Global warming will create increasingly
challenging thermal environments in Northwest rivers for salmon and trout this
century, a recent study concludes.
The study documents long-term temperature
trends in large rivers across the region and describes the biological
implications for salmon and trout this century.
If warming occurs as predicted (1 to 3 degrees
Centigrade over the baseline period 1993 - 2011), warming would expose sockeye
salmon to 5 to 16 percent warmer conditions (3 to 143 degree days) and reduce
suitable river habitat for trout by 8 to 31 percent, while also causing trout
to move upstream to seek cooler water.
As dire as that may seem, the authors of the
study say that path is less of a “road to ruin” for the fish and more like a journey
“For some populations or fisheries that are
heavily exposed and vulnerable, an additional 1–3 C of warming accompanied by
those changes may well prove to be the road to ruin,” the study says. “But for
the majority of salmon and trout populations and species, we believe a more apt
metaphor is a path through purgatory as these fish continue attempting to adapt
by tapping their remarkable stores of diversity and resilience.
“Current greenhouse gas emission rates may
make their purgatory last much of the 21st century, so concerted, ongoing, and
strategic efforts by the conservation and management communities will be needed
to assist in that adaptation,” the study says.
“Global Warming of Salmon and Trout Rivers in
the Northwestern U.S.: Road to Ruin or Path Through Purgatory?” was published
online March 27, 2018 in Transactions of the American Fisheries Society (https://onlinelibrary.wiley.com/doi/abs/10.1002/tafs.10059).
Authors – all from the U.S. Forest Service,
Rocky Mountain Research Station in Boise, Idaho – are Daniel Isaak, fisheries
research scientist, Charles Luce, research hydrologist, David Nagel, spatial
analyst, and Dona Horan, Gwynne Chandler and Sherry Wollrab, all fisheries
Isaak said the study is different than
previous studies about projected changes in warming streams, “We thought it was
important to first describe what the historical trends have been for river
temperatures. We therefore compiled the best available long-term monitoring
records from state, federal, and tribal agencies, which consisted of data from
391 sites in the region’s 56,500 kilometers (35,107 miles) of rivers in
Washington, Oregon, northern California, Idaho, Montana, and Wyoming.”
The result they got suggests that warming
trends in rivers were prevalent during summer and early fall months in recent
20-year and 40-year periods (0.18–0.35 C per decade from 1996 to 2015 and
0.14–0.27 C per decade from 1976 to 2015). The warming paralleled air
temperature trends and were mediated by discharge trends at regional and local
levels, he said.
Next, those trend estimates were used to
inform selection of future river temperature scenarios and assess changes in
thermal exposure of adult sockeye salmon migrating to four population areas as
well as thermal habitat shifts for resident brown trout and rainbow trout
populations throughout the region.
“Effects of those changes on population
persistence and fisheries are likely to be context dependent and strategic
habitat restoration or adaptation strategies could ameliorate some biological
impairments but effectiveness will be tempered by the size of rivers, high
costs, and pervasiveness of thermal effects,” Isaak said. “Most salmon and trout
rivers will continue to provide suitable habitats for the foreseeable future
but it also appears inevitable that some river reaches will gradually become
too warm to provide traditional habitats.”
Although warming of the region’s rivers is
evident and temperatures during some months are now about 1 C warmer than 20–40
years ago, it is encouraging to note that salmon and trout populations remain
widespread in the Northwest, the study says.
“Less encouraging is that the Earth is
probably in the initial decades of a long-term warming period, and that
temperature increases will act synergistically with regional trends in
hydrology, non-native species invasions, human population growth and water use,
and less favorable ocean conditions to negatively affect cold-water fishes,”
the study continues.
The authors say there are options to ease the
predicted thermal stress on salmon and trout populations, but that the remedies
are “context dependent and will require strategic decision making.”
For example, efforts to keep rivers cool could
include minimizing the number of flow diversions, increasing shade provided by
riparian vegetation, reconnecting rivers to floodplains to enhance habitat
diversity, and increasing channel roughness to encourage more water exchange
between the channel and cooler hyporheic flows, the study says.
Other, “(m)ore aggressive measures have also
been discussed such as excavating deep pools adjacent to warm rivers to access
cool groundwater or the construction of wingwalls upstream of cold tributary
inflows to limit mixing and create microrefugia,” the study says. In river
reaches where flows can be regulated, releases of cold water during warm
periods could cool reaches lower in the river and managers could coordinate
actions among multiple dams to maximize effects.
“Where salmon populations are a primary
concern, dam breeching or installation of fish passage systems could provide
access to significant lengths of cold rivers in portions of historical ranges
that have been blocked,” the study says. “Where fish passage structures already
exist, designs might be optimized to minimize migration delays that increase
Isaak said the authors drew on data already
recorded by local biologists:
“New data were not collected to conduct this
study. Instead, we simply mined the regional NorWeST database (https://www.fs.fed.us/rm/boise/AWAE/projects/NorWeST.html) to identify the best long-term monitoring sites, then contacted
the local biologists to obtain the most recent years of data from those sites,”
Biologists were from the Idaho Department of
Fish and Game, Montana Fish, Wildlife and Parks Department, Oregon Department
of Fish and Game, Washington Department of Fish and Wildlife, Washington
Department of Ecology, King County, Burns Paiute Tribe, Confederated Salish and
Kootenai Tribes, Confederated Tribes of the Umatilla Indian Reservation, Nez
Perce Tribe, Yakama Nation, Idaho Power, U.S Forest Service, U.S. Geological
Survey, U.S. Bureau of Reclamation, U.S. Army Corps of Engineers, Bureau of
Land Management, and the National Oceanic and Atmospheric Administration.
“This research approach suggests that valuable
information can be developed inexpensively from well-maintained databases and
coordinated efforts among resource agencies,” Isaak concluded.