperature was not a limiting factor. Multivariate redundancy analyses reveal the combination of increased habitat patchiness and reduced light levels best explain distributions with depth. These results suggest reductions in herbivorous fish populations across shallow to upper mesophotic depths are likely the result of non-consumptive predator effects and behavioral choices rather than abiotic constraints or resource limitation.