Consumption of discards by Herring Gulls Larus argentatus and Lesser Black-backed Gulls Larus fuscus off the Belgian coast in the breeding season

Fishery discards in the Belgian part of the North Sea are a source of food for
Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus. To
understand the importance of discards for local L. argentatus and L. fuscus
populations, single-item discard experiments were performed at four offshore
distances from the gullery of the Port of Zeebrugge, at four different stages of
the breeding season (May to August 2011). We compared flock composition
during discarding with the distribution of Herring and Lesser Black-backed Gulls,
with respect to offshore distance from the colony as reflected by an 11-year
(2002–2013) dataset of standardised ship-based surveys. Consumption of
discards depended on the type of fish that was discarded, but prey selectivity by
adults was reduced during the chick rearing stage. A generalised linear mixed
model identified the number of scavengers following the vessel, the proportion of
adults and of Herring Gulls in the flock and the frequency of food robbery events
interacting with the stage of the breeding season as affecting the variation in flatfish
consumption. Shifts in scavenger flock composition and discard consumption
between stages of the breeding season are likely linked to variation in food
requirements of the gull population along the season and to dispersal patterns
towards the end of summer. Nutrient requirements of breeding adults peak
during the chick rearing stage, making this a key period in terms of dependence
of the breeding parents on discarded fish as food source.