WHITEFISH – A new research paper published in Fisheries magazine explores how a warming climate is affecting trout streams in the Flathead River basin and throughout the Rocky Mountains, and urges quick action if native trout populations are to persist in diminishing cold-water habitats.
The report examines the climate histories of five river basins in the Rocky Mountains, including the Flathead River, which is home to robust populations of native bull trout and westslope cutthroat trout. In every case, stream characteristics have been adversely affected by warming trends, which have led to higher stream temperatures and habitat fragmentation.
The report’s authors include Clint Muhlfeld, an aquatic ecologist at the U.S. Geological Survey’s Glacier National Park field office, and Daniel Isaak, of the U.S. Forest Service’s Rocky Mountain Research Station in Boise, Idaho. The researchers said the retrospective assessment helps develop a trajectory for future warming based on historical trends in watersheds monitored and recorded during the past six decades.
To better understand the potential consequences, the report also offers empirical evidence of threats that already are occurring, and which will only be exacerbated by future climate change.
“We are using the past as a guide to the future,” Muhlfeld said. “We are developing predictive models to forecast changes to streams, ecosystems and species, but with this study we wanted to look at the empirical data that has already been collected throughout the western United States to see what changes have occurred in these watersheds of interest. This isn’t speculation, this is happening.”
The evidence indicates that as winter snowpack declines, spring runoff occurs earlier, peak summer flows dwindle and stream temperatures spike, trout in the western United States increasingly will be squeezed into shrinking cold-water refuges, and not all populations will survive.
“Despite the best intentions, we will not be able to preserve all populations of native trout in the Rocky Mountains this century,” the report states. “However, it should soon be possible to have the tools and information to know when and where resource commitments are best made under a given set of assumptions about future climate change.”
Isaak said the report, titled “The Past as Prelude to the Future for Understanding 21st-Century Climate Effects on Rocky Mountain Trout,” will help inform management decisions so that resources are used efficiently and not squandered on populations of species that are too fragmented to save.
“We are not going to be able to save all populations everywhere, and that is a pretty significant shift in our thinking,” Isaak said. “Previously we were trying to hang on to every last cutthroat population and every last bull trout population, and I don’t see that happening realistically given the direction we are going. We are going to have to make some choices and the sooner we can make those choices the better. But at the same time we need to be able to look before we leap.”
The article was published in the December edition of Fisheries magazine, a publication of the American Fisheries Society, and is available online at fisheries.org.
In addition to the Flathead River basin, the study also focused on the Greater Yellowstone Ecosystem, the Green River, the Rio Grande and the Boise River, and examined data recorded between 1950 and 2009.
Muhlfeld said certain actions that may offset future climate effects – habitat degradation, hybridization, rising temperatures and the threat of wildfire – include maintaining or restoring in-stream flows, increasing riparian vegetation to shade streams, and maximizing summer habitat volume.
Although bull trout are less susceptible to hybridization than cutthroat, the species is more susceptible to climate warming. Because they spawn in the fall, their egg nests, or redds, are vulnerable to scouring by high winter streamflows, which are swelling as autumn precipitation falls as rain instead of snow.
“We have some of the last of the best trout populations in the northern Rockies, but when you look under the hood it reveals some critical issues that we have to address,” Muhlfeld said. “There are existing stressors and changes happening right now that will increasingly challenge our trout populations given the future of climate change, and we’re calling for comprehensive management efforts. We are not going to save them all, but we can use these analyses to inform how we spend our money, invest our resources and direct our management.”
Reporter Tristan Scott can be reached at (406) 531-9745 or at firstname.lastname@example.org.