Warmer temperatures and altered stream flows increase the struggle of Pacific salmon

by Steve Toub

Part 1 of a series on the Endangered Species Coalition’s report “America’s Hottest Species“, 11 endangered species impacted by climate change.

Pacific salmon struggle to swim hundreds of miles to return from the ocean to the freshwater streams where they were born. But this struggle pales in comparison to external threats, which have led to a severe decline in Pacific salmon populations. Five populations of Pacific salmon are endangered and 23 are threatened under the Endangered Species Act, including populations of Chinook salmon, Chum salmon, Coho salmon, Sockeye salmon, and steelhead trout, which are true salmon despite their name. The primary historical cause of the decline as been overfishing, but loss of freshwater habitat has been increasing as a factor. Dam-building, logging, and pollution are among the causes of salmon habitat loss in the past century; looking forward, increased climate change will further stress Pacific salmon populations.

In 2007, the Independent Scientific Advisory Board (ISAB) for the Northwest Power and Conservation Council, Columbia River Basin Indian Tribes, and National Marine Fisheries Service published a major report on Climate Change Impacts on Columbia River Basin Fish and Wildlife. It predicted that by 2090, more than 40% of salmon habitat in Oregon and Idaho would be lost. Since the impact of of climate change on salmon habitat is more severe at higher elevations, Washington state, which has overall lower elevations, would lose 22% under the worst case.

Warmer air temperatures alter precipitation and water flows in the region, affecting salmon in several ways. More winter precipitation falling as rain than snow (and more overall precipitation) means heavier stream flows and floods that damage spawning nests in gravel stream beds and wash away incubating eggs. Additionally, the lighter snowpack and its earlier snow melt makes peak river runoff earlier, taking them out to sea before plankton blooms are ready, depriving them of a primary food sources. More significantly, reduced stream flows in summer and fall, making the shallower or drying smaller streams up altogether, forcing them into smaller and less diverse habitats and reducing the likelihood that salmon pass the physical obstacles that can prevent them from completing their upstream migration back to where they were born.

Warmer stream and estuary temperatures pose multiple challenges. A report by Light in the River on the impact of global warming on Pacific salmon states that the optimal temperatures range for juvenile and adult salmon is 55-64° F and that stream temperatures over 70° F are extremely stressful; it later notes that “recent summer water temperatures in the Columbia River have averaged 68-70° F” and that global warming trends are expected to continue. Warmer temperatures increase the metabolism of the salmon, forcing them to find more food to survive. However, their food sources may be scarcer: when eggs are hatching earlier young salmon may be out of sync with the insects they eat and other species better adapted for warmer water are better able to compete for the same food sources. Diseases and parasites increase in warmer water, adding an additional stress, more potent when salmon are already thermally stressed. Also, higher water temperatures accelerate embryo development, causing to eggs hatching earlier in the year and leaving salmon fry less developed and therefore more vulnerable to predators.

Rising sea levels, warmer ocean temperatures and ocean acidification may also stress salmon, but less is known about how these changes will affect the species.

Studies have shown the direct correlation between temperature and health of salmon. Warm periods have fewer salmon; cooler periods have relatively higher numbers. One study of four populations for Snake River spring/summer chinook, cited in the Light in the River report, projected that a 22% decline in October streamflow and a 5.5 degree Fahrenheit increase in average June temperature led to a 37-50% decline in population.

Implementing strong habitat restoration plans may be the only way to limit population declines. In one study of how Chinook salmon in the Snohomish River fare under different climate change models and different restoration scenarios, by 2050 populations decline by an average of 20% in one climate change model and 40% in another. Under an aggressive habitat restoration model populations increase by 19% under the less severe climate change model and decline by only 5% in the more sever model. Moderate restoration scenarios under both models resulted overall declines.

Environmental organizations have been pressing the federal government in to adopt a strong salmon recovery plan for years. Save Our Wild Salmon reports that the Obama plan released recently was only marginally better than the plan proposed by the Bush administration and that “runs counter to the science and advice of experts in the field-regional Forest Service, Department of Fish and Wildlife and American Fisheries Society scientists.” After a November 23 hearing, a U.S. District Court judge is expected to rule very soon on the status of this latest salmon recovery plan.

Under threat for years and under increasing stress from climate change, Pacific salmon need your help to survive. Donate to the Endangered Species Coalition to help save America’s imperiled wildlife for future generations.

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