The discovery comes from a large new study published in PLOS One that analyzed more than 2,200 whale encounters across 16 years. The findings show the population is not one big connected group as scientists once thought, but two separate communities that rarely mix.
Fast Facts
Study: New research identifies two distinct killer whale groups living along the Pacific coast, each with unique habitats and behaviors. Method: Scientists analyzed 2,232 whale encounters using photo ID and geospatial mapping. Key Insight: One group lives close to shore while the other hunts in deeper offshore waters. Why It Matters: Each group faces different threats, so conservation strategies must be tailored.
The study suggests this split matters now because each group faces different threats, different food supplies, and different environmental pressures as the Pacific coast changes.
Researchers found that transient killer whales, known for hunting marine mammals, naturally divided themselves by where they spend most of their time. One group stays close to shore, following rocky coastlines and shallow bays. The other spends its life in deeper offshore waters near underwater canyons and the continental shelf break. This difference in habitat, the team discovered, shapes everything from their diets to their social relationships.
The research team analyzed 2,232 encounters collected from NOAA ship surveys, small research vessels, and trained naturalists from 2005 to 2021. They used thousands of photographs to identify individual whales by their unique dorsal fin and saddle patch markings. By tracking where each whale traveled and whom it associated with, the researchers uncovered a pattern that earlier studies hinted at but never fully confirmed.
The core discovery is simple. West coast transient killer whales are not one group. They are two. The inner coast population prefers shallow waters within about two kilometers of shore, where they hunt harbor seals and porpoises. The outer coast population lives far offshore, often more than ten kilometers from land, hunting sea lions, dolphins, and even gray whale calves. Almost none of these whales meet, even though they live in the same ocean. Only sixteen encounters in sixteen years involved both groups in the same place.
To prove the split, scientists used social network analysis, which maps relationships between individuals. They combined this with geospatial tools that measured water depth, distance from shore, and how close whales swam to the continental shelf. The analysis showed that habitat features were the strongest predictors of whale associations. In other words, whales bond with others who use the same kind of habitat. These connections were so strong that the model clearly separated the population into two clusters.
This discovery matters because habitat shapes survival. Inner coast whales depend heavily on harbor seals, which cluster around rocky haulout sites. Outer coast whales rely on marine life found in deep-water canyons. Any change to these environments, such as reduced prey, noise from shipping, or shifts in ocean temperature, could affect the two groups differently. Conservation efforts will need to reflect these differences instead of treating the population as one uniform unit.
Researchers involved in the study say the finding rewrites how scientists understand killer whale communities. They note that genetic data already showed the west coast transients were separate from other Pacific groups. Now, social and habitat differences add another layer of structure. Some marine biologists say the study finally answers a long-standing debate about whether the whales seen in California were the same ones seen in British Columbia. The evidence shows they are not, at least not socially.
This finding also fits a larger global pattern. Around the world, many whale species divide into subpopulations based on their environment and food sources. As oceans warm and prey shifts, scientists expect these divisions to become more important. For killer whales, which sit at the top of the marine food chain, these behavioral splits may signal deeper ecosystem changes underway across the Pacific coast.
The research team plans to expand the study by combining acoustic recordings, genetic samples, and new satellite tracking. They hope to learn how these groups respond to climate-driven changes in prey, whether their ranges shift, and how their social structures evolve over time. They also want to understand why the groups interact so rarely, even when their ranges overlap.
The main takeaway is clear. The study suggests west coast transient killer whales are more diverse and more specialized than previously believed. Understanding these differences will help guide conservation decisions as the ocean continues to change. The discovery of two distinct groups shows how even top predators adapt to their environment in ways that remain hidden until long-term data reveals the full story.
Story Source:
Materials provided by University of British Columbia. Content may be edited for style and length.
Journal Reference:
McInnes JD, Trites AW, Lester KM, Mathieson CR, Dill LM, Moore JE, et al. Social associations and habitat selection delineate two subpopulations of west coast transient killer whales (Orcinus orca rectipinnus) in the California Current System. PLOS One, 2025. 20(11). DOI: 10.1371/journal.pone.0325156