Age-related differences in foraging behaviour at sea and interactions with fishing vessels in an opportunistic urban gull

Fishing activity generates high amounts of fishing discards, a predictable anthropogenic food subsidies used by seabirds. Although the use of discards by these predators has been well studied, there is a lack of knowledge about the ontogenetic differences in their use. We contributed to filling this gap for the yellow-legged gull (Larus michahellis), an opportunistic predator that extensively exploits anthropogenic food subsides. We investigated its foraging behaviour during the early breeding season deploying GPS devices on adults, immatures, and juveniles from the urban population of Barcelona (northwestern Mediterranean Sea) and examining the effect of fishing vessels on their spatial movements using a Vessel Monitoring System. The results revealed age-related differences in distribution and foraging behaviour at sea and an interaction with fishing vessels in this seabird. Age-related differences in behaviour were explained by the reproductive constraints of adults and the ontogenetic differences associated with lower foraging ability in immature and juvenile individuals. We did not find apparent preferences for a specific type of fishing vessel between ages. These results suggest that the reform of the Common Fisheries Policy might affect the entire population of this species, that could lead to an increase in the use of urban environments, increasing the conflicts with human activities.