Abstract

In the Salish Sea, a 16,925 km² inland sea shared by Washington, USA, and British Columbia, Canada, the Pacific harbor seal (Phoca vitulina richardii) population (totaling around 50,000 individuals) has been at equilibrium for over two decades.^1,2,3 Pre-exploitation harbor seal population size in the Salish Sea is unknown, but after the cessation of bounty programs in the 1960s and the adoption of protective measures in the 1970s, harbor seal numbers on both sides of the international border increased exponentially until reaching presumed carrying capacity in the mid-1990s. This stable population of marine mammals offered a novel opportunity to evaluate whether disease acts in a density-dependent manner to limit population growth.

We conducted a retrospective analysis of harbor seal stranding and necropsy findings in the San Juan Islands sub-population to assess age-related stranding trends and causes of mortality. Between January 01, 2002 and December 31, 2018, we detected 882 harbor seals that stranded and died in San Juan County and conducted necropsies on 244 of these animals to determine primary and contributing causes of death. Age-related seasonal patterns of stranded animals were evident, with pups found in the summer, weaned pups recovered primarily during fall, and adults and sub-adults recovered in summer and fall. Pups were the most vulnerable to mortality (64% of strandings). Pups predominantly died of nutritional causes (emaciation) (70%), whereas sub-adults and adults presented primarily with clinical signs and gross lesions of infectious disease (42%) and with non-anthropogenic trauma (27%). Primary causes of weaned pup mortality were distributed equally among nutritional, infectious, non-anthropogenic trauma, and anthropogenic trauma categories. Nutritional causes of mortality in pups were likely related to limitations in mid- and late-gestational maternal nutrition, post-partum mismothering, or maternal separation possibly related to human disturbance. Infectious causes were contributing factors in 33% of pups dying of nutritional causes (primarily emaciation-malnutrition syndrome), suggesting an interaction between poor nutritional condition and enhanced susceptibility to infectious diseases. Additional primary causes of harbor seal mortality were related to congenital disorders, predation, human interaction, and infections, including zoonotic and multidrug-resistant pathogens.

Bottom-up nutritional limitations for pups, in part possibly related to human disturbance, as well as top-down predatory influences (likely under-represented through strandings) and infectious disease, are important regulators of population growth in this stable, recovered marine mammal population.

Acknowledgments

We thank the numerous San Juan County Marine Mammal Stranding Network volunteers who assisted with carcass collection and necropsy and Wolf Hollow Wildlife Rehabilitation Center for their efforts in stranded harbor seal rehabilitation and release. All samples were collected under permits from the National Marine Fisheries Service Marine Mammal Health and Stranding Response Program Permit #18786. Harbor seal stranding response and necropsies were performed using funding from the John H. Prescott Marine Mammal Rescue Assistance Grant with in-kind support from The Whale Museum and the SeaDoc Society, a program of the Karen C. Drayer Wildlife Health Center, University of California, Davis School of Veterinary Medicine.

*Presenting author
+Student presenter

Literature Cited

1.  Jeffries S, Huber H, Calambokidis J, Laake J. 2003. Trends and status of harbor seals in Washington State: 1978–1999. J Wildlife Manage 67:207–218.

2.  DFO. 2010. Population assessment Pacific harbour seal (Phoca vitulina richardsi): Canadian Science Advisory Secretariat, Science Advisory Report 2009/011. Vancouver, BC: Fisheries and Oceans Canada.

3.  Majewski SP, Ellis GM. 2019. Abundance and distribution of harbour seals (Phoca vitulina). British Columbia, VIC: Prep Publishing.