DOI: 10.5061/dryad.q83bk3jnn
title: Estimated mortality of the Highly Pathogenic Avian Influenza (HPAI) pandemic on Northern gannets (Morus bassanus) in southwest Ireland

description: The 2022 Highly Pathogenic Avian Influenza (HPAI) outbreak that occurred
 in 2022 in many European countries affected several seabird species. Among
 them, Northern gannets (Morus bassanus), were particularly impacted. We
 conducted aerial surveys in waters around the two largest gannet colonies
 in Ireland (Little Skellig and Bull Rock, together representing 87% of the
 national population) in September 2022. During surveys, 184 dead gannets
 were recorded on survey effort, representing 3.74% of the total number of
 gannets recorded. We estimated the abundance of dead gannets in the
 surveyed area at 1,526 (95% CIs 1,299–1,793) individuals. The percentage
 of dead gannets observed was used to estimate a minimum local population
 mortality of 3,118 individuals across both colonies. Aerial surveys proved
 a reliable methodology providing key information on gannet mortality
 derived from HPAI at sea. The study provides the first estimate of gannet
 mortality in the two largest gannetries in Ireland.
description: Aerial surveys were conducted along the
 southwest coast of Ireland encompassing the two largest gannet colonies in
 the country, Little Skellig (35,294 breeding pairs) and Bull Rock (6,388
 breeding pairs), and surrounding waters used for foraging. These two
 colonies account for 87% of the country’s breeding gannet population
 (Cummins et al., 2019). Surveys
 were conducted in September 2022 over an area of approximately 12,036
 km<sup>2</sup> using a twin-engine fixed-wing aircraft
 equipped with bubble windows to afford observers an unrestricted view of
 the sea below the aircraft. Transects were spaced approximately 3.7 km
 apart (Figure SM1, supplementary material) and flown at a height of 75m
 above sea level and a speed of approximately 185 km/h using strip-transect
 methodology (Eberhardt, 1978) in the target sea state of Beaufort ≤3 and
 &gt;2km visibility. Two observers recorded all seabirds within 200
 metres on either side of the aircraft along the transect lines noting
 species, number of birds, and behaviour, with each sighting date/time
 linked to location using a Garmin GPS 60. Gannets could be distinguished from other seabird
 species due to their larger size and colouration, and dead individuals
 identified by posture; floating sideways with an extended neck and the
 wings not folded against their body, while live individuals were in flight
 or sitting on the water with the wings folded against the body.
 Survey effort, and density of gannets were
 calculated in QGIS 3.22.7 (http://www.qgis.org) using a 4x4km grid across
 the study area. The total area surveyed per cell was estimated by
 multiplying the distance flown in each cell by the strip width (0.4 km if
 both observers were on effort, 0.2 km if one observer was on effort, 0 if
 both observers were off effort). The number of dead and alive gannets
 within each cell was counted and divided by the survey effort to obtain
 the density (individuals/km<sup>2</sup>) of dead and alive
 gannets. Estimated total abundance across the survey area was obtained by
 multiplying density by cell area and summing across all cells (Jessopp et
 al., 2018). Poisson 95% Confidence intervals (CIs) were calculated
 following the exact method: qchiscq(0.025, 2*x)/2 , qchiscq(0.975,
 2*(x+1))/2 To assess
 the representativeness of the survey area in relation to the foraging
 ranges of gannets from Little Skellig and Bull Rock, tracks from 23
 individuals (9 from Little Skellig and 14 from Bull Rock) previously
 published by Wakefield et al (2013) were used to calculate a 50% Kernel
 Utilization Distribution (UD), representing the core foraging
 range, for each colony using the R Package Adehabitat HR (Calenge, 2006).
 Separate colonies' UDs were merged in QGIS to obtain a combined
 representative foraging range for both colonies, and the percentage of
 overlap with the surveyed area was calculated. We made an initial estimate
 of the population level impact of HPAI in southwest Ireland calculated
 as: Estimated population mortality
 (EPM) = breeding gannets X (mean dead gannets estimate/mean alive gannets
 estimate) Maximum and minimum
 estimates were calculated as: Min EPM = breeding gannets X (min
 dead gannet estimate X (max dead gannet estimate/min alive gannet
 estimate)) Max EPM = breeding gannets X (max
 dead gannet estimate X (min dead gannet estimate/max alive gannet
 estimate))   REFERENCES: Calenge C. 2006 The package ‘adehabitat’ for the R
 software: A tool for the analysis of space and habitat use by animals.
 Ecol Modell 197, 516–519. (http://dx.doi.org/10.1016/j.ecolmodel.2006.03.017) Cummins S., Lauder C., Lauder A., Tierney D. 2019 The Status of Ireland’s Breeding Seabirds: Birds Directive Article 12 Reporting 2013 – 2018. Eberhardt L.L. 1978 Transect Methods for Population Studies. J Wildl Manage 42, 1-31. (http://dx.doi.org/10.2307/3800685) Jessopp, M., Mackey, M., Luck, C., Critchley, E., Bennison, A., Rogan, E., 2018. The seasonal distribution and abundance of seabirds in the western Irish Sea.  Wakefield E.D., Bodey, T.W., Bearhop, S., Blackburn, J., Colhoun, K., Davies, R., Dwyer, R.G., Green, J.A., Grémillet, D., Jackson, A.L., Jessopp, M.J., Kane, A., Langston, R.H.W., Lescroël, A., Murray, S., Le Nuz, M., Patrick, S.C., Péron, C., Soanes, L.M., Wanless, S., Votier, S.C., Hamer, K.C. 2013 Space partitioning without territoriality in gannets. Science 341, 68–70. (http://dx.doi.org/10.1126/science.1236077)