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