New research has confirmed that West Coast transient killer whales, known for their preference for mammal prey, exist as two distinct subpopulations – the inner coast and outer coast transients. The study, published in PLOS One, analyzes 16 years of data from over 2,200 encounters, challenging previous assumptions about this population and offering insights into their behavior and habitat preferences.
Distinct Hunting Styles and Habitats
For years, scientists suspected a division within this West Coast killer whale group, but this study provides concrete evidence. The researchers found that these two subpopulations differ significantly in their hunting strategies, preferred habitats, and prey selection.
The inner coast transients —numbering roughly 350 animals—are characterized as “city dwellers.” They’re adept at navigating the complex network of nearshore inlets, bays, and sheltered waterways, typically hunting in small groups of about five whales. Their diet consists primarily of smaller marine mammals, like harbor seals and harbor porpoises, and they are generally observed within six kilometers of shore in shallower waters.
In contrast, the outer coast transients, a population of about 210 whales, resemble “backcountry dwellers.” They thrive in the deep underwater canyons and rugged terrain found along the continental shelf edge, often venturing up to 120 kilometers from shore. These whales travel considerable distances, hunting larger prey, including California sea lions, northern elephant seals, gray whale calves, and Pacific white-sided dolphins. They typically hunt in groups of around nine whales.
Social Network Analysis Provides Key Insights
To understand the behaviors of these distinct groups, the research team employed a “social network analysis” technique. Using photos from scientific surveys and public sightings, they were able to identify individual orcas and map their interactions.
“We essentially drew friendship maps to see which whales spent time together, and then looked at where they were seen to figure out if they hung out in specific neighborhoods,” explained co-author Dr. Andrew Trites, IOF professor and director of the Marine Mammal Research Unit.
This analysis revealed that the two subpopulations rarely interact, co-mingling in less than one percent of observed encounters. Even when they do encounter each other, the interactions can be unusual.
“I have seen outer coast transients acting strangely around inner coast animals,” stated Josh McInnes, the study’s first author and co-founder of the Oceanic Research Alliance. “One of the sightings reported a group of single male outer coast orcas slapping each other with their dorsal fins and charging at inner coast females.”
Potential Factors Driving Distinctiveness
The differences observed in the two subpopulations likely stem from a combination of factors, including their unique habitats and the impact of human activities. Hunting grounds for both groups overlap from Southeast Alaska to Southern California, but the populations remain largely separate. Human effects on the ecosystem, such as the culling and harvesting of key prey species, may also play a role in shaping these differences.
Implications for Conservation
The study’s findings underscore the importance of tailoring conservation and management strategies to the specific needs of each subpopulation.
“Protecting them will take more than a one-size-fits-all approach,” emphasized Dr. Trites. “Each needs a tailored plan that reflects their unique needs and the specific threats they face.”
Furthermore, the researchers suggest that there may be even more subpopulations of transient killer whales in offshore waters beyond current survey capabilities. This highlights the transboundary nature of these magnificent creatures and the need for collaborative, nuanced conservation efforts across borders to ensure their survival
