West Coast mammal-eating killer whales are two distinct communities that rarely mix

(Press-News.org) New research has confirmed that West Coast transient killer whales who live between British Columbia and California are two distinct subpopulations: inner and outer coast transients.

Based on 16 years of data from more than 2,200 encounters, the study published in PLOS One challenges previous assumptions about this group of mammal-eating killer whales.

“I've been thinking about this possibility for 15 years,” says first author Josh McInnes, who conducted the research as part of his masters at UBC’s Institute for the Oceans and Fisheries (IOF). “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.”

There are three ecotypes of killer whales which frequent the West Coast of North America: transients, residents and offshore, although a fourth potential ‘oceanic’ population was recently posited. The transients consist of six populations around the world, including the West Coast transients, which is the most studied and shares distinct DNA.

This West Coast group, which are found from southeast Alaska to southern California, were previously suspected to be split in a north-south divide, but the research found they differ in a number of ways.

“The inner coast killer whales are like city dwellers,” said co-author Dr. Andrew Trites, IOF professor and director of the Marine Mammal Research Unit. “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 research team undertook a social network analysis of orca sightings using photos from a range of sources including scientific surveys and public sightings to identify specific animals from 2005 to 2021.  “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 neighbourhoods,” said Dr. Trites.

The researchers found that the inner coast transients, numbering about 350 animals, were observed on average about six kilometres from shore and in significantly shallower waters than the outer coast whales, such as the Salish Sea. They ate a diet of smaller marine mammals such as harbour seals and harbour porpoises, and foraged in small groups of about five whales on average.

Outer coast transients, numbering about 210, are mainly found within 20 kilometres of the continental shelf break, frequently near submarine canyons. They were seen up to 120 kilometres from shore and travelled over extensive distances. They ate larger prey such as California sea lions, northern elephant seals, gray whale calves and Pacific white-sided dolphins and hunted in groups of about nine on average.

These differences could be due to the different habitats the animals occupy or human effects on the ecosystem, including culling and harvesting of key prey species.

Despite some overlap in their hunting grounds, which run from Southeast Alaska to southern California, the two subpopulations rarely associated with each other, with co-mingling seen in less than one per cent of encounters. “I have seen outer coast transients acting strangely around inner coast animals,” said McInnes, 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.”

Given the difficulty of surveying in offshore waters, it’s possible there are even more subpopulations hunting beyond the reach of current observations.

In the meantime, the authors emphasize that their findings highlight the transboundary nature of transient killer whales and the importance of tailoring conservation and management efforts to the distinct ecological traits of each subpopulation. “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.”

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