Harassment and killing of porpoises (“phocoenacide”) by fish‐eating Southern Resident killer whales (Orcinus orca)

Abstract

Endangered Southern Resident killer whales (Orcinus orca) are fish‐eaters that preferentially prey on adult Chinook salmon (Oncorhynchus tshawytscha). Despite being salmon specialists, individuals from all three killer whale pods (J, K, L) have been observed harassing and killing porpoises (family Phocoenidae) without consuming them. Retrospectively, we identified and analyzed 78 episodes of Southern Resident killer whales harassing porpoises between 1962 and 2020, of which 28 resulted in the porpoise's death (“phocoenacide”). Fifty‐six episodes involved harbor porpoise (Phocoena phocoena), 13 involved Dall's porpoise (Phocoenoides dalli), and the porpoise species was unreported for nine episodes. Southern Resident killer whales often targeted young porpoises that were similar in size to adult Chinook salmon. Both sexes participated in porpoise harassment. Juveniles engaged in the behavior the most; however, their rates of engagement were not found to differ significantly from most other age classes. The behavior was passed through generations and social groupings, as it was first observed in L pod and spread to the other two pods. Killer whales are highly complex animals known to exhibit social learning and cultural transmission of learned behaviors, but the reason(s) for this behavior is unknown. Hypotheses include the social and developmental benefits of play, hunting practice, or displaced epimeletic behavior.

A shortfin mako shark circling a finless porpoise with damaged caudal fin

“…Taken together, natural predators such as killer whales and sharks, rather than artificial or diseased factors, likely were responsible. Although killer whales may leave porpoises they attack without eating them (Giles et al, 2024), there were no sightings of a pod of killer whales in the Seto Inland Sea during the period of this observation. Considering that only a short time had passed since the finless porpoise lost its caudal fin, it is more plausible that the mako shark injured the porpoise, rather than other sharks, such as white sharks, attacking and then ignoring the immobilized porpoise.…”