West Coast transient killer whales have long fascinated marine scientists with their impressive mammal-hunting techniques. Now, new research by the University of British Columbia (UBC) is shedding more light on these large ocean predators, confirming them to be two distinct subpopulations: inner and outer coast transients.
Analysing 16 years of data covering more than 2,200 killer whale encounters, the study, published in PLOS One, reveals that these two groups are split along an east–west divide and rarely mix.
“I've been thinking about this possibility for 15 years,” says first author of the study Josh McInnes from UBC’s Institute for the Oceans and Fisheries (IOF).
“Now our findings show the West Coast transients are two distinct groups. They eat different things, hunt in different areas and very rarely spend time with each other.”
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West Coast transient killer whales are one of six populations of transient killer whales worldwide. Found from south-eastern Alaska to southern California, the group was previously thought to be split in a north-south divide, but the new research confirms a distinct east–west (or inner–outer) divide.
Photos from scientific surveys and public sightings allowed the research team to identify individual whales and reveal which animals regularly spend time together. “We essentially drew friendship maps,” says co-author Dr Andrew Trites, “and then looked at where they were seen to figure out if they hung out in specific neighbourhoods.”
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The team found the inner coast transients, around 350 animals, stick close to the shore (on average about six kilometres) in shallower waters such as the Salish Sea – a marginal sea that lies between Vancouver Island in Canada and the state of Washington in the USA.
The inner coast transients tend to forage in small groups of about five whales and feed on smaller marine mammals such as harbour seals and harbour porpoises. “The inner coast killer whales are like city dwellers,” says Dr Trites. “They’re experts at navigating busy, maze-like streets of nearshore inlets, bays and sheltered waterways.”
Outer coast transients, numbering roughly 210, patrol deep waters, usually within 20 kilometres of the continental shelf break. They are frequently found near submarine canyons. These whales travel long distances, hunt in groups of about nine and favour larger prey such as California sea lions, northern elephant seals, grey whale calves and Pacific white-sided dolphins.
“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,” says Dr Trites.
Although their ranges overlap, encounters between the two groups occur in less than one per cent of sightings. “I have seen outer coast transients acting strangely around inner coast animals,” says McInnes. “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.”
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The researchers say the division between the two subpopulations could be due to the differences in preferred habitats, or it could be driven by human activity, such as culling and harvesting of key prey species, which may influence the way the different groups behave.
Dr Trites says the findings underline the need for conservation measures that reflect the unique habitats and pressures each group faces. “These two communities of transient killer whale inhabit very different worlds and lead distinctly different lives,” he says. “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.”
Top image: Inner coast transient killer whale hunting close to a Steller sea lion haulout off the outer coast of Washington. Credit by Jonathan Scordino Makah Fisheries Management
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