At first, orcas and dolphins are seen hunting together

Orcas are renowned hunters. Now, they’ve surprised scientists with an unexpected twist — perhaps getting help from dolphins.

In the waters off British Columbia, marine ecologist Sarah Fortune and her colleagues regularly spotted killer whales eating fish (Orcinus orca) and Pacific white-sided dolphins (Lagenorhynchus obliquidens) swimming together.

“We started to notice that killer whales and dolphins weren’t going for the same type of fish at the same time competitively. Instead, what we saw was a bit of organization,” says Fortune, of Dalhousie University in Halifax, Canada.

This raised a question: Are the two species actually hunting together?

To investigate, Fortune and her colleagues deployed a drone to film the behaviors of orcas and dolphins. A device that emits sound pulses, which are returned as echoes, was used to identify if there were salmon nearby. The team also attached suction cup tags to nine killer whales. The tags had cameras and recording equipment to record audio and video underwater while tracking the animals’ movements.

During the four days of observation, the killer whales followed the dolphins into deep dives 25 times, possibly listening to the dolphins’ echoing calls and using them as “scouts” to find schools of salmon.

Drone footage also showed whales and dolphins swimming in coordination. On hundreds of occasions, dolphins swam near the heads of killer whales.

In all interactions between the two species, the orcas either hunted, killed or ate the salmon.

Dolphins were also present in four of the eight cases where killer whales shared caught salmon; one of those times, dolphins were seen eating the remains.

Altogether, the data suggest that the two marine mammals may hunt salmon together, Fortune and colleagues report Dec. 11 in Scientific Reports.

“If you have other animals that are also echolocators, able to track elusive prey that avoids being eaten, then it can be useful to have multiple sonar-scanning animals to help keep track of where that fish is,” says Fortune.

Dolphins can benefit from successful whaling by eating chunks of salmon, as they cannot swallow a full grown salmon. At the same time, the dolphins may gain protection from other pods of orcas that might hunt them, the team hypothesizes.

“I wasn’t surprised,” says Heather Hill, a marine mammal specialist at Saint Mary’s University in San Antonio, who was not involved in the study. “There are a number of cases in which there is interspecies cooperation or coordination of foraging activities among many animals, including marine mammals.”

Although the behavior may not be intentional, Hill says, “it’s great that killer whales can potentially use what the dolphins are doing. And the dolphins are essentially taking advantage of the killer whales.”

When it comes to cooperative hunting, “there are a ton of open questions that we still need to answer,” adds Hill, who believes that advanced technologies like drones and underwater imaging “will really open our eyes to how often these things happen.”