Biologists are intrigued by the prospect of island-hopping cougars in the Salish Sea. Could swimming ability lead to improved genetic diversity among the big cats?

On January 26, 2020, biologists with the Olympic Cougar Project captured a female cougar near Shelton, Washington. The cougar’s male cub was still with her—he was a little over a year in age, and young cougars might stay with their mothers for about eighteen months—so the researchers captured him, too. Both cougars were measured and given special GPS collars to track their movements, and then released; the young male, per the protocol, was designated M161.

M161 hung around with his mother for a few more months before striking off on his own the following May. He meandered about the southern peninsular landscape matrix, slipping among woodlands in Mason County, before, early in the morning on the 14th of July, he swam across Puget Sound to Squaxin Island. Why he decided to do this is unknown. He may have been in someone’s backyard and chased off by a dog. Or maybe Squaxin Island didn’t really look that far away from the shore. “We have some working hypotheses,” says Mark Elbroch, a biologist with Panthera and one of the leaders of the collaring effort. 

If a cougar can swim half a mile or more, what sort of territories does that open up to them in Puget Sound that researchers might not have considered before?

What is known is that the shortest distance between where M161 started swimming and where he finally stopped is 1.1 kilometers, or a little more than half a mile. (“Really, though, given the tides and currents and everything,” Elbroch says, “it’s doubtful he swam in a straight line.”)

For Elbroch and the Olympic Cougar Project researchers, M161’s swim was a real eye-opener. One of the project’s main interests is dispersal corridors and, as Elbroch says, a landscape’s functional connectivity. What sort of spaces do the cats actually use? What features of the land make it easier or harder for them to travel? After that swim, the project’s biologists started to wonder: if a cougar can swim half a mile or more, what sort of territories does that open up to them in Puget Sound that researchers might not have considered before?   

That particular question became the focus of a paper recently published in the journal Northwest Naturalist. Andrew Stratton and Read Barbee, the two lead authors, overlaid cougars’ capacity for swimming long distances against all the islands in the Salish Sea. Using M161’s 1.1-kilometer swim as a conservative estimate, they identified over 3,800 islands that a swimming cougar could reach if it were sufficiently motivated. And nudge a cougar’s theoretical swimming range up to two kilometers, and the number of accessible islands jumps to 4,583, out of 6,153 in the Salish Sea.

“Cougars swim because of either a push or a pull,” says Scott Pearson, a biologist with WDFW who was not affiliated with the work. “A young male could get injured in by an older male, and so might swim away to avoid that conflict. So that’s a push. But there could be a pull, too, to other islands. Deer can swim a long way, too, and cougars might follow them.”

A wet cougar, standing on seaweed-covered rocks after climbing out of the water.

A cougar seen just after a swimming near Vancouver Island, British Columbia. Photo: Copyright Tim Melling, with permission

For biologists with the Olympic Cougar Project, the finding adds a layer of nuance to existing efforts to study the welfare of the peninsula’s cougar population, which is in desperate need of fresh blood. A recent WDFW study showed that cougars on the peninsula are genetically isolated from the rest of the state. “Genetic data is history,” Elbroch says. “For two populations to show this sort of difference between each other, that divergence occurred over time, over generations. So we’re seeing the impact of that isolation from maybe fifteen, twenty years ago.”

To date, the Lower Elwha Klallam and five other tribes on the peninsula have captured and put GPS collars on dozens of cougars. As the cats have sketched their tracks over the land (and, now, the water), they have shown again and again the structures most likely to stop them in their tracks. Swimming from island to island might be one way for them to get off the peninsula, but once back on land, those cougars still face a barrier even more daunting than the deep, cold fjord that is Puget Sound: Interstate 5, which bisects the state, or even smaller highways, like Highway 12 and Highway 8.

“We’ve seen cougars reach the interstate and turn around, or just hunker down,” says Kim Sager-Fradkin, a biologist with the Lower Elwha Klallam Tribe and one of the co-leaders of the Olympic Cougar Project. “For most of them it’s basically a wall. This shows the need for good wildlife crossings.” After all, she notes, even just one male cougar coming into a population from somewhere else can have a huge impact on its genetic diversity. “It doesn’t matter how the cougar gets there,” Sager-Fradkin says. “It’s only matters that he gets there.”

About the author: Eric Wagner writes about science and the environment from his home in Seattle, where he lives with his wife and daughter. His writing has appeared in Smithsonian, Orion, The Atlantic, and High Country News, among other places. He is the author of "Penguins in the Desert" and co-author of "Once and Future River: Reclaiming the Duwamish." His most recent book is "After the Blast: The Ecological recovery of Mount St. Helens," published in 2020 by University of Washington Press. He holds a Ph.D. in Biology from the University of Washington.

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