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Upper Columbia Basin Bull Trout Study Finds Small Populations Declining, At Risk
Posted on Friday, December 21, 2018 (PST)

Bull trout populations in the upper Columbia River basin in Idaho and Montana are small and some are continuing to decline, according to a recent study.


The study concludes that the vast majority of bull trout populations have less than 20 redds (nests) per year and that a subset of these populations has significantly declined over the last 20-30 years. Only a couple of the populations have grown, despite protections as a threatened species under the federal Endangered Species Act (1999), according to researcher Ryan Kovach, a biologist and ecologist with the U.S. Geological Survey’s Northern Rocky Mountain Service Center in Missoula, Montana.


Although an assessment of extinction risk was not the study’s focus, “theory from conservation biology and genetics suggests that small populations are at risk of extirpation,” he said, defining a small population as one with 20 redds or fewer per year.


Risk varied widely across the study area, he continued. For example, bull trout populations in Lake Coeur d’Alene (Idaho), Rock Creek (Montana) and Flathead Lake (Montana) tended to be at higher risk, while populations in the South Fork Flathead above Hungry Horse Reservoir (Montana) and Lake Pend Oreille (Idaho) tended to be at lower risk.


The study, “Long-term population dynamics and conservation risk of migratory bull trout in the upper Columbia River basin,” was published in the Canadian Journal of Fisheries and Aquatic Sciences (


Kovach’s co-authors are Jonathan Armstrong, Department of Fisheries and Wildlife, Oregon State University; David Schmetterling, Montana Fish, Wildlife & Parks; Robert Al-Chokhachy, Northern Rocky Mountain Service Center, USGS, West Glacier (all are biologists/ecologists); and Clint Muhlfeld, professor at the University of Montana.


The study evaluated long-term population dynamics and conservation risk of 88 bull trout populations in upper Columbia River basin headwaters. According to the study, its objectives are to:

(1) determine abundance and trend (declining, stable, or increasing) of local populations and metapopulations,

(2) describe patterns in population synchrony both within and among watersheds,

(3) describe portfolio effects across metapopulations, and

(4) compare various measures of conservation risk to determine whether prioritization may vary depending on the metrics used to assess status.


The study found that over half of the bull trout populations studied (51 percent) had, on average, fewer than 20 redds per year, and 74 percent of populations had fewer than 50 redds per year. Redd counts varied substantially, the study said. Several metapopulations had fewer than 63 redds per year (Priest Lake, Lake Coeur d’Alene, Cabinet Gorge), but the most productive metapopulations had 500 or more redds per year (Hungry Horse Reservoir, Lake Pend Oreille).


Bull trout populations were stable in 85 percent of locations, significantly declining in 13 percent of locations, and significantly increasing in only 2 percent of locations, the study said.


It went on to say that there “were two metapopulations with two or more declining local populations, Flathead Lake and the Blackfoot River, and only one metapopulation where the overall population growth rate was significantly declining (Rock Creek). There were no metapopulations where the overall population growth rate was significantly increasing.”


Although most populations appeared stable, according to the study, there was limited evidence for increasing trends in abundance.


“This is concerning given that many bull trout populations in the upper Columbia River basin are depressed relative to historic levels and most populations will be subject to increasing stress from future climatic conditions,” the study said.


A number of factors are responsible for the small populations and the general lack of upward population trends.


“We did not assess the effects of climate change in this paper,” Kovach said. However, bull trout are extremely intolerant of warm temperatures and as such it should come as no surprise that ongoing and future climate change poses a significant risk to this species.


“That being said, other factors, especially invasive species, habitat fragmentation, and habitat degradation all negatively impact bull trout, and these are all problems that can be targeted by management.”


Some of those suggested management actions, according to the study, are the suppression of lake trout, a non-native species, habitat restoration, translocation (moving bull trout to cold water habitats that are free of invasive species), and the strategic removal of barriers to bull trout movement.


Some of these actions have shown a positive benefit. For example, suppression of lake trout in Lake Pend Oreille has benefitted bull trout, as has extensive habitat restoration in the Blackfoot River basin. Translocation in Glacier National Park is also planned for the Blackfoot basin, and removal of the Milltown Dam near Missoula benefited bull trout and other native species in both the Clark Fork and Blackfoot river basins.


“Long-term redd count data generally emphasize that many migratory bull trout populations in the upper Columbia River basin remain small and of significant conservation concern,” the study concludes. “Ultimately, the variability in conservation risk across metrics highlights that bull trout are differentially influenced by environmental and biotic conditions operating on a continuum of scales, thereby requiring management actions or strategies that vary across core areas and local populations. In turn, conservation of migratory fish like bull trout will require ecosystem-level approaches that target stressors in headwater habitats as well as critical habitats used during subadult and adult life stages.”

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