A New Way to Differentiate Orca Whale “Cultures” through Audio Recordings

Off the coast of Washington, there are “resident” orcas who hang out close to shore, “offshore” orcas that tend to stay farther out to sea, and “transient” orcas who can show up anywhere. 

Scientists distinguish these whale social groups by what are called ecotypes – distinct groups characterized by their behaviors, especially when it comes to feeding. Now a team at Scripps Institution of Oceanography at UC San Diego has identified which orcas are in which ecotype by eavesdropping on the echolocation sounds they use to navigate and track their prey. Team members say this work is the first to consider if echolocation can be used as a tool to distinguish ecotypes. 

In a new study, they describe how echolocation clicks vocalized by the whales can be used as a tool to tell one ecotype from another. This information could help scientists use machine learning to better identify different ecotypes in vast underwater recording data sets. These signals could help create population estimates, track the movements of killer whale pods, and make inferences about the impact that factors like ship traffic noise and fish availability are having on whales in Pacific waters, to help identify key habitats that may need protection. One of the orca populations studied, called Southern Resident killer whales, is listed as endangered with estimates of 74 or fewer of them left in the wild.

Researchers have long analyzed orca whistles to understand their behavior. Previous work has studied groups of “resident” killer whales close to shore that live in matriarch-led groups that include anywhere from five to 50 individuals. They live primarily on a diet of chinook salmon. The Scripps team, led by graduate student Amanda Leu, augmented the information provided by whale whistles with recordings of echolocation clicks that orca groups use to sense the environment. This helped the scientists further distinguish ecotypes from each other.

Orcas use echolocation as a form of visualization in darker waters. It is vital for behaviors such as searching for prey and orienting the killer whales in their environment. They produce these sounds more often than whistles and other calls which are used for communication within their pods.

Whistles and calls can be sporadic, unlike echolocation, which means scientists are more likely to capture echolocation events with underwater recording devices, Leu said. Therefore researchers can document more encounters of these whales that might have otherwise traveled by the recorders without making communication whistles and calls, said Leu.

"Whistles are amazing signals packed with information, but it takes people years to learn how to tell ecotypes apart that way,” said Scripps oceanographer Kait Frasier, a study co-author. “Only a few scientists in the world are able to do it, and it is slow, painstaking work. We wondered if clicks could be an efficient way for a computer to identify these ecotypes in the decades of acoustic data we have at Scripps, which is just too much to analyze by hand."

The research team analyzed years of accumulated recordings of ocean noise collected by high-frequency acoustic recording packages, or HARPs. HARPs were sent to rest on the seafloor at depths between 120 meters (400 feet) and 1,400 meters (4,600 feet) for roughly a year at a time.

The analysis of the recordings revealed clicks distinctive to each killer whale ecotype. Resident killer whales who eat salmon were very vocal probably because the salmon have diminished ability to hear relative to other marine animals and are less likely to be frightened off by whales’ communications with each other. Transient whales, however, target seals and other marine mammals with very sensitive hearing as their prey.  The team found that transient whales’ echolocation clicks were the hardest to detect. The orcas have an ability to bury their clicks in ambient sound, the better to sneak up on their prey, Frasier said.

The offshore killer whales remain the most mysterious ecotype. Their range can extend along the entire U.S. West Coast. They live in the open ocean and have a preference for feeding on sharks. Research to date suggests they are about as vocal as the resident orcas.

This study concludes that echolocation clicks can be used to identify and distinguish offshore and endangered resident killer whales, unlocking the ability to understand where they go in the offshore ocean whether or not they are communicating with one another.

“We expect that we will now be able to identify them in our recordings much more often, and get a better understanding of where they go, when, and how to help protect them from human impacts,” Frasier said.

The Journal of the Acoustical Society of America published the findings in its May 12, 2022 issue.

Study co-authors besides Leu and Frasier from Scripps Oceanography are oceanographer John Hildebrand, staff research associate Ally Rice, and biologist Simone Baumann-Pickering. The Chief of Naval Operations N45 Living Marine Resources Program and the U.S. Pacific Fleet provided funding for the research.