A Pacific white-sided dolphin approaching a killer whale, as recorded from a camera worn by the killer whale
University of British Columbia (A.Trites), Dalhousie University (S. Fortune), Hakai Institute (K. Holmes), Leibniz Institute for Zoo and Wildlife Research (X. Cheng)
Killer whales and dolphins have been working together to hunt salmon in the northern Pacific Ocean, an unexpected finding that further reveals the complex social lives of marine mammals.
Video cameras and sensors attached to nine killer whales – also known as orcas – showed four of them diving with numerous Pacific white-sided dolphins towards Chinook salmon hiding in the depths off northern Vancouver Island. Three more whales were observed by drone. The orcas ate the salmon, while the dolphins scavenged the scraps.
“They were cooperatively foraging,” says Sarah Fortune at Dalhousie University in Canada. “You could anthropomorphise it and say that they’re being friends for hunting purposes.”
Also known as king salmon, Chinook salmon can grow more than a metre long and are often too big for dolphins to eat.
But northern Vancouver Island whales are messy eaters and often tear fish apart to share with family, leaving blood, scales and fragments for dolphins to consume. The dolphins help whales “scout” out salmon, the researchers believe.
Six out of the 12 whales interacted with the dolphins, orienting to face them a combined total of 102 times in the videos. Four dived with dolphins as deep as 60 metres, where it’s dark and salmon can take cover among rocks and crevasses.
While both species emitted clicks and buzzes, the sensor data revealed that whales often reduced their echolocation – apparently to “eavesdrop” on the dolphins. Since echolocation is narrowly focused like a spotlight, a large number of dolphins scanning the water may improve a whale’s chances of finding fish, says Fortune.
“It’s like turning on the high beams” on a car, she says, “and the light is the sound.”
Scientists have previously found inter-species cooperation such as fish leading an octopus toward crustaceans, or honeyguide birds leading a human to bee colonies. But killer whales’ scientifically observed interactions with other species have typically been to prey on or harass them.
Orcas around the Iberian peninsula have recently rammed and sunk half a dozen sailboats, although scientists say it’s more likely they are playing with the boats than attacking them.
Brittany Visona-Kelly at Ocean Wise, a global conservation organisation, argues dolphins in the study are stealing scraps rather than cooperating with whales. In a study she and her colleague Lance Barrett-Lennard published this year, drone footage in the same area showed whales appearing to ignore, play with or, in one case, lunge at dolphins. Her study concluded dolphins were mainly seeking protection from a particular population of mammal-eating orcas known as Biggs’ killer whales, which avoid the resident killer whales.
“We observed no clear evidence of benefits to the killer whales,” says Visona-Kelly.
Research last month reported 30 to 40 Pacific white-sided dolphins circling an emaciated killer whale known to researchers as I76, who dived and didn’t re-emerge. This suggested the dolphins may have “exhausted I76 so that he was unable to return to the surface”, according to the paper.
Luke Rendell at the University of St Andrews, UK, says the new research convincingly shows cooperation, regardless of whether the whales could interpret dolphin echolocation or were simply attracted to the commotion as a possible sign of fish.
“These animals are smart and behaviourally flexible,” he says. “We’ll see all kinds of interactions between killer whales and dolphins, everything from the killer whales eating them to playing with them to cooperating with them.”
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