CAMPHUYSEN CJ, KELDER L, ZUHORN C & FOUCHIER R (2023) Avian influenza panzootic leads to mass strandings of Northern Gannets Morus bassanus in the Netherlands, April-October 2022. LIMOSA 96 (2): 49-59.

In April 2022, reports were received of an unprecedented stranding of dead Northern Gannets along the Dutch North Sea coast, especially on the Wadden Sea islands. Some 221 individuals washed ashore within one month, where around 22 dead birds would have been normal, considering an average of 0.06 ± 0.07 stranded gannets per km coastline from systematic beached bird surveys (1980-present; or 10x background levels in April 2022). In addition, 15 birds were found within the Wadden Sea, where 6 would have been normal (0.02 ± 0.09 km-1, or 2.4x background levels). Numbers along the North Sea coast decreased again in May (still 6x background levels) and the incident was therefore not investigated, but it was established that the stranded birds were mostly adults, with a high body mass and that they were (subcutaneous) very fat, with no indications for an unnatural death such as oiling or entanglements. Around the North Sea, the first indications for what would turn out to be a major avian influenza panzootic all over the Northern Hemisphere affecting colonial seabirds, became apparent in May 2022: high mortality of breeding gannets on nests, large numbers of corpses of gannets afloat under several of the major breeding colonies and widespread in the North Sea basin. Within the Netherlands, stranding frequencies increased again in June (17x background levels), soared to a peak of 2.19 gannets km-1 in July (63x background levels), and subsequently declined somewhat in autumn (Fig. 2). Strandings occurred all over the Dutch coastline (Fig. 3), with at least 2837 casualties in total, as an extrapolation based on monthly densities between April and October (236, 116, 430, 1131, 465, 247, and 212 respectively), where about 170 dead birds in total would have been normal given the long-term dataset (1980- 2021; Tab. 1). The data suggest that adults suffered disproportionally during the current panzootic, with immature birds (2nd to 5th plumage types) thus far relatively being spared. Between Apr-Oct 2022, 89.5% of all gannets found dead were mature birds (n= 344), while only 60.1% were adults in that period over 1980- 2021 (n= 759, X²₁=95.1, P< 0.001). In the spring, during the first strandings (Apr-May), the difference was not (88.7% adult over 1980-2021, n= 221, 91.7% in 2022, n= 60, X²₁= 0.18, n.s.), but in that season adults normally always outnumbered immatures. Both during the peak in summer (May-June, 63.1% versus 94.7% adult), and in early fall (Aug-Oct, 44.8% versus 85.4% adult), the age distribution was significantly different (X²₁= 31.2, P< 0.001 and 7.2, P< 0.01 respectively; Fig. 4). Against a background of long-term survey data, the significance of the mortality inflicted in Northern Gannets could clearly be appreciated. The results suggest that birds were infected whilst underway towards their breeding grounds (in April), but also that many birds returned to the south, or at least abandoned the breeding grounds early, in mid-summer. The April-October 2022 event can be seen as an exceptional outlier in the long-term database (Fig. 5), in terms of overall densities, age composition and apparent cause of death. Of 19 tested individuals sampled on the Wadden Sea island Vlieland (June-July), 18 individuals (95%) tested positive for avian influenza, of which at least 16 cases involved the H5 subtype. Of these, five were characterised as HPAI H5N1, and these were genetically closely related to viruses found in gannets sampled in England, Scotland, Wales, and Sweden in 2022. Individual gannets sampled in Wieringen (1x July, 1x August, 1x September), also tested positive for the H5 subtype, but the virus found in the gannet sampled in August was genetically closer to that found in Dutch geese than to the virus found in other gannets sampled in Europe. The panzootic of highly pathogenic avian influenza virus (HPAIV) H5N1 in summer 2022 posed an enormous challenge to colonial (pelagic) seabird populations. The emergence of H5N1 infections in the marine environment signalled a significant change in the dynamics of the virus, with unprecedented mortalities affecting dozens of seabird species in seabird colonies throughout the North Atlantic. High densities of H5N1 susceptible birds (at roosts, foraging grounds or, now, in colonies) are at risk to become infected and to subsequently infect conspecifics. Well-known routes of transmission are through virus excreted in faeces, or, for scavengers and predators, following consumption of infected birds. High pathogenic H5N1 viruses are exceptional, because they can also replicate in and be shed from the respiratory tract of infected birds. If the virus can survive sufficiently long under abiotic conditions, for example in freshwater basins, the frequent virus outbreaks in wetlands affecting swans, geese and various ducks are easily understood. Pelagic seabirds were apparently not at risk, or not thus far, until the most recent outbreaks in the marine environment. Current pressing questions therefore include: how persistent is avian influenza in salt water and how and where come piscivorous, strictly marine, Northern Gannets in contact with the virus? It is therefore critical to understand the incidence and mortality risks of HPAIV H5N1 for seabirds, including the scale and timing of inflicted damage.

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