Orcas are killing porpoises—but not eating them. Why?

In 2005, Deborah Giles saw something she’ll never forget—a dead porpoise, riding the snouts of a pod of orcas off Washington State.

“What on Earth is happening?” wondered Giles, the science and research director for the nonprofit Wild Orca, based in Friday Harbor. “It didn't make any sense.”

Scientists first recorded this behavior in southern resident killer whales in 1962, and since then, eyewitnesses have observed more than 70 such incidents, peaking in 2005 at 10.

Often working as a team, these fish-eating orcas push porpoises with their snouts, hold them in their mouths, and carry them above the water. Sometimes orcas toy with their victims, allowing them to escape before recapturing them. Some are more aggressive, tossing the porpoise, slapping it with their flukes, or shaking it in their mouths. Most of the time, the porpoises presumably die following such rough treatment.

Because of noise pollution, contamination, and low prey availability, only 75 southern resident killer whales remain, and they were listed as endangered in the U.S. in 2005. Northern resident and Alaska resident killer whales, the other two populations in the eastern North Pacific, have only been seen engaging in such harassment a handful of times.

For a study published last month in the journal Marine Mammal Science, Giles and colleagues reviewed 78 incidents of southern resident killer whales harassing or killing harbor or Dall’s porpoises in the Salish Sea between 1962 and 2020, grouping them by characteristics like pod, sex, and age.

They found the behavior was mostly consistent across age and sex—though four individuals, including three females, showed a particular affinity for interacting with porpoises, with five observed incidents per animal.

“It does make you wonder, from the human perspective, what is influencing this,” says study co-lead author Sarah Teman, a research assistant with UC Davis' SeaDoc Society, a marine conservation nonprofit based in Washington. “Or maybe it's just a fad, and these are their mom jeans that they're bringing back in style.”

Adapting to new prey?

After observing so many instances of orcas harassing and killing porpoises, a million-dollar question emerged, Giles says: “Are they going to start eating porpoise?”

Southern resident killer whales eat chinook salmon, which are also endangered due to habitat degradation and other threats. Even though other orca populations in the western North Atlantic consume porpoises, the southern residents never have. “It's an important question to ask, because [the orcas] are starving,” Giles says.

But even though the orcas sometimes carried the porpoises in their mouths, they never ate them.

“To me, that's the most important thing that came out of this study—is yet another spotlight shining on the fact that we need to be focusing on recovery of their prey in order to make sure that these whales have enough to eat,” Giles says.

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Various efforts to restore salmon populations—including NOAA’s Puget Sound salmon recovery plan—are underway.

Play, prey practice, or parenting

This orca behavior mystifies scientists—but they identified a few potential explanations, such as play.

Orcas are curious, affectionate animals, and they often play to bond, communicate, and have fun, for instance lifting their calves out of the water. “If I had to choose one of the of the [possible] behaviors … I would say it's playtime,” Giles says.

Anaïs Remili, a postdoctoral researcher at McGill University in Montreal who was not involved in the study, agrees that’s most likely explanation. “Play’s something that we definitely observe in the other populations as well,” she says. She notes that orcas near Iceland have been seen harassing birds on occasion—“but it's the same thing; we can never really confirm that they actually ate the bird.”

She also sees a “similar potential parallel” with the orcas off Spain that have been attacking boats in the last few years. “We have so much more to learn about how culture is being spread in marine mammal populations,” she says.

Orcas might also be practicing their hunting skills, using porpoises as “moving targets,” Giles says. Even though the orcas don’t eat the porpoises, their victims are mostly calves that are about the same size as chinook salmon.

Another possible explanation is “mismothering,” or a misplaced attempt to care for weakened or sick young, researchers say.

Nearly 70 percent of southern resident killer whale pregnancies end in miscarriage or calf death, according to Giles, and female orcas have been seen carrying their deceased calves for miles, like J35, who carried her dead baby for 17 days.

The porpoise harassment “might be due to their limited opportunities to care for young,” Giles says.

Scientists have observed orcas mismothering before—an orca took in a baby pilot whale in 2021, for instance—but in these prior cases, the animal usually survives.

“We think that we had it all figured out, but there's so much to learn,” says Teman. “It makes me really excited to think about all we're going to know about the southern residents in 50 years that we're blind to right now.”