Abstract
1. Seabirds and pinnipeds are vulnerable to reductions in prey availability, especially duringthe breeding season when spatial constraints limit their adaptive capacity. There are growingconcerns about the effects of fisheries on prey availability in regions where large commercialfisheries target forage fish.2. For breeding seabirds and pinnipeds, prey availability depends on a combination of abundance,accessibility, patchiness and distance from the colony. An understanding of the aspects of prey availabilitythat determine foraging success is essential for the design of effective management responses.3. We used a mechanistic individual-based foraging model based on observed data for two seabirdspecies, the Peruvian Booby Sula variegata and Guanay Cormorant Phalacrocoraxbougainvilliorum, to simulate the foraging patterns of seabirds feeding on schooling fish. We ranthe model over simulated prey fields representing eight possible combinations of high or lowprey abundance, shallow or deep prey, and broadly distributed or spatially concentrated prey.4. The results highlight the importance of the accessibility of prey. Depth distribution wasthe primary factor determining modelled foraging success for both species, followed by abundance,and then spatial configuration.5. Synthesis and applications. The individual-based foraging model provides a spatially explicitframework for assessing the effects of fisheries on the foraging success of seabirds andother central place foragers, and for evaluating the potential effectiveness of marine-protectedareas and other fisheries management strategies for safeguarding central place foragers indynamic ecosystems. Our analysis indicates that broad-scale fisheries management strategiesthat maintain forage fish above critical biomass levels are essential, but may need to be supplementedby targeted actions, such as time–area closures, when environmental conditionslead to low prey abundance or reduce prey accessibility for seabirds or pinnipeds of conservationconcern. The individual-based foraging model is adaptable and could be reconfigured forapplication to other species and systems.