Great Skuas and Northern Gannets on Foula, summer 2022 - an unprecedented, H5N1 related massacre

• Main Authors: Camphuysen, Kees, Gear, Sheila
• Other Authors: Leonne van der Weegen, NIOZ Royal Netherlands Institute for Sea Research in cooperation with Utrecht University
• Language: unknown
• Published: NIOZ 2022
• Subjects: Earth and Environmental Sciences; highly pathogenic avian influenza (HPAIV) H5N1; pandemic; marine environment; pelagic seabirds; mass mortality; population decline; symptoms; Stercorarius skua
• Online Access: https://doi.org/10.25850/nioz/7b.b.gd

In summer 2022, a highly pathogenic avian influenza virus (HPAIV) H5N1 pandemic struck, affecting numerous species of colonial seabirds all over the Northern Hemisphere. Great skuas Stercorarius skua and Northern Gannets Morus bassanus were especially badly affected by the outbreak. This report documents the exceptional mortality levels, poor reproductive success and a major population decline in Great Skuas breeding in Foula (Shetland), and a drop in numbers of breeding Northern Gannets on the island. Systematic searches (May-August) produced over 1500 corpses of Great Skuas of which 109 had been ringed earlier in life. The distribution of corpses was non-homogeneous, with clusters of corpses in relatively wet areas, mostly known as clubs and bathing sites. Mortality became apparent in early May, peaked in the second half of May and early June, but continued through the summer at lower levels. In June, Apparently Occupied Territories (AOT) of Great Skuas were counted and it appeared that numbers had declined by 57% since 2015, the most recent earlier survey (from 1820 to 778 AOT). Given the ongoing mortality in July and August, a decline in the order of magnitude of 60-70% in occupied territories is more likely. Great Skuas commenced breeding as usual in May and June, but most attempts failed, mainly because one or both partners got infected and died. The rapid spread of infections over Foula has probably been facilitated by the habit of Great Skuas to bath and socialise at freshwater lochs and pools; sites where close conspecific interactions occur and where all major pathways of infection could play a role: through accumulating of virions in water, aerosols shed from the respiratory tract, via dropped faeces, or by scavenging. How the gannets have picked up the virus is a mystery, but further infections in other marine birds and marine mammals indicate that the virus does cause problems in the marine environment. Gannets bred, but the population declined by 20% during the season (less dramatic than in other ...