Bathymetry Structures Predator-prey Dynamics between Zooplankton and Fish in Marine Pelagic Ecosystems

Light decays exponentially with depth in aquatic systems, and vertical movement is a widespread strategy among zooplankton for avoiding visual predation. Topographical features may, however, constrain descents and trap zooplankton in light exposed habitats favorable for fish predation. We present an extensive dataset on the vertical distribution of zooplankton in the topographically rugged Barents Sea, which hosts some of the largest fish stocks in the world. Combining data analyses with theoretical modelling of visual search, we show that foraging opportunities for planktivorous fish are structured by the vertical zooplankton distributions and constraints from the bathymetry. During daytime, zooplankton are exposed to four orders of magnitude more light above shallow banks (< 200 m) than in the deeper water surrounding the banks. Furthermore, we predict that fish may encounter significantly more prey over banks even though zooplankton is more abundant in the deeper areas. Stomach data from a key planktivore support these predictions and suggest that fish foraging on copepods have been more successful over shallow bathymetries. The spatial pattern in foraging success for fish can deviate much from the spatial pattern in prey abundance. Our results bring new perspectives to fish biomass distributions and regional variation in trophic coupling between fish and zooplankton.