Postmortem interpretation of drowning-related findings in seabirds: Implications for forensic casework involving prolonged freezing

Seabird bycatch in commercial fisheries is a major conservation concern worldwide, yet pathological investigations of confirmed drowning cases remain limited, particularly in carcasses subjected to prolonged frozen storage prior to necropsy. This study describes the gross and histopathological findings of three seabirds representing different species that were accidentally captured and drowned in bottom-set gillnets off Hokkaido, Japan, in 2024, and subsequently stored frozen before examination. Necropsy and histopathological analyses were performed to identify lesions potentially associated with drowning while considering the influence of freeze–thaw artifacts on tissue preservation and interpretation. Common findings included pulmonary congestion, venous distension, and congestion of internal organs, particularly the lungs and kidneys. In contrast, severe liquefaction and structural disruption of brain tissues were considered strongly influenced by prolonged freezing, limiting reliable histopathological interpretation. Skeletal muscle tissues appeared comparatively well preserved, with minimal degenerative or inflammatory changes observed. These findings suggest that certain vascular and congestion-related lesions may remain macroscopically and histologically recognizable despite frozen storage, whereas delicate neural tissues are substantially more susceptible to freeze–thaw artifacts. Given the limited number of confirmed pathological investigations of seabird bycatch drowning, this exploratory study provides baseline postmortem reference data for future comparative investigations involving larger sample sizes and multiple seabird species. Furthermore, the study contributes to improving forensic interpretation in delayed or frozen carcasses by distinguishing findings that appear relatively resistant to freeze–thaw artifacts from those that may be substantially altered by prolonged freezing.