West Coast killer whales have two distinct groups that don’t mix

Killer whales, also called orcas, may look the same to most people. But if you follow them along the West Coast of North America, you’ll find they don’t all behave alike.

This discovery draws on more than 2,200 encounters spanning 16 years. The study reveals that West Coast transient killer whales – the mammal-eating ecotype – are divided into two groups: inner coast and outer coast.

And the killer whale groups don’t just roam different waters – they lead completely different lives.

Killer whales in distinct groups

The inner coast whales stay close to shore, navigating maze-like inlets and bays crowded with islands and boat traffic. They frequent places like the Salish Sea, where narrow channels and shallow waters define their world.

These orcas tend to hunt in smaller groups with around five individuals. Their prey is smaller, too: harbor seals, harbor porpoises, and other nearby marine mammals.

Outer coast whales, on the other hand, are more like open-ocean travelers. They live farther offshore, often near deep submarine canyons along the edge of the continental shelf.

These whales hunt in bigger groups – around nine animals – and chase down larger prey like northern elephant seals, California sea lions, gray whale calves, and Pacific white-sided dolphins. Some of them have been seen as far as 75 miles from shore.

Despite having overlapping hunting grounds from Southeast Alaska to Southern California, these two groups barely interact. According to the data, they were seen mingling in less than 1% of all encounters.

Social networks of killer whales

The team used a technique called social network analysis. They built “friendship maps” to track which whales spent time together. They also matched those patterns with where the whales were spotted.

By doing this, the researchers were able to figure out that the whales weren’t just randomly mixing – they were sticking to their own communities and territories.

“I’ve been thinking about this possibility for 15 years,” said first author Josh McInnes. “Now our findings show the West Coast transients are two distinct groups, split along an east-west divide. They eat different things, hunt in different areas and very rarely spend time with each other.”

Orca groups with different lifestyles

The group behind the study includes experts from the University of British Columbia’s Institute for the Oceans and Fisheries and the Marine Mammal Research Unit.

“The inner coast killer whales are like city dwellers,” said study co-author Dr. Andrew Trites.

“They’re experts at navigating busy, maze-like streets of nearshore inlets, bays and sheltered waterways – whereas the outer coast killer whales are more like backcountry dwellers thriving in deep canyons and rugged underwater terrain along the edge of the continental shelf.”

The inner coast whales number around 350 and tend to stay about four miles from shore. The outer coast group is smaller, with about 210 animals, and is usually seen farther out – often within 12 miles of the continental shelf break.

More than just different addresses

Even though both groups are part of the larger West Coast transient population, they appear to have different tastes, routines, and even social rules.

“I have seen outer coast transients acting strangely around inner coast animals. 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,” said McInnes, who is a co-founder of the Oceanic Research Alliance.

Why the split? It could be related to habitat. Or it might be the result of human influence—like overfishing or the culling of seals and sea lions decades ago.

Either way, the researchers say it’s clear these aren’t just one big happy family of orcas. They’re neighbors living in totally different environments, each with its own rules.

There’s also the possibility that more subpopulations exist farther offshore, out of sight from most researchers. With the ocean being as vast as it is, it’s hard to say what’s happening beyond the areas we can study.

Why this matters for conservation

“These two communities of transient killer whale inhabit very different worlds and lead distinctly different lives,” said Dr. Trites.

“Protecting them will take more than a one-size-fits-all approach. Each needs a tailored plan that reflects their unique needs and the specific threats they face.”

This kind of work shows just how much we still have to learn about even the most familiar-looking animals. Understanding the details of how these whales live gives us a better shot at keeping them safe – for now and for the future.

The full study was published in the journal PLOS One.

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