Sociality and seabird diving: lower dive effort for solitary gannets compared with those in groups and at fishing boats

Sociality shapes many aspects of marine predator foraging, but impacts on diving behaviours are poorly understood. For instance, group foraging may provide benefits in terms of information on food location, but increased competition may reduce prey capture success, with divergent consequences for diving. We used bird-borne cameras and GPS to compare dive behaviour of adult northern gannets Morus bassanus under 3 scenarios: (1) solitary, (2) social natural, and (3) social scavenging (i.e. at fishing vessels). Of 195 dive bouts, 40.5% were solitary, 22.6% were social natural, and 36.9% were social scavenging. Dive sociality was repeatable within individual foraging trips. Solitary dives were less frequent and shorter compared with social scavenging and shorter than social natural dives. Longer dives were associated with longer inter-dive periods but only when gannets were alone. Social diving effort tended to increase with distance to the colony, perhaps because of a prey-depletion halo effect, but this was not the case when birds were scavenging. Together, our results show how diving behaviour is related to sociality and how this may be influenced by intra-colony competition and the presence of fisheries.