Abstract
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.
Highlights
Harbor porpoises (Phocoena phocoena) are found in nearshore waters along the U.S West Coast from north of Point Barrow, Alaska (AK), and south to Point Conception, California (CA) [1,2,3], with very few sightings in offshore waters [3]
American shad was responsible for the majority of harbor porpoise asphyxiations
It is striking that American shad has not been documented as harbor porpoise prey in the Atlantic, where the fish is native
Summary
Harbor porpoises (Phocoena phocoena) are found in nearshore waters along the U.S West Coast from north of Point Barrow, Alaska (AK), and south to Point Conception, California (CA) [1,2,3], with very few sightings in offshore waters (beyond the 200 m isobath) [3]. Harbor porpoises on the U.S West Coast are a nearshore species, unlike some harbor porpoises that are seen in offshore waters in the Atlantic (e.g., off Greenland [5]). Oceans 2020, 1 the U.S West Coast, harbor porpoises are known to mainly prey on small fish species, such as Pacific herring (Clupea pallasi), walleye pollock (Theragra chalcogramma), Pacific hake (Merluccius productus) northern anchovy (Engraulis mordax) and Pacific sandlance (Ammodytes hexapterus), that average less than 30 cm in length [6,7,8,9,10,11,12], a similar finding to other populations worldwide [13,14,15,16,17,18,19,20,21,22,23].
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