High prevalence of head and neck lesions in stranded seals: cause of death?

Authors: Jan Haelters, Francis Kerckhof & Sophie Brasseur

Abstract: In 2021 Belgian beaches witnessed 101 deceased, washed ashore seals. This number is extremely high compared to the one in previous decades, with figures never exceeding 50. Of these animals, 21 grey seals (Halichoerus grypus), 13 harbour seals (Phoca vitulina) and 2 unidentified seals were collected for necropsy, and photographs were available for an additional 49 seals. Of the 90 animals that were necropsied or for which information was available, such as clear photographs, 58 (64%) presented severe head and neck lesions, which in 27 cases we described as a circular lesion around the neck, known in forensic medicine as ligature marks. Most of the animals with circular neck lesions were juveniles. We suspected that bycatch was the most probable cause of death in these 27 cases, though the characteristic lesions often seemed to have occurred post-mortem. Due to the nature of the lesions, we could exclude other possible backgrounds such as knife cuts or predation marks left by grey seals. We hypothesize that the animals were caught in nets and died due to asphyxia, while the hauling of the net caused the typical lesions. Some seals, however, survived: three live seals with similar neck lesions were found, still bearing parts of a nylon monofilament fishing net. The observations warrant a further investigation of the phenomenon, including of the system of bycatch, the spatial and temporal extent, the number of seals bycaught and the type of fishing vessel involved.