Abstract
Harbor seals (Phoca vitulina) are among the most common marine mammal species presented to rehabilitation centers along both North American coasts. Despite disease surveys of stranded harbor seals in other regions, a comprehensive review of the causes of mortality for live-stranded animals presented to rescue facilities has not been undertaken in western Canada. Stranded seals in other areas present with malnutrition, infectious disease, trauma, or human interaction.1 Previous reviews of wild harbor seal mortality have identified infectious disease as the most significant cause of mortality in North America.2,3 In particular, Phocid herpesvirus and enteric pathogens have been associated with significant morbidity.4-6 To better understand factors that may impact the conservation and management of this species in western Canada, a retrospective study was conducted to categorize and describe the causes of mortality in live-stranded harbor seals from 2011–2020.
Medical records including necropsy reports for 966 predominantly perinatal live-stranded harbor seals recovered along the British Columbia coast from 2011–2020 and presented to the Ocean Wise Marine Mammal Rescue Centre (MMR) were reviewed. Approximately 17.1% (165 individuals) of these animals died while at MMR with interannual variation ranging from 12.5–20.4%. Most of the seals were admitted to MMR in July–September (83.2%) which correlates with the regional pupping season. Mortality occurred at a mean of 11.9±5.16 days after admission with a mean body mass of 10.8±1.58 kg at the time of death. Necropsies were performed by a board-certified veterinary pathologist and consisted of gross examination, histopathology, and ancillary diagnostic testing. Of the 165 deceased individuals, seven records were excluded due to incomplete or missing data. Seals were classified into the following age groups: pups (<2 months; n=135), weaned pups (2–12 months; n=18), subadults (12–48 months; n=1), or adults (>48 months; n=4).7 The primary cause of mortality was classified into one of nine categories:7
1. Congenital
2. Infectious
3. Metabolic
4. Nutritional
5. Parasitic
6. Human interaction
7. Non-anthropogenic trauma
8. Other
9. Undetermined
As with prior studies, infectious disease was the most common cause of death accounting for 64% of mortality (101/158). Phocine herpesvirus-1 infection was identified in 15% of the mortalities with the highest prevalence occurring in 2019 (n=3/13, 23%). Other common pathogens included Clostridium difficile and Pseudomonas aeruginosa. There were two cases of pulmonary mycosis due to Cryptococcus gattii. Pups were the most common age class (85% of mortalities, 135/158) and predominantly died of an infectious process (67%). Approximately 25% of the seal pup mortalities (34/135) were considered premature animals based on the presence of a lanugo coat. In all age classes, non-anthropogenic trauma (7.6%) and nutritional-related deaths (7.0%) were the next most common causes of mortality. In six cases, mortality was directly linked with congenital disorders. Two of these cases involved axial skeletal malformities that are not currently described in the literature.8 More recently, sporadic idiopathic megaesophagus has been detected in neonates.
This is believed to be the first study describing the causes of mortality in harbor seals undergoing rehabilitation in BC, Canada. Understanding these causes may result in earlier diagnosis leading to targeted medical treatment and, potentially, improved outcomes. Mortality studies can lead to the identification of new diseases that affect wild populations and are an important source of information on the health of the marine ecosystem.1
Acknowledgements
The authors wish to thank the Vancouver Aquarium Marine Mammal Rescue Center for their contributions to this study and their efforts for marine mammal rescue and rehabilitation. The technical staff at the Animal Health Center, Abbotsford, BC, Canada is gratefully acknowledged. Marine mammal rescue and rehabilitation performed under agreement with the Department of Fisheries and Oceans Canada (DFO) contract #F5211-190227.
*Presenting author
+Student presenter
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