New fisheries research suggests that the headwaters of many Teton County streams will hold up as important spawning grounds for cutthroat trout deep into the 21st century, when waters are expected to heat up as the world warms.
Under the most extreme climate change scenario for 2080, the high reaches of the Gros Ventre River, Polecat Creek and Big Elk Creek are among the watersheds that will likely contain juvenile cutthroat and serve as “climate refuge streams,” according to a study by the U.S. Forest Service’s Rocky Mountain Research Station. That means those streams will have temperatures that will harbor breeding cutthroat, but at the same time remain too cold for non-native, competing rainbow and brown trout, fisheries scientist Dan Isaak said.
“Cold water is protecting these fish, and now we’ve got an accurate stream temperature model across all the streams in the region,” said Isaak, who led a study published in the journal Global Biology Change. “Brook trout are the one exception to the invasive trout issue — they can live in colder streams than brown trout or rainbow trout.”
The study, titled “The cold-water climate shield: delineating refugia for preserving salmonid fishes through the 21st century,” was more precise than previous studies because its models used actual stream temperature data and factors such as stream slope rather than relying on “crude surrogates” like air temperature and elevation, Isaak said.
In total the “climate shield” research assessed nearly 311,000 stream miles and projected the likelihood of juvenile cutthroat and bull trout occupancy in 2040 and 2080. The Forest Service’s Mike Young and David Nagel were collaborators.
Across the Northern Rockies, Isaak and his colleagues found that 90 percent of the climate refuge streams are on public lands. Only 15 percent, however, flow through highly protected lands such as national parks or wilderness areas.
In the models some high-elevation Jackson Hole area streams actually improve as habitat for juvenile cutthroat. That’s because they’re projected to warm up into being suitable habitat, Isaak said.
“For cutthroat, you can have streams that are too cold for them,” Isaak said. “You get up[stream] far enough and they can’t recruit and pull off a year class.
“The best streams for them are the ones that are right near that 11 degree [Celsius] mark and maybe down to 8 or 9 degrees,” he said.
The Cub Creek drainage in the Teton Wilderness is one example of a stream that goes from being unsuitable cutthroat spawning habitat now to having a 90 percent or better chance of occupancy in a much-warmer 2080, the Rocky Mountain Research Station’s maps show.
But the vast majority of watersheds in the Jackson Hole area fall off as cutthroat spawning habitat with a warmer climate, the maps show.
Suitable breeding grounds shift uphill and tend to be smaller forks of creeks rather than whole drainages.
The North Fork of Fall Creek and Cache Creek, for example, are wholly eliminated as juvenile cutthroat habitat by 2080, according to model’s predictions.
Factor the spread of non-native brook trout into the equation, and the Forest Service’s models predict that suitable cutthroat habitat dwindles further yet.
“The only time that we can see them being bad enough that they can cause the extinction of a local native population is if you’re in a small, low-gradient meadow type of stream,” Isaak said.
On steeper and larger streams, cutthroat and brook trout are more likely to coexist, he said.
Isaak’s models predict that the South Fork of Fish Creek — deep in the Gros Ventre drainage and accessed from Union Pass — is one example of a cutthroat climate refuge stream that could be highly influenced by the occupation of brook trout.
That meadow fishery goes from having a 90 percent or better chance of occupancy in 2080 without brook trout to a 25 to 50 percent chance with them.
Isaak said he sees the climate shield project as a useful tool for fisheries managers that could help them “concentrate their firepower rather than spreading things out.”
The mapping effort, he said, highlights the streams that are cold enough so “that if you’re a manager then you don’t have to worry about keeping invasives out because the natural temperature regimes will do it for you.”
The climate shield study piggybacks on 20 years of research that attempts to determine how climate change will mix up the composition of salmonids in the West, said Rob Gipson, the Wyoming Game and Fish Department’s regional fisheries supervisor.
“It’s definitely a starting point for thinking about what to protect first,” Gipson said of the research.
Brook trout eradication efforts are underway in places such as Yellowstone National Park’s Elk Creek and the Eagle Creek Meadows area in the Shoshone National Forest’s Washakie Wilderness.
No projects are planned to wipe out brook trout in the Jackson Hole area, Gipson said.
Local populations of brookies exist in places such as the Upper Gros Ventre, Spread Creek and Flat and Fish creeks. They dominate the fishery in the Cub Creek drainage, Gipson said.
“What is creating the current distribution?” Gipson asked. “Why are they spreading into some areas and other areas they are not?”
The abundance of interconnected trout habitat in the Snake River watershed and the minimal number of natural barriers to fish movement complicates pulling off projects that would target brook trout, he said.
“One of the things that make our cutthroat population so strong is also one of the things that in some ways jeopardizes it,” Gipson said of the region’s interconnected habitat.
“That also means it becomes more difficult to have small, isolated populations of brook trout where you’d have any chance of removing the whole population.
“Without pretty good guaranteed success rates, we wouldn’t go after any population because the brook trout would go ahead and recolonize,” he said.
A synthesis of Isaak’s climate shield research is presented at http://www.fs.fed.us/rm/boise/AWAE/projects/ClimateShield.html.