The ongoing study of harbor porpoises from bluffs overlooking Burrows Bay in Anacortes contributed recently to new findings regarding the species along the West Coast.
Katrina MacIver of the local organization Pacific Mammal Research presented the findings at the first digital Salish Sea Ecosystem Conference in late April.
Observations and photos from the Anacortes researchers were combined with observations and photos from San Francisco Bay researchers and the analysis of stomach contents of dead porpoises from Cook Inlet in Alaska.
For the first time, the researchers collectively documented harbor porpoises interacting with and likely eating salmon and a fish called American shad — both much larger species than the porpoises have been documented eating in the past.
“Diet analysis studies available along the U.S. West Coast and Salish Sea spanned over 30 years (1981 to 2012) and documented variation in both prey type and frequency of consumption yet did not document salmonids or American shad,” the researchers wrote in a paper published in March in the research journal “Aquatic Animals.”
While the Anacortes researchers observed harbor porpoises in an ongoing effort to identify individual animals, some were seen chasing salmon to the surface in Burrows Bay.
“We saw them swim rapidly in circle, dive, surface headfirst and then catch the fish just under surface,” MacIver said. “Sometimes it was dramatic with splashing.”
Similar behavior was observed by the San Francisco researchers, although from their observation site on the Golden Gate Bridge they saw porpoises chasing American shad.
“The sequence of events here was very similar to that seen for the harbor porpoise pursuing the salmon in Burrows Bay. The porpoise is clearly accelerating after the fish, turning tightly at the surface as the chase continues,” they wrote in the paper. “After the fish is caught, the porpoise surfaced multiple times, carrying the fish cross-wise in its mouth as did the harbor porpoise carrying the salmon in Burrows Bay.”
Several photos captured by the Pacific Mammal Research team in Anacortes showed porpoises with salmon. In one photo from July 25, 2019, researchers identified the fish as a coho.
“Most of our information about porpoises comes from dead animals or from animals in captivity, so this type of observational report is very, very important,” MacIver said. “We’ve been doing a long-term harbor porpoise identification study and part of that is to study the foraging activity and behavior of individual porpoises.”
The Pacific Mammal Research team has since 2014 been studying harbor porpoises in Anacortes, led by founder and Director Cindy Elliser. The recent glimpse into a porpoise-salmon relationship tells them more work needs to be done.
“Wild harbor porpoise behavior remains poorly understood,” the recent publication states. “Reports such as this are important to improve our knowledge of this difficult to observe species and highlight the need for further research and monitoring to fully understand their behavioral repertoire and ecological relationships.”