Causes of Mortality in Harbor (Phoca vitulina) and Gray (Halichoerus grypus) Seals in Denmark

Determination of the causes of mortality in stranded marine mammals can contribute valuable information for conservation of wild populations, as well as contribute to risk assessments for different pathogens, hosts, and environmental conditions. This study examined necropsy reports for harbor (Phoca vitulina; n = 213) and gray (Halichoerus grypus; n = 40) seals stranded in Denmark in the period 2014 to 2021 to determine the causes of mortality where feasible. The likelihood that human interactions did or might have contributed to the mortality was also assessed. Infection with lungworms, heartworms, gastrointestinal roundworms, and influenza virus was tested for each seal in the data. Parasitic bronchopneumonia was the most common cause of death in both harbor (n = 68) and gray (n = 8) seals, and significantly more juveniles than adults died as a result of parasitic infections in the harbor seal cohort. Starvation was also a major cause of death in juvenile seals. Cause of death, death class (found dead, euthanized, or culled), and whether human interactions played a role in mortality did not vary significantly between the two species. Traumatic causes of death, resulting from confirmed or probable human interactions, were associated with adult and subadult seals of both species. Culling was the cause of death for 13.6% of harbor seals and 17.5% of gray seals.