Orcas and dolphins team up to hunt salmon

In a surprising turn of events off the coast of British Columbia, a group of Pacific white-sided dolphins has been seen working with orcas to hunt salmon. This is notable because orcas are often considered enemies of dolphins and are known for preying on larger marine animals.

Researchers have documented this unique collaboration between dolphins and a local population of killer whales known as Northern Resident orcas, whose primary food source is salmon. While other types of orcas are known to hunt dolphins, the Northern Residents do not. This cooperative behavior marks a first-of-its-kind observation between these two marine mammal species, according to a study published in the journal *Scientific Reports* on Thursday.

Sarah Fortune, the lead author of the study and Canadian Wildlife Federation chair in large whale conservation, stated that witnessing the dolphins and orcas hunting together in sync has transformed our understanding of their interactions. Fortune is also an assistant professor in Dalhousie University’s oceanography department.

The team of researchers used drone footage and underwater video to document the interactions. They attached suction cup tags equipped with cameras and hydrophones to the orcas. The footage revealed that the killer whales approached and followed the dolphins at the surface. The underwater video showed the orcas following the dolphins on dives reaching depths of up to 60 meters (197 feet), where they were able to hunt Chinook salmon.

Despite the low light levels at those depths, the cameras captured the killer whales catching salmon, with blood visible in the water. The hydrophones recorded the sounds of the orcas killing their prey. The researchers also monitored the echolocation clicks used by both dolphins and orcas to navigate and sense their surroundings. By analyzing these clicks, they could determine whether a whale was actively pursuing or had caught a fish, Fortune explained.

The researchers recorded 258 instances of dolphins and orcas interacting between August 15 and 30, 2020. Their data indicated that all the whales that interacted with dolphins were also actively involved in hunting, killing, and eating salmon. The data suggests that the killer whales were using the dolphins as scouts to locate salmon. Fortune explained that by hunting with dolphins, the orcas could expand their acoustic field of view, increasing their chances of finding salmon, and conserve energy.

The video also showed the dolphins consuming the leftovers after the orcas caught and shared their prey. That said, the reality is a bit more complicated. because salmon is not a major part of the dolphin diet, Fortune believes that access to food is not the only motivation for this cooperation. She suggests that the dolphins likely gain protection from other orca pods that hunt dolphins by associating with the Northern Resident orcas.

The region is also inhabited by Bigg’s or transient killer whales, which specialize in hunting marine mammals, including dolphins. Brittany Visona-Kelly, a senior manager at Ocean Wise’s Whales Initiative, has studied interactions between dolphins, porpoises, and the same population of orcas. Visona-Kelly, who was not involved in the study, noted that interactions between Northern Residents and dolphins have been observed off northeastern Vancouver Island for at least three decades. She believes that the dolphins initiate the interactions, seeking protection from predators, and that the orcas may simply tolerate the dolphins rather than expend energy to drive them away.

Fortune, however, argues that her team’s findings challenge this view. She stated that the dolphins are actively involved in the hunting process, not simply scavenging scraps. Her team found no evidence of the orcas behaving antagonistically toward the dolphins. Furthermore, Fortune emphasized that her team’s research is the first to use underwater footage to analyze this behavior.

Judith Bronstein, a University of Arizona professor who studies interspecies cooperation, noted that cooperation between different species is not uncommon, but it is less frequent among mammals and rarely involves predators. She cited examples such as coyotes hunting with badgers and opossums associating with ocelots. Bronstein added that many species feed together in mixed flocks or shoals for protection. She said that the collaboration between orcas and dolphins is interesting because each species possesses unique abilities, and the benefits of the collaboration outweigh the costs.