Jeglinski, J. W. E. | bioRxiv Science

bioRxiv2026

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.

ecology↗

bioRxiv2023

Strong breeding colony fidelity in northern gannets following High Pathogenicity Avian Influenza Virus (HPAIV) outbreak

High pathogenicity avian influenza virus (HPAIV) caused the worst seabird mass-mortalities on record in Europe across 2021-2022. The northern gannet (Morus bassanus) was one of the most affected species, with tens of thousands of casualties in the northeast Atlantic between April-September 2022. Disease outbreaks can drastically modify the movement ecology of animals and diminish spatial consistency, thereby increasing the potential for disease transmission. To detect potential changes in movement behaviour, we GPS-tracked breeding adults following the initial HPAIV outbreak, at three of the largest gannet breeding colonies where major mortality of adults and chicks occurred (Bass Rock, Scotland, UK; Grassholm, Wales, UK; Rouzic island, Brittany, France). Crucially, GPS-tracked birds remained faithful to their breeding sites and did not prospect other breeding colonies. They performed regular foraging trips at sea, similar to their behaviour before the outbreak. Gannet foraging effort was nonetheless lower than in 2019, thus surviving birds may have benefited from reduced intra- and interspecific food competition. Breeding colony fidelity of adult northern gannets following HPAIV mass-mortalities suggests limited long-term capacity to virus spread, which may contrast with the behaviour of adults during the disease outbreak, or with that of younger individuals.

ecology↗