Combining bird-borne tracking and vessel monitoring system data to assess discard use by a scavenging marine predator, the lesser black-backed gull Larus fuscus

Abstract

A step-wise reduction of fishery discards in the North Sea may have cascading effects on scavenging seabirds that rely on discards as a food resource. Understanding the interactions between seabirds and fisheries is thus crucial for seabird management and conservation. Using bird-borne GPS loggers, we investigate to what extent incubating lesser black-backed gulls Larus fuscus at Helgoland overlap on a spatio-temporal scale with fishing vessel positions obtained through the vessel monitoring system, as well as the distance around vessels at which birds adopt a non-random search behaviour that is indicative of the utilization of discards. We characterize foraging trips and discuss our findings in view of potential effects of the change of the European discard policy on lesser black-backed gulls. Individuals overlapped on a spatio-temporal scale with fishing vessels inone-third of the foraging trips (17 of 48 trips). Within <5 km of fishing vessels, birds exhibited lower flight speeds and higher path tortuosity values, indicating non-random search behaviour and thus the utilization of discards. To feed on discards, birds travelled greater total distances and spent more time away from the colony than birds feeding on non-discards, which is mainly due to the distant location of fishing grounds from the colony. Our results support previous studies showing the spatio-temporal overlap between birds and fishing vessels and behavioural switching from commuting to foraging. The inferred use of both discards and non-discards highlights the dual feeding strategy in this population, but a long-term reduction in discards may negatively affect the population on Helgoland, if sufficient alternative high-energetic food resources are not available within their foraging range.