Targeted prey fields determine foraging effort thresholds of a marine diver: important cues for the sustainable management of fisheries.
Understanding functional relationships between seabirds and prey can play an important role in the sustainable management of fisheries that compete for the same resources yet data linking the foraging performance of seabirds directly with concurrent information on prey supplies remain limited. We assess the influence of prey availability on the foraging performance of breeding African penguins Spheniscus demersus using data from instrumented birds during 14 pelagic fish surveys carried out over 3 years around their two largest breeding colonies, St Croix and Bird islands in Algoa Bay. Characteristics of prey fields targeted by African penguins were identified by assessing the response of foraging birds to the acoustically determined distribution of small pelagic fish. Path- and dive-derived foraging performance metrics were modelled as a response to overall and targeted prey abundance estimates to assess potential nonlinear functional relationships between penguin foraging effort and prey availability. African penguins displayed marked spatial heterogeneity in their selection of foraging sites, but birds from both colonies invariably foraged on fish schools <40 m deep. Generalized additive model fits improved for all foraging performance responses when using targeted prey abundance (i.e. only fish in areas frequented by penguins and at depths <40 m, as opposed to estimates using all prey). Models for trip duration (r2=0.45) and foraging path length (r2=0.44) showed a negative nonlinear form in their response to targeted prey abundance, with significant improvement in penguin foraging effort when prey abundance was greater than 45% of the maximum prey biomass recorded. Synthesis and applications. Functional relationships between seabird foraging performance and prey supplies have the potential to inform the sustainable management of fisheries that compete for the same prey. Our findings highlight the importance of quantifying prey that is available to seabirds when assessing critical thresholds in prey abundance below which seabird foraging performance is adversely affected. These findings can be integrated into dynamic ocean management strategies to mitigate resource competition around seabird colonies. A framework for flexible, colony-centred closures is proposed as a more dynamic alternative to time/area closures with the potential to respond to prevailing habitat conditions in near real-time.