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Study Details Challenges for Northwest Salmon, Trout as Region's Rivers Face Warming Temperatures by Staff Columbia Basin Bulletin, April 13, 2018

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 through "purgatory."

"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.

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 biologists.

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 thermal exposure."

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 to identify the best long-term monitoring sites, then contacted the local biologists to obtain the most recent years of data from those sites," he said.

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.

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