Why does a group of B.C. killer whales harass and kill porpoises without eating them?

A group of orcas that inhabit Canadian waters are known to harass and kill porpoises without eating them. Known as the southern resident killer whales, the predominately salmon-eating community spends much of its time off the coast of British Columbia.

"Fish-eating killer whales have a completely different ecology and culture from orcas that eat marine mammals — even though the two populations live in the same waters," Deborah Giles, the research director of conservation non-profit Wild Orca, said in a news release. "So we must conclude that their interactions with porpoises serve a different purpose, but this purpose has only been speculation until now."

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In a new study, Giles and other researchers attempt to explain the behaviour, which has puzzled scientists for decades. By analyzing 78 documented cases of porpoise harassment between 1962 and 2020, they offer three plausible explanations: social play, hunting practice, and so-called "mismothering" behaviour.

"Southern Resident killer whales were regularly documented harassing or killing porpoises over the last 45 years, and this behaviour appears to be both increasing in frequency and spreading among the population," the study states. "Even though this endangered population is prey-limited, consumption of porpoises was never observed."

While other groups of orcas are known to hunt and eat porpoises, the southern residents exclusively eat fish. An endangered population of only 75 animals, their survival is intricately linked to another endangered species, the Chinook salmon, which makes up the bulk of their diet. Travelling between California and Alaska over the course the year, the whales can usually be found in the sheltered waters of the Salish Sea in British Columbia and Washington state from spring through fall.

Like many intelligent species, orcas may engage in playful activities to communicate, bond or simply have fun, the researchers hypothesize. Harassing porpoises could also help orcas hone their salmon-hunting skills.

"Differentiating killer whale hunting practice from play is not always possible, and the two are likely interrelated," the study explains. "For (southern resident killer whales), porpoise-harassing behavior could provide both practical salmon hunting practice and play that benefits group cohesion, coordination, and bonding."

Mismothering could mean the whales are trying care for porpoises perceived to be weak or ill. Females in the community have been seen keeping deceased calves afloat similarly to how they carry porpoises. 

“Mismothering behaviour — also known as ‘displaced epimeletic behaviour’ to scientists— might be due to their limited opportunities to care for young,” Giles said. “Our research has shown that due to malnutrition, nearly 70 per cent of Southern Resident killer whale pregnancies have resulted in miscarriages or calves that died right away after birth.”

The southern resident orcas made headlines in 2018, when a member J35 tragically carrier her dead calf for 17 days and 1,600 km. A separate population off the coasts of Spain and Portugal have also been making news for attacking and even sinking boats.

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The study was co-led by Sarah Teman from the SeaDoc Society, a program of the University of California Davis School of Veterinary Medicine, and included funding in part from Fisheries and Oceans Canada. It was published Thursday in the peer-reviewed journal Marine Mammal Science.

"We found that porpoise-harassing behaviour has been passed on through generations and across social groupings. It’s an amazing example of killer whale culture," Teman said in the news release. "Still, we don’t expect the Southern Resident killer whales to start eating porpoises. The culture of eating salmon is deeply ingrained in Southern Resident society. These whales need healthy salmon populations to survive."