Flexible foraging tactics by a large opportunistic seabird preying on forage- and large pelagic fishes

Generalist and opportunistic marine predators use flexible foraging behaviour to exploit prey bases that change in diversity and spatial and temporal distributions. Behavioural flexibility is constrained by characteristics such as individual cognitive and physical capabilities, age, reproductive condition and central place foraging. To assess flexibility in the foraging tactics of a marine bird, we investigated the diets and foraging behaviour of the largest seabird predator in the North Atlantic Ocean. Northern gannets Sula bassana exploit a broad spectrum of pelagic prey that range in mass by more than 2 orders of magnitude. We investigated their foraging activity at their largest offshore colony in the western Atlantic Ocean during 1998 to 2002, when they preyed primarily on shoals of spawning and post-spawning capelin Mallotus villosus, a small forage fish (~15 g), and also on a much larger pelagic fish, post-smolt Atlantic salmon Salmo salar (~200 g). Inter-annual dietary variation is associated with gannet and prey fish distributions. Landings of capelin at the colony by gannets were correlated with returns of larger foraging flocks from inshore, whereas landings of Atlantic salmon were associated with smaller flocks returning from offshore. Maximum foraging trip distances ranged from 20 to 200 km and averaged 57 ± 12 (SE) km, consistent with distances to inshore capelin aggregations. When capelin abundance was low (in 2002), more gannets foraged offshore, preyed on large pelagic fishes (mostly Atlantic salmon) and exhibited the greatest dietary diversity. Though the outbound portions of foraging trips were more sinuous than inbound routes, individual gannets exhibited general fidelity to foraging sites. These large avian predators used flexible foraging tactics to adjust to changing prey conditions and generate longer-term strategies to take advantage of diverse trophic interactions over a range of ocean ecosystems.