Prey-related asphyxiation in harbor porpoises

A paper published in the journal Oceans in 2020 describes cases of prey-related asphyxiation in harbor porpoises along the U.S. West Coast. The findings suggest that a majority of cases involve non-native American shad and that asphyxiation tends to occur more with reproductively active females than other age and sex classes.

A surfing harbor porpoise in Burrows Pass, off Fidalgo Island, WA. Photo: Copyright Cindy R. Elliser, Pacific Mammal Research
A surfing harbor porpoise in Burrows Pass, off Fidalgo Island, WA. Photo: Copyright Cindy R. Elliser, Pacific Mammal Research


Harbor porpoise (Phocoena phocoena) diets are predominantly comprised of small fish species (<30 cm) and squid. However, predation on larger species (up to 63 cm) occurs, raising the question of increased risk of asphyxiation associated with this behavior. Literature was reviewed and stranding data from 1983 to 2020 from the U.S. West Coast (including California, Oregon and Washington) were searched for cases of prey-related asphyxiation of harbor porpoises and analyzed in relation to age, sex, reproductive status and prey species. Twenty-nine cases were documented. Twenty-seven cases involved large prey; non-native American shad caused the asphyxiation in 87% of the cases where the prey species was identified. The majority (92%) of harbor porpoises were females, and at least 83.3% were pregnant or recently post-partum. Reproductively active females may be more likely to attempt potentially risky behavior in order to compensate for their increased energetic needs. Increasing numbers of non-native American shad may pose a unique danger in this region for harbor porpoises not adapted to deal with the challenges of that prey. This may be a cause for concern, as there is likely an interaction between location, age and reproductive status on the diet composition and foraging strategies of harbor porpoises.


Elliser, C.R., J. Calambokidis, D.N. D'Alessandro, D.A. Duffield, J.L. Huggins, J. Rice, I. Szczepaniak, and M. Webber. 2020. Prey-Related Asphyxiation in Harbor Porpoises (Phocoena phocoena) along the U.S. West Coast: Importance of American Shad (Alosa sapidissima) on Adult Female Harbor Porpoise Mortality. Oceans 2020 1(3), 94-113. doi:10.3390/oceans1030008

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About the Author: 
Cindy R. Elliser 1, John Calambokidis 2, Dalin N. D’Alessandro 3, Deborah A. Duffield 3, Jessica L. Huggins 2, James Rice 4, Isidore Szczepaniak 5,6 and Marc Webber 5,6 1 Pacific Mammal Research, Anacortes, WA; 2 Cascadia Research Collective, Olympia, WA; 3 Department of Biology, Portland State University, Portland, OR; 4 Marine Mammal Institute, Oregon State University, Newport, OR; 5 The Marine Mammal Center, Sausalito, CA; 6 Department of Ornithology and Mammalogy, California Academy of Sciences, San Francisco, CA