Foraging success in planktivorous fish increases with topographic blockage of prey distributions

Abstract

Banks and shelves are productive zones of the ocean, and often home to large fish stocks. Can shallower bottom topographies improve foraging opportunities for pelagic fish by blocking zooplankton from hiding in deep, darker water? We use mechanistic principles of visual prey search and an extensive dataset of zooplankton depth distributions to model foraging success in planktivorous fish across a large marine ecosystem. Our results show that zooplankton distribute deeper with deeper bathymetry, and that fish find exponentially less food with increasing bottom depths. Over shallow banks, zooplankton are forced into higher light exposure, providing higher prey encounter rates for fish despite lower abundance of prey. Stomach data analyses from a key planktivore support these predictions and suggest that fish foraging on copepods are more successful over shallower grounds. Our study demonstrates that prey availability for planktivorous fish is not proportional to zooplankton abundance, while the bottom depth is an important factor in fish foraging success and zooplankton mortality rates.