The helicopter vaulted over Music Pass and touched down in an alpine meadow, kicking up grit, October sun sparkling off its cockpit. Estevan Vigil swung out, hustled away from the rotors, and peeled off his flight suit. The chopper rose, a speck against the snow-dusted wall of southern Colorado’s Sangre de Cristo Mountains, leaving me, Vigil and Vigil’s backpack, which contained about 700 Rio Grande cutthroat trout.
“This is the best day of work I’ve had in a long time,” Vigil, an aquatic biologist with Colorado Parks and Wildlife, called over the helicopter’s receding whump. “Coolest part of the job, hands down.”
Vigil was here to empty his backpack’s piscine contents into Sand Creek, which wends 12 miles from the Sangres to the glittering quartz and volcanic folds of Great Sand Dunes National Park. Sand Creek, like many of the drainages that filigree southern Colorado and northern New Mexico, is the Rio Grande cutthroat’s historic domain. For centuries, “Rios” — which, like their eponymous river canyon, come in a range of ruddy pastels — fed the Taos Pueblo, various Apache bands and Spanish-speaking shepherds, including Vigil’s great-grandfather, Felipe. “He’d be up there all summer with his flock and his rifle, catching and eating these same fish,” Vigil said. “If you’d asked me my favorite fish when I was 5 years old, I would’ve said Rio Grande cutthroat trout.”
But Vigil’s beloved fish has fallen on hard times. Cities and ranches drained southwestern streams, roads and logging operations fragmented rivers, and humans loaded watersheds with nonnative species. Today, Rios occupy approximately 12% of their erstwhile habitat. One of the places from which they likely vanished was Sand Creek, which was colonized, early on by brook trout — feisty Eastern fish, likely planted by miners — and, later, a distantly related cutthroat subspecies that state biologists stocked for anglers. The interlopers overwhelmed Rios, and some interbred with them, swamping their gene pool. By the 21st century, the native trout were gone.
In 2020, after years of planning, Vigil and his colleagues began the laborious process of returning Rios to Sand Creek. That September, a team of biologists and technicians dosed the stream’s upper reaches and its headwater lakes with a fish-killing poison. This mass piscicide wasn’t fun — “you get kind of depressed when you’re killing fish,” Vigil acknowledged — but eliminating the invaders was a necessary prelude to restoring the native inhabitants. A year later, biologists stocked thousands of genetically pure Rios in the lakes. Now, Vigil planned to release more, this time in Sand Creek itself. “When you consider the size and remoteness, this is probably the biggest restoration project we’ve ever done,” Vigil said.
“When you consider the size and remoteness, this is probably the biggest restoration project we’ve ever done.”
Yet Sand Creek’s reclamation is about more than returning Rios to their former kingdom; it’s also about securing their future. Cutthroat need cold water, and climate change is shrinking their range; Rios, the southernmost of the West’s dozen cutthroat subspecies, are especially vulnerable. Sand Creek is a refuge from the warming. Several icy tributaries pump snowmelt and spring-fed water into the stream, keeping its mainstem cool even as ambient temperatures climb. Right now, these feeder tributaries are fishless, too frigid for eggs to develop, and boulders prevent trout from swimming up some of them. But as Sand Creek warms, biologists may someday release Rios into them. “Species need us in the climate change era to get them into the habitats where they’ll be safe,” Fred Bunch, Great Sand Dunes’ chief of resource management, told me later.
The National Park Service and its collaborators haven’t yet stocked the chilly tributaries, or decided if they ever will, Bunch said. Elsewhere in the West, though, biologists have begun to plant trout in once-fishless waters that may become climate strongholds. Such foresightful translocation has long been a hot topic among foresters, who refer to it as assisted migration and have contemplated transplanting everything from California’s sequoias to Florida’s torreya conifers. But while the assisted migration of trees in the U.S. has mostly remained “a mere research subject,” as the journalist Lauren Markham put it, the assisted migration of fish is already happening, with less fanfare. To many biologists, the rapidity of climate change necessitates it. Yet this well-intentioned meddling recalls a disquieting history. Trout, unlike trees, are predators, with a track record of transforming ecosystems wherever they’re put. A golden rule in conservation biology, Clint Muhlfeld, a research aquatic ecologist with the U.S. Geological Survey, told me, is to do more good than harm.
“The result is a hybrid,” lamented one manager in 1937, “less productive than either, in which the rainbow type predomin-ates and the native charact-eristics soon disappear.”
WESTERNERS HAVE ASSISTED TROUT migration for more than a century. Beginning in the late 1800s, fishing clubs, miners, railroad workers and biologists dumped millions of trout into lakes and creeks to promote angling: brown trout from Germany, rainbows from the California coast, Northeastern brook trout. Stocking techniques were rudimentary — one newspaper recommended conveying trout in “milk cans or some such suitable receptacle” — yet the consequences were profound. Nonnative fish gobbled frogs, salamanders and insects. The West’s native cutthroat, named for the vivid slash-marks along their jaws, were particularly harmed. Browns ate them, brookies outcompeted them, and rainbows interbred with them. “The result is a hybrid,” lamented one manager in 1937, “less productive than either, in which the rainbow type predominates and the native characteristics soon disappear.”
Over time, climate change exacerbated the crisis. Megafires choked creeks with ash, and warmer, lower flows allowed nonnative fish to overtake rivers. Cutthroat and bull trout, another cold-loving Western fish, fled upstream. One 2009 study found that several cutthroat subspecies could lose substantial portions of their habitat within 50 years. Like pikas and whitebark pines, the West’s endemic fish were being driven upslope toward oblivion.
As the situation worsened, scientists considered dramatic measures. Some of the West’s coldest, most climate-proof streams and lakes naturally lay in high elevations near the headwaters and were thus off-limits to fish. Road culverts, dams and various human-made blockades impeded access to others. Trout couldn’t reach these refugia on their own, but, if humans moved them, they might survive. Assisted migration, a 2010 report declared, could “establish or reestablish self-sustaining populations.”
So, managers acted. In 2014, Glacier National Park worked with Muhlfeld to rescue bull trout from Logging Creek, where non-native species threatened them with extirpation. Muhlfeld and his crew zapped the stream with electric probes, netted the stunned bull trout, and hiked them in backpacks to Grace Lake, above a fish-blocking waterfall. Muhlfeld hadn’t removed the fish from their home watershed, merely given them a lift upstream. Still, it showed that human intervention could salvage a troubled population. “All indications are that those fish are growing really well and spawning,” Muhlfeld said.
Other scientists took more drastic measures. In 2017, biologists in Yellowstone National Park poisoned the Upper Gibbon drainage system to exterminate the rainbow and brook trout that the Park Service itself once stocked above and around a waterfall. Rather than letting the Upper Gibbon remain fishless, likely its historic state, the agency then planted cutthroat and grayling, salmonids native to elsewhere in the park. Brian Ertel, a Park Service fisheries biologist, told me that the agency also plans to eliminate invasives and release cutthroat in upper Buffalo Creek, cold refugia whose natural waterfalls will thwart reinvasion. “The climate modeling, the diversity of habitat, and the barriers all show that, hey, this is going to be a great place for cutthroat trout to survive and thrive,” Ertel said.
Of course, icy headwaters aren’t just refuges for trout; they also harbor trout food. Alpine lakes in many Western states sustain boreal toads, a sensitive and toxic species whose tadpoles trout attack. Streams in Montana and Wyoming support the western glacier stonefly, a threatened insect whose reliance on snow and glacial melt makes it as vulnerable to climate change as any fish. Studies have shown that after cutthroat were released into fishless lakes in the Canadian Rockies, mayfly populations plummeted, while midges and roundworms exploded. “(A) liberal policy on assisted colonization,” researchers concluded, “could cause broad irreversible damage.”
Despite the potential repercussions, research has been scant. One forthcoming review found that just two of more than 200 peer-reviewed studies concerning assisted migration involved fish. Still, biologists have worked to avoid collateral harm. Before moving bull trout to Grace Lake, Muhlfeld surveyed its insects, amphibians and plankton to ensure the release wouldn’t inflict disaster. Rainbow trout have occupied Yellowstone’s Upper Gibbon River for so long that cutthroat couldn’t make things worse. “It’s already been impacted by nonnative fish, so it’s not like we’re dumping trout on top of amphibians or other aquatic animals,” Ertel said. We’ve broken the world so thoroughly that we’ve created the conditions for its repair.
Yet even proponents acknowledge assisted migration’s no panacea. As Muhlfeld noted, skinny, high-elevation refugia aren’t necessarily trout’s preferred habitat, but where they’ve been exiled. One 2019 study found that the best strategy for preserving Rio Grande cutthroat long-term was to prioritize “large watersheds with intact habitat” and natural downstream barriers — places exactly like Sand Creek. The lesson: While assisted migration can buy trout time, we also need to restore entire ecosystems — reconnect rivers to floodplains, demolish dams, reduce withdrawals, eliminate invasives. The gravest danger from assisted migration may not be its side effects, but that we wrongly conclude it’s sufficient.
AS ASSISTED MIGRATION HAS SPREAD, the term’s meaning has blurred. The Forest Service says that it means any human-directed movement of a species in response to climate change. But when the Utah Division of Wildlife Resources recently produced a video touting its “assisted migration” of cutthroat, it never mentioned climate. Absent the motivation of global warming, was the state practicing assisted migration? Or was it just stocking fish, as states have always done?
Some of this ambiguity is built into assisted migration, which is less a discrete strategy than an umbrella term that encompasses all manner of interventions. Assisted range expansion, for example — where you move an organism just outside where it’s currently and historically found — is fundamentally different from assisted species migration, where you transplant something to a distant locale. Critics who deride assisted migration as ecological gambling sometimes elide these vagaries, making the concept seem more radical than it is. It’s one thing to move Rio Grande cutthroat into fishless tributaries within their historic range, another to consider shipping them to, say, Montana.
“As some of the areas they’re reproducing in become too warm, these tributaries are going to provide places for them to go.”
Assisted migration in the Sand Creek drainage will proceed slowly and with a light touch, if at all. “Those fishless streams are a good potential option that we’ll revisit down the line,” Estevan Vigil told me after the helicopter left. Cutthroat may also move into the cold creeks on their own. “As some of the areas they’re reproducing in become too warm, these tributaries are going to provide places for them to go,” Vigil predicted. This possibility struck me as a promising glimpse into the future: By restoring organisms to their former domains, biologists also give them the opportunity to make their own adaptive decisions, a range expansion both human-assisted and self-directed.
Vigil and I hiked down to Sand Creek, his precious cargo jouncing on his back. The stream tripped over boulders; snow clung to its banks. Vigil unslung his backpack and pulled out the bag of 4-month-old cutthroat. The trout, no longer than a pinkie joint, swirled in their plastic bubble like county-fair goldfish. Vigil tore open the bag and spilled a few into a shallow pool — cautiously at first, to make sure they’d acclimate. The fish righted themselves and swam off. Satisfied, Vigil emptied the bag, and we watched fish dart to and fro like campfire sparks. One by one they drifted toward the current beyond the pool. They faced upstream for a moment, Sand Creek flowing through their open mouths and over their gills, and then they were gone.
Ben Goldfarb is a High Country News correspondent and the author of Eager: The Surprising, Secret Life of Beavers and Why They Matter. His next book, on the science of road ecology, will be published by W.W. Norton in 2023.
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This article appeared in the print edition of the magazine with the headline Assisting the Rio Grande cutthroat.
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