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SEATTLE — One might think of local southern resident orcas as sea pandas, playfully spy hopping through their day. The local porpoises will tell you different.

Scientists for decades have observed fish-eating orcas mauling and killing Dall’s and harbor porpoises. But why do they do it? It’s not to eat them, the southern residents that frequent Puget Sound eat only fish, primarily Chinook salmon. Even when Chinook are scarce, they do not switch to eating the abundant sea mammals, including porpoises all around them, as transient, or Bigg’s killer whales, do. So what are they up to?

In a study published Thursday in Marine Mammal Science, researchers analyzed 78 documented interactions between southern residents and porpoises in the Salish Sea, mostly around the San Juan Islands, beginning in 1962.

Their paper is an amazing catalog of observations.

Orcas chasing, pushing, ramming, tossing, capturing, releasing and recapturing, and slapping porpoises with their flukes; thrashing them in their mouth, and carrying them around. They balance porpoises on their head, and jam them in their mouth, harass, puncture and drown them. But one thing they never do is eat them, not even a one bite — though they will rake them with their long, pointed teeth.

Some porpoises survive these encounters. Others do not — and the orcas have been seen still playing with them, diving down after the carcass to bring it up and knock it around some more. Some of these encounters involve just one orca, but sometimes as many as 15 to 25 orcas will gang up and carry on, the scientists reported.

Sometimes the orcas shoulder the porpoise, carrying it around together between them, squeezed between their flanks, seeming to engage in a game of seeing see how long they can keep it out of the water. They often would play with the porpoise for hours. Even long after it was dead. So killing it is not the sole, or even primary, objective.

The behavior, an example of which is captured in a YouTube video by nonprofit Wild Orca, is not unique to the southern residents; northern and Alaska resident killer whales also mess with porpoises. But not nearly as much as the southern residents do, the scientists reported.

The first time she saw this going on in the waters of the San Juan Islands in 2005, Deborah Giles of Wild Orca, and lead author on the paper, said she couldn’t figure out what the orcas were doing. “It was like, ‘What on earth is that?’” Giles said. She began asking around and learned other researchers had witnessed similar interactions, too. A highly collaborative paper was born of observations and a deep literature dive, ultimately involving 20 scientists across 16 organizations, different disciplines, widespread geography and generations of scientists and orcas.

While it is still a head scratcher, some theories are emerging.

Scientists have three primary hypothesis, among many, about what’s going on. One is that the whales are practicing their salmon-hunting techniques. Porpoises make great moving targets for hunting practice, particularly harbor porpoise: They are abundant, and the juveniles are about the size a Chinook salmon used to be, at 50 pounds.

Another is that the orcas are just horsing around. Play among orcas as with other intelligent, social animals is part of bonding, communicating and just having fun. This play might also benefit group communication and teamwork, too, the scientists posited.

Or maybe the southern resident females of reproductive age — who lose most of their pregnancies due to nutritional stress — are engaging in displaced mothering behavior, attempting to care for porpoises they perceive as weaker or ill.

Southern residents are known to carry their calves for a time when they die, as most famously mother orca Tahlequah, J35, did when she carried her hours-old dead calf for 17 days and more than 1,000 miles through the Salish Sea in summer 2018, raising worldwide concern for the plight of the endangered southern resident orcas.

Mismothering has been observed in both genders and even juvenile orcas. An orca will swim over to a porpoise mother and snatch their baby away and go play with it, just as orcas will with their own young, Giles said. “They will actively pursue the mother, and go get it,” she said of the baby.

A lot of what goes on looks like orca family play, Giles said.

“They are very tactile, there is a lot of playing and carrying and that is a lot of what we see here,” Giles said. “We would see them pec to pec with the porpoise between them, just surfing in the water.”

Very often a porpoise toyed with winds up without a scratch on it. “In many cases if you didn’t know there had been an interaction with a killer whale, you would not know it, very often there are no visible markings,” Giles said. But of course such a small animal with tiny lungs would have no chance when a killer whale dives with it, Giles noted. “No one likes to see that.”

The southern residents are struggling to survive, with only 75 counted in the 2023 census. They face multiple challenges, including lack of sufficient Chinook salmon throughout their foraging range. The biggest caloric bang for the hunting effort, Chinook and the southern residents co-evolved in the Salish Sea and eating salmon is a deeply ingrained part of orcas’ culture.

Southern resident females lose about 70% of their pregnancies, according to a 2017 study. That work showed the high rate of pregnancy failure was linked to nutritional stress, due to low Chinook salmon abundance.

When you are an orca, you are what you eat. Orcas live in every ocean of the world, and there is only one species, Orcinus orca, but many types, defined by their cultures including language, primary home range and diet.

So even as the number of sea mammals of all types in the northeastern Pacific has surged, due to cessation in hunting, Chinook is still 50% to 100% of the southern residents’ diet, depending on the season. The southern residents also will eat coho and chum, steelhead and even occasionally lingcod and halibut, research by Brad Hanson, scientist at the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center in Seattle, has shown.

The fact the orcas can so easily catch and kill porpoise — but emphatically will not even taste it, shows how vital salmon are to their survival.

“The diet of these whales has been reinforced genetically and culturally for thousands of years, they really need salmon to survive,” said Sarah J. Teman, second author on the paper who worked on it while a research assistant at the nonprofit SeaDoc Society. “What I see from this is how important it is to restore our salmon runs and their habitat and to protect them against climate change, so the southern residents have a chance to rebound.”

But while porpoises won’t be dinner, the southern residents’ harassment behavior is increasing in frequency and spreading, the scientists reported, as they teach it to one another among the J, K and L pods and pass it on to the next generation.

It’s not universal however. Some of the matrilines within the J, K, and L pods, including the family of L25, the oldest mother orca, have never been seen roughing up porpoises.

Yet.