Sea-ice perturbation and mass starvation of Thick-billed Murres in the Northwest Atlantic Ocean

Mass mortality among seabirds has been associated with ocean heatwaves. Though less commonly reported, cold water events can also be devastating. During spring 2022, thousands of Thick-billed Murres ( Uria lomvia Linnaeus 1758) died on the southern Labrador and northeastern Newfoundland coasts. We describe the extent of mortality, its timing, spatial extent, and associated ocean-climate conditions. We diagnose mortality and compare carcasses to healthy birds. The rapid-onset, 5-week event lasted from 18 March to 25 April 2022, emanating from an impact site in southern Labrador. Extreme winds compacted and reduced sea ice area, which exposed sub-zero water temperatures and limited feeding options. Neither sex nor age were associated with mortality, and a subset of carcasses tested negative for influenza A. Carcass masses averaged 65.5% of the body mass of healthy birds, indicative of a threshold for stage III starvation (protein catabolism). Photographs and measurements showed significant pectoral muscle wasting, and pectoral mass was less than that of healthy birds. The probable cause of the die-off is starvation accelerated by a rapid drop in water temperature and limited foraging options driven by a sea-ice perturbation. Ocean-climatic variability and extreme events are expected to increase, posing challenges for polar seabirds.