An Updated Population Estimate For Northern Gannets Across Their North-East Atlantic Breeding Range Following The 2022 Outbreak Of High Pathogenicity Avian Influenza

Northern gannets (Morus bassanus) have been regarded as a seabird success story, due to population increases throughout the 20th and 21st century contrasting with global seabird declines. However, in 2022 gannets experienced a severe outbreak of High Pathogenicity Avian Influenza (HPAI) across their global distribution, leading to an urgent need to reassess their population status. This study presents breeding gannet census numbers for 2023/24 from all colonies across the North-East Atlantic metapopulation (Great Britain, Ireland, the Channel Islands, Iceland, Norway, the Faroe Islands, France, Germany, Russia). Gannet numbers decreased by 17% across the North-East Atlantic metapopulation between the 2013/14 and 2023/24 census from 414,598 to 345,854 apparently occupied sites (AOS), a global decrease of at least 13%. The bulk of the reduction in AOS was driven by the largest colonies (>10,000 AOS) each losing tens of thousands of AOS. These figures likely underestimate the impact of the HPAI outbreak worldwide, since most colonies will have increased between the last census in 2013/14 and the 2022 HPAI outbreak, and the Canadian breeding population was last counted pre-HPAI outbreak. Scotland still holds the largest proportion of both the North-East Atlantic metapopulation (59%), and the world population (46%), while Great Britain, Ireland and the Channel Islands together hold 83% of the North-East Atlantic metapopulation and 64% of the world population. This study not only presents an updated population census for gannets in the North-East Atlantic but illustrates the large-scale impacts of a disease outbreak on a seabird species across its global range and highlights the importance of more regular census efforts to better quantify the demographic consequences of such events.