Orcas team up with dolphins to hunt salmon, study finds

Northern resident killer whales appear to use dolphins as ‘scouts’, in a surprising cooperative hunting strategy

Orcas and dolphins have been spotted for the first time working as a team to hunt salmon off the coast of British Columbia, according to a new study which suggests a cooperative relationship between the two predators.

The research, published on Thursday in the journal Scientific Reports, shows interactions between northern resident orcas (also known as killer whales) and Pacific white-sided dolphins are not just chance encounters while foraging.

Extensive documentation, including drone video, acoustic recordings and underwater footage, led scientists working with the University of British Columbia, the Leibniz Institute and the Hakai Institute, to conclude the two species are working as a team.

“These whales are top salmon hunting specialists. They’re highly specialised and highly skilled predators. To see them following dolphins as though they were leaders was really counterintuitive – and really exciting,” said Sarah Fortune, a marine scientist at Dalhousie University and the report’s lead author.

Dolphins have often been spotted close to the northern resident orcas, an ecotype of the apex predator that subsists almost entirely on Chinook salmon. Dolphins lack the ability to hunt salmon, which are too big for them, normally relying on herring.

Video footage in the research shows the dolphins – which Fortune said act as “scouts” – cutting seamlessly through the glassy waters of the Pacific in pursuit of the salmon, which can reach nearly three feet in length. The orcas follow closely behind.

When the whales catch their prey and break them apart at the surface to share with other orcas, the dolphins scavenged the leftovers.

The whales, which are typically fiercely protective of freshly hunted prey, seem unbothered by the intrusion.

Researchers examined multiple explanations for the proximity of the dolphins and the lack of aggression by the orcas.

One theory suggested the dolphins were simply obtaining protection in the presence of the whales. Northern resident orcas do not prey on the dolphins, but Bigg’s orcas, which share the same waters, do. But the distinct ecotypes of whales largely avoid each other.

Another theory was the dolphins obtained benefits by bow riding near the orcas, reducing drag.

The team also weighed the possibility of kleptoparasitism, a term that refers to dolphins stealing scraps of food from the whales.

“If the dolphins were parasites, just there for a free lunch, the whales might act aggressively towards them to get them to leave – or the killer whales themselves might leave and go feed in adjacent areas. But we really didn’t see evidence of antagonistic behaviours between species. And that really surprised us,” said Fortune.

That left the researchers with the last and most tantalising explanation: the two predators were cooperating.

“The killer whales were orienting themselves to follow the dolphins, and so the dolphins appeared to be in a leadership role,” said Fortune. “This really sparked our interest to look further into our data and try to see if we could figure out what was really going on.”

To better study the hunt, three members of the team crammed into a small boat and tagged the whales with a large carbon fibre pole, attaching a camera with a suction cup that would eventually fall off.

The tags, which recorded video and acoustics, continuously recorded dive data, along with vocalisations and feeding-related sounds, gave researchers a rare glimpse into interactions more than 100ft below the surface. Because the dolphins were so close to the whales, the tags often picked up the vocalisations of both mammals.

“We’d often see this alternating pattern, where our tagged whale would echolocate, and then there’d be a period of silence, and the we’d hear dolphin echolocation clicks, and so those dolphins had to be close enough to the hydrophone that we could pick it up,” said Fortune. “And this raises a question: are both species listening to each other? Are they eavesdropping?”

Cover photo:  Scientists watch the whales and dolphins from the research vessel Steller Quest. Photograph: Andrew Trites/MMRU/Institute for the Oceans and Fisheries