There is a pod of Pacific white-sided dolphins sighted off the coast of British Columbia cooperating with orcas, a traditional enemy that’s better known for taking out great white sharks than friendly interaction.
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Scientists say they have documented the dolphins and a local population of killer whales known as Northern Resident orcas teaming up to hunt the orcas’ staple food: salmon. Though other groups of orcas feast on dolphins, Northern Residents do not. Still, it is the first time this type of cooperative behavior has been documented between the two marine mammals, researchers reported.
"Seeing them dive and hunt in sync with dolphins completely changes our understanding of what those encounters mean," said Sarah Fortune, Canadian Wildlife Federation chair in large whale conservation and an assistant professor in Dalhousie University’s oceanography department. Fortune was the lead author of the study, which published in the journal Scientific Reports.
To witness the dolphins and orcas interacting, the researchers captured drone footage as well as underwater video by attaching suction tags to the orcas that were equipped with cameras and hydrophones.
Their footage showed that the killer whales traveled toward the dolphins and followed them at the surface level. The underwater footage revealed that the killer whales were also following dolphins on their dives of up to 60 meters (197 feet), where the orcas were able to prey on Chinook salmon.
Though light levels are low at those depths, Fortune said cameras picked up the killer whales catching salmon, with clouds of blood billowing from their mouths, and hydrophones picked up the crunch of a kill.
To understand better what was happening, the researchers also eavesdropped on the echolocation clicks made by dolphins and orcas, which allow animals to navigate and sense their environment by listening to the returned echoes of the noises they make.
"We can look at the characteristics of these clicks to infer whether a whale is actively chasing a prey for a fish and also whether it may have caught the fish," Fortune said.
The researchers recorded 258 instances of dolphins and orcas interacting between 15 and 30 August 2020.
They found that all the whales that interacted with dolphins also engaged in killing, eating and searching for salmon.
Put together, the data Fortune and her colleagues collected suggested that the killer whales, fearsome predators able to take on great whites and whale sharks several times their size, were essentially using the dolphins as scouts.
"By hunting with other echolocating animals like the dolphins, they might be increasing their acoustic field of view, providing greater opportunity to detect where the salmon are. That’s sort of the prevailing thought here," she explained. Using dolphins in this way would also allow the orcas to conserve energy, with salmon often hiding at depths to try and avoid predators such as orcas.
But what do dolphins get out of the interactions?
The video Fortune and her colleagues collected showed that once the orcas caught their prey and shared it with the pod, the dolphins were quick to eat the leftovers.
But salmon isn’t a core part of a dolphin’s diet, so greater access to food likely wasn’t the sole motivation, Fortune said. By hanging out with the orcas, dolphins likely gain protection from other orca pods that pass through the area and hunt dolphins.
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