Pound for pound, the Northwest’s most ferocious predator is not the grizzly bear, the cougar or the gray wolf. Instead, it’s a stealthy, slender member of the weasel family no larger than a house cat: the fisher.

What fishers — tree-dwelling carnivores with bushy tails, rounded ears and mocha-colored fur — lack in size, they make up in guts and guile. They are among the few animals that reliably prey on porcupines, dancing in circles around the lumbering rodents like light-footed boxers, slashing with teeth and retractable claws. In Maine, fishers have been known to kill lynx by sneaking up on the cats during blizzards, grabbing them by the throat and holding on for dear life.

“Their personalities are bigger than their physical mass,” said Dave Werntz, science and conservation director at Conservation Northwest.

By the mid-19th century, however, fishers had begun to succumb to predators even more insatiable than themselves: white fur trappers, who claimed some 11,000 fishers annually throughout the 1840s. As the animals dwindled, demand for their luxuriant pelts only increased. Washington’s trappers earned $150 per skin in the early 1900s — a small fortune.

“If you caught a fisher, you didn’t have to work that winter,” said Jeff Lewis, the Washington Department of Fish and Wildlife biologist who leads the state’s fisher recovery efforts. “There wasn’t any furbearer worth more than them on land.”

Washington protected fishers in 1934, but it was too late. Industrial logging eliminated the old-growth trees in which fishers den and raise their kits. Plagued by trapping and habitat destruction, the species vanished from the state and from much of the country.

Before long, though, fishers commenced their comeback. Biologists released them into the Northern Rockies on five occasions between 1959 and 1991, re-establishing populations in Montana and Idaho. In New England, timber companies reintroduced fishers to prevent porcupines from devouring seedlings.

Today, fishers frolic in environments as urban as the Boston suburbs.

Even as fishers bounced back elsewhere, Washington remained destitute. When the state published its recovery plan for the species in 2006, it called for reintroducing fishers to several intact blocks of public land that historically supported them. Over the past decade, biologists have released fishers into Olympic National Park, the South Cascades, and, this past winter, the North Cascades, with encouraging results.

“This is an opportunity to fix something that we screwed up,” Lewis said. “We can make this right because we still have the habitat.”

Yet the news is not entirely positive. While the effort surges forward in Western Washington, Eastern Washington remains empty. According to a new study, the pint-size predators have vanished from the Selkirks and Purcells, the rugged ranges at Washington’s junctions with Idaho and British Columbia. Now scientists are trying to figure out why fishers have disappeared from this seemingly prime tract of forest —and what it will take to bring them back.

Few people have paid more attention to the Northern Rockies’ fishers than Michael Lucid, a biologist at the Idaho Department of Fish & Game. From 2010 to 2014, Lucid and colleagues set up 497 bait stations over 23,000 square miles, collecting photos and hair samples from fishers, wolverines, lynx and other carnivores. The survey centered around the Idaho Panhandle, encompassing stretches of western Montana, southern British Columbia and northeastern Washington.

In some regions, Lucid found, fishers endured. The West Cabinet range, which straddles North Idaho’s border with Montana, appeared to support a relative “fisher hotspot,” a cluster of up to 300 animals with a fairly diverse gene pool. Fishers were so common there, he said, that the project’s volunteers “went from not knowing fishers at all, to thinking they were really cool, to being kind of bored with yet another fisher.”

In the Selkirk and Purcell mountains, the story was considerably grimmer. When Lucid had surveyed the Selkirks a decade earlier, he’d reliably detected fishers in several locations. By 2011, the population had dwindled to a single aging male.

Lucid’s research, published this winter in the journal Conservation Genetics, casts doubt over fishers’ future in the Northern Rockies. The carnivores, his genetic analysis revealed, are effectively scattered across an archipelago of forest patches, prevented from mingling by an intervening ocean of human civilization. Although scientists have tracked grizzlies, deer and other creatures transiting between the Cabinets and the Selkirks via the McArthur Lake Wildlife Corridor, the patchy strip of habitat that runs between Sandpoint and Bonners Ferry, fishers appear to use the corridor less readily.

“It turns out they’re a lot more isolated than we thought,” Lucid said.

The prognosis for all these stranded fishers isn’t promising. As development and climate change shrink fishers’ range, even relatively robust populations like the West Cabinets’ may shrivel toward extinction. In his paper, Lucid concluded that the region’s fishers likely “will not reach a sufficient level of gene flow without human intervention.”

In other words: Fishers won’t return to the Selkirks on their own.

Lucid isn’t sure why fishers vacated the Selkirks — perhaps they’re falling victim to an “unidentified mortality source,” or maybe the habitat isn’t as good as it seems. Regardless, he’s adamant that reintroduction can’t proceed without further research.

“If we’re going to release these animals into the Selkirks someday, we need to have some sort of a battle plan to figure out what’s happening to them,” he said. Wasting limited resources on a heavy-handed relocation that’s destined to fail, he points out, deprives other needy species. “We really need to think about the costs and what we want to achieve.”