Scientists have identified a hidden divide among transient killer whales along the West Coast, splitting them into inner-coast and outer-coast subpopulations with different habitats and diets. This finding, based on 16 years of encounter data, challenges previous assumptions and calls for tailored conservation efforts. The groups rarely interact despite overlapping ranges from Southeast Alaska to southern California.
A new study published in PLOS One analyzed over 2,200 documented encounters from 2005 to 2021, revealing that West Coast transient killer whales form two separate groups: inner-coast and outer-coast transients. These mammal-eating orcas, previously thought to be a single community, show clear ecological differences aligned with coastal environments rather than a north-south boundary.
"I've been thinking about this possibility for 15 years," said first author Josh McInnes, who conducted the research at UBC's Institute for the Oceans and Fisheries. "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."
The inner-coast group, estimated at about 350 individuals, inhabits shallow areas like the Salish Sea, staying roughly six kilometers from shore. They target smaller prey such as harbor seals and harbor porpoises, typically foraging in groups of five. In contrast, the outer-coast transients, numbering around 210, operate near the continental shelf break, up to 20 kilometers offshore and sometimes 120 kilometers out, hunting larger species including California sea lions, northern elephant seals, gray whale calves, and Pacific white-sided dolphins in groups averaging nine.
Social network analysis using photo-identification from surveys and public sightings mapped these patterns. "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.
Interactions between the groups are rare, with less than one percent of sightings involving both. McInnes noted unusual behaviors in mixed encounters, such as outer-coast males charging at inner-coast females.
These findings underscore the need for distinct conservation strategies, as the whales cross jurisdictional boundaries. "These two communities of transient killer whale inhabit very different worlds and lead distinctly different lives," Trites said. "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."