What will the future hold for western river systems and the cold-water species that depend on them? To answer this question, scientists at the USDA Forest Service Rocky Mountain Research Station compiled temperature records from nearly 400 monitoring sites along large rivers in the northwestern United States.
They found that average river temperatures during the summer and early fall months rose about 1°C from 1976 to 2015 and that if trends continue, western rivers may warm by an additional 1°C by 2050. This warming trend is likely to result in habitat shifts and population changes for many trout and salmon species in these large river systems. The good news is that these changes are not happening as fast as previously predicted, fish have some ability to adapt, and there are ways to offset warming in portions of some rivers.
The study, “Global warming of salmon and trout rivers in the northwestern U.S.: Road to ruin or path through purgatory?” recently published in Transactions of the American Fisheries Society, uses historical monitoring data compiled from more than a dozen agencies to describe historical trends in an effort to better predict the future. The scientists also looked at other trends over the same period showing warming air temperatures and declining water flows. Those trends were then translated to potential biological impacts that may occur later this century. They focused on species such as adult salmon migrating to spawning grounds and resident trout species, like brown trout and rainbow trout, in nearly 35,000 miles of rivers. This is important because healthy trout and salmon populations are major economic drivers in the region for both recreational and commercial fisheries, and other fish populations that have declined are listed as endangered species.
The impetus for the study was the summer of 2015. The region experienced record low snowpacks and abnormally warm June temperatures that led to extensive salmon die-offs in the Columbia River and the temporary closure of many popular trout sport-fisheries. Continuation of trends observed in historical river temperature records suggest that years like 2015 will become increasing common later this century. At the same time, salmon and trout are adapting. They are moving into cooler micro-refuge sites during heat waves and altering their migration timing. There is also a gradual, long-term distribution shift to upstream, cooler areas that are predicted to remain as cold water refuges.
The paper suggests several options that managers could use to help offset warming and preserve cold-water river habitats. These include minimizing water withdrawals from rivers, increasing shade provided by riparian vegetation, enhancing habitat diversity and the number of deep pools that often tap into cold groundwater, releasing cold water from deep storage dams during heat waves, and improving fish passage at dams that block access to cooler river sections.
“The data show that global trends are having local impacts on our western rivers,” said Dan Isaak, lead author of the study with the U.S. Forest Service Rocky Mountain Research Station. “River temperatures are increasing more slowly than air temperatures across the region and cold-water fish should be able to adapt for the foreseeable future; however, some river reaches will gradually become too warm to provide traditional habitats and if the warming trend continues throughout the century, we could see some very different fish communities.”
The Rocky Mountain Research Station (RMRS) is one of seven units within the U.S. Forest Service Research and Development. RMRS maintains 14 field laboratories throughout a 12-state territory encompassing parts of the Great Basin, Southwest, Rocky Mountains, and the Great Plains. RMRS also administers and conducts research on 14 experimental forests, ranges and watersheds and maintains long-term research databases for these areas. While anchored in the geography of the West our research is global in scale. To find out more about the RMRS go to www.fs.usda.gov/rmrs. You can also follow us on Twitter at www.twitter.com/usfs_rmrs.
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