| Summary: | Documenting human interaction (e.g., boat collisions, gunshot wounds and fishery interactions) in marine mammal strandings is necessary to understand anthropogenic impacts and can be used to inform stock assessments as well as policy and management decisions. Boat propellers can inflict serious harm to marine mammals and the associated injuries are often fatal, however little is known about their impact on harbor seals in the region. A retrospective analysis of harbor seal (Phoca vitulina) strandings in San Juan County, WA revealed a 1.1% prevalence (8/718) of propeller strike mortality since the first clearly documented case in 2006. All eight cases were investigated by individuals trained to respond to marine mammal strandings and six freshly dead carcasses received complete necropsies. Diagnoses were based on established evidence of propeller strike injuries including cleanly incised wounds, uniform spacing between lacerations, and one eye-witness account. Propeller strike injuries were found in both males (50%) and females (37.5%) and one animal of unknown sex (14.2%). All eight cases involved nursing or weaned pups, which is consistent with previous findings suggesting immature age classes are particularly vulnerable to this type of injury. Propeller strikes have been associated with an Unusual Mortality Event (UME) of pinnipeds in the United Kingdom and may have the potential to impact species at the population level. Baseline data on prevalence of propeller strikes in a harbor seal population at carrying capacity serves as a reference for struggling pinniped stocks where little data on propeller induced mortality is available, but could potentially impact population recovery.
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