In the narrow channels of Johnstone Strait off northeastern Vancouver Island, northern resident killer whales appear to have discovered an unusual foraging strategy: follow the dolphins.
In a rare example of two marine mammal species working together, scientists have documented northern resident killer whales and Pacific white-sided dolphins co-operatively hunting chinook salmon. The sophisticated partnership was revealed through aerial drones and biologgers - specialized suction-cup cameras attached to killer whales that captured underwater video, acoustic and motion data.
"What we noticed a lot of the time was that the killer whales are following the dolphins, so they would orient towards them at the surface, and then they would follow them on their dives," said Sarah Fortune, the Canadian Wildlife Federation chair in large whale conservation at Dalhousie University and lead author of the study published Thursday in the journal Scientific Reports.
Co-operative hunting is well-documented among marine mammals of the same species. Pack-ice killer whales co-ordinate wave-washing attacks to knock seals off ice floes, bottlenose dolphins herd fish into a "bait ball" for easier capture, and humpback whales use co-ordinated bubble-net feeding, blowing bubbles and vocalizing to trap schools of prey at the surface.
But examples of different marine mammal species co-operatively foraging are more rare, Dr. Fortune says.
Earlier research suggested dolphins or porpoises opportunistically steal food scraps from killer whale hunts without providing clear benefits in return - a behaviour known as "kleptoparasitism." But the latest body of research suggests a more complex, co-operative relationship.
Research published earlier this year in Ecology and Evolution studying the same species in Johnstone Strait between 2018 and 2021 documented frequent interactions between killer whales with either dolphins or porpoises, but from their aerial vantage point, researchers argued the relationship was largely one-sided. While dolphins and porpoises benefited from possible protection from predators, saving energy by drafting alongside the whales, and potentially scavenging food (though the researchers did not observe feeding), the interactions provided little to no apparent benefits to the whales in return. The only exception was occasional play behaviour between young killer whale calves and dolphins.
However, an observational study released in mid-November in Marine Mammal Science, documenting the August 2025 death of a northern resident killer whale in Johnstone Strait, argues these interactions can come at a cost to the whales. When conducting a health check on the emaciated male, known as I76, researchers found him and family repeatedly surrounded by Pacific white-sided dolphins. The whales frequently dive deep or change their behaviour to avoid persistent dolphin approaches. During one of these encounters, I76 dove deep with his pod and never resurfaced.
"These interactions do not appear to benefit northern resident killer whales, whose responses to Pacific white-sided dolphins range from neutral to antagonistic," says Jared Towers, a cetacean researcher contracted by Fisheries and Oceans Canada and co-author of the study.
But the Thursday study - conducted in August, 2020, and funded by the Natural Sciences and Engineering Research Council of Canada - was able to go beneath the surface using Customized Animal Tracking Solutions, or CATS tags, to record fine-scale movements from the whale's point of view.
Dr. Andrew Trites, director of the Marine Mammal Research Unit at the University of British Columbia and co-author of the study, says the light bulbs came on when the research team observed the two species having co-ordinated diving and echolocation behaviour, with the whales relying on the dolphins as scouts.
"They're down there searching for these huge fish, which they are incapable of catching themselves. But they can find them. They just can't catch them," Dr. Trites said of the dolphins.
"They're too big, and so they locate them. The whales use the same technology, these on-board radar systems, and they can take advantage of the dolphins' radar system and use them to help them find fish at depth."
At depth and in low light, the killer whales appear to eavesdrop on dolphin echolocation signals to help locate large chinook salmon - fish that can weigh 30 to 50 pounds and are too big for dolphins to capture and swallow whole.
"Because we see both species at depth, and we hear that they're both producing these echolocation clicks to presumably find fish, it also shows that they're expending time and energy to try to gain access to a prey resource," Dr. Fortune said.
When the whales caught these massive salmon, they brought them to the surface and tore them apart to share with pod members, while the accompanying dolphins scavenged the leftovers.
The absence of aggression in the new study provides further evidence of co-operation, Dr. Fortune says.
"We don't have evidence of kleptoparasitism, and we also don't have any evidence of animosity. We don't see killer whales getting aggressive, like charging dolphins or biting at their flippers, and we don't see them changing direction, and leaving the area. It's actually the reverse, and they're heading toward the dolphins."
One of the earlier studies did observe some mildly antagonistic behaviours. "We did see some tail slapping and lunging of some whales at Pacific white-sided dolphins," said Brittany Visona-Kelly, lead author of that study and senior manager of whale health and monitoring with OceanWise. However, the researcher said, "None of those behaviours seem to deter any of the dolphins or porpoises from continuing to travel and associate with the whales."
Northern resident killer whales, like southern resident killer whales, are fish eaters that pose no predation threat to dolphins or porpoises. While both resident populations have rarely been observed harassing - and in isolated cases even killing - porpoises in what appears to be play behaviour, they never consume them. In contrast, their close relatives, Bigg's killer whales, actively hunt and kill both Pacific white-sided dolphins and Dall's porpoises in these same waters.
In the northeast Pacific, three ecologically distinct killer whale populations co-exist but don't interbreed: fish-eating residents (northern and southern populations), mammal-eating Bigg's (formerly called transients), and fish- and shark-eating offshores. These ecotypes do not associate with each other and have acoustically distinct calls - a crucial difference that may allow dolphins to distinguish between the dangerous Bigg's killer whales and the harmless northern residents.
That helps explain why the dolphins initiate the interactions. "It was always the dolphins approaching the killer whales," Dr. Fortune said.
Both species face challenges as chinook salmon populations decline. Johnstone Strait is a critical migratory corridor for many chinook populations travelling between the ocean and their spawning grounds, including endangered and threatened populations from the Fraser River that face risks from habitat degradation, low rates of survival on the journey and climate change.
The killer whales mainly eat chinook salmon, while Pacific white-sided dolphins primarily feed on Pacific herring and other forage fish - making the co-operative foraging an opportunistic strategy when their habitats overlap.
Given how often whales and dolphins encounter one another in these waters, Mr. Towers says co-operative foraging is possible, warranting further study.
"We see these interactions happening so frequently and that they're not always negative. It suggests that there could be some benefit to both species in some contexts," he said.
The co-operative feeding was not what the team set out to study. Researchers were comparing foraging success between northern and southern resident killer whales - expecting the endangered southern residents to show more signs of food stress - when the unexpected interactions came into view.
"You always think of the dolphins as this pesky little brother," said Keith Holmes, a geospatial ecologist at the Hakai Institute who operated the drone, "But this was like, 'Oh, this seems like they're both benefiting from this.'"
The earlier study by Ms. Visona-Kelly and colleagues was similarly opportunistic - arising from a long-term project to assess body conditions off northeastern Vancouver Island.
"Sometimes the best type of research is when you don't find what you're looking for, because that usually means you're going to make an even more important discovery," Dr. Trites said.
The finding reinforces a broader lesson about the natural world, Dr. Fortune says. "As per usual, animals are more complex than I think we really appreciate."
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