The Predator's Dilemma: Investigating the responses of seabirds to changes in the abundance and distribution of small pelagic prey

Abstract

Understanding relationships between predators and their prey is a central question in ecology and ecosystem management. For seabirds, relationships between food availability and breeding success and survival are typically non-linear. Reduced food availability has been identified as a potential threat to seabirds and other central place foragers (CPFs), especially in regions where large commercial fisheries target forage fish. For CPFs, food availability can be defined in terms of abundance, accessibility, concentration, and distance from the colony. The design of effective conservation responses depends on understanding the effects of changes in food availability on foraging success. The goal of this research was to strengthen understanding of the effects of changes in various aspects of prey availability on the foraging success of seabirds and other CPFs. A spatially-explicit individual-based foraging model (IBFM) was developed, guided by the foraging ecology of two seabird species, the Peruvian Booby (<italic>Sula variegata</italic>) and Guanay Cormorant (<italic>Phalacrocorax bougainvillii</italic>). The prey field in the IBFM was derived from geostatistical simulations of the distribution of Peruvian anchoveta (<italic>Engraulis ringens</italic>) based on acoustic survey data representing contrasting foraging conditions. Seabird tracking data were partitioned into different movement modes to guide simulation of movement processes. Decision rules in the IBFM were informed by analysis of foraging site selection by Peruvian Boobies and Guanay Cormorants in terms of the abundance and depth distribution of their prey. The effects of different assumptions about search strategies used by seabirds to locate ephemeral prey patches were explored in the IBFM. The IBFM was then used to investigate the effects of changes in the abundance and distribution of prey on the foraging success of Peruvian Boobies and Guanay Cormorants. The results highlight the importance of the depth distribution of prey for surface-foraging seabirds, and provide insights into differences in the vulnerabilities of the two seabird species to changes in the abundance and distribution of their prey. More broadly, the results have implications for the potential effectiveness of marine protected areas and other fisheries management strategies in safeguarding the foraging success of CPFs in the context of environmental variability that affects the distribution of prey.