Salish Sea Ecosystem Conference

Harbor Porpoises (Phocoena phocoena vomerina) Catching and Handling Large Fish on the U.S. West Coast

The harbor porpoise is a cryptic species, and information on their behavior is limited. This study describes the chase and capture of large fish by harbor porpoises (Phocoena phocoena vomerina) in the Salish Sea off Fidalgo Island, Washington, which were identified as salmonid species (2017/2019). Similar large fish chase/capture events of American Shad (Alosa sapidissima) have also been documented in San Francisco, CA (2016/2017). In all capture events consistent behavior was observed: the porpoise accelerated after the fish, swimming in a circle at the same spot, diving and coming out of the water head first carrying the fish cross-wise in its mouth. While the catch of the large prey was visible just below the surface, it remains unknown whether the porpoise eventually consumed the fish. We also document a harbor porpoise that drowned in drift net gear that had fed on salmonid species in Cook Inlet, Alaska (2014). Salmonid species and American Shad have not been documented as prey items for harbor porpoises along the U.S. West Coast and Salish Sea, despite diet studies that have spanned over 30 years. These species are, on average, larger than typical prey species known to be consumed by harbor porpoises. The morphology of the upper respiratory tract in odontocetes may make them more vulnerable to an esophageal obstruction by large prey items that can lead to asphyxiation, and indeed cases of harbor porpoises dying from asphyxiation have been documented. As harbor porpoises have a very high metabolism, the high nutritional pay off may be worth the possible risk of catching a larger prey item. Little is understood about wild harbor porpoise behavior and reports such as this helps to better understand their behavioral repertoire, ecological relationships, and the role they play in Salish Sea trophic energy flow.