Distribution and foraging interactions of seabirds and marine mammals in the North Sea: multispecies foraging assemblages and habitat-specific feeding strategies

The top-predator community in the northwest North Sea consists of 50 species of seabirds and marine mammals, most of which are piscivorous. Sandeels are important prey for many species, and reduced sandeel abundance has had detectable consequences for breeding success, most notably in surface-feeding seabirds. In recent years, breeding success and population trends of seabirds nesting along the east coast of Britain have differed among species, suggesting species-specific responses to fluctuating prey stocks. A large-scale, multi-disciplinary study of top-predator distribution patterns and at-sea foraging behaviour was conducted in the northwest North Sea to investigate some of the behavioural mechanisms underlying these species-specific population responses. This approach provided new insights into the ways in which marine predators utilize a shared prey resource. At-sea distributions of some of the smaller seabirds, such as black-legged kittiwakes, suggested individuals avoided feeding in inshore areas used by the larger Larus gulls. This resulted in an apparently counter-intuitive, positive relationship between annual breeding success and foraging range, with productivity tending to be lower in years when oceanographic conditions led to good foraging areas occurring closer inshore. Combining distributional data with information on activity patterns showed that northern gannets used different foraging strategies in nearshore and offshore habitats and that chick-rearing common guillemots utilized spatially segregated, colony-specific feeding areas. Many surface-feeding and plunge-diving seabirds relied heavily on facilitation by pursuit-diving predators, such as auks and cetaceans.