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.
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.
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.
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.
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 (http://www.nrcresearchpress.com/doi/abs/10.1139/cjfas-2017-0466#.XBkoEGhKjIU).
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.
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:
determine abundance and trend (declining, stable, or increasing) of local
populations and metapopulations,
describe patterns in population synchrony both within and among watersheds,
describe portfolio effects across metapopulations, and
compare various measures of conservation risk to determine whether
prioritization may vary depending on the metrics used to assess status.
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).
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.
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.”
most populations appeared stable, according to the study, there was limited
evidence for increasing trends in abundance.
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
number of factors are responsible for the small populations and the general
lack of upward population trends.
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.
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.”
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
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.
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.”