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Warm river temperatures lead to drop in Steelhead survival rates

The Aberdeen Times of Aberdeen, Idaho

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While some may enjoy hot August nights along the Columbia River, migrating steelhead take exception. Many of the fish, uncomfortable in the Columbia's warm summer water, have been seeking refuge in cool-water tributaries along their migration route.

This behavior intrigued a University of Idaho fisheries research group, who say some upriver steelhead populations are at risk as anglers cash in on the high numbers of steelhead concentrating in cool tributaries in western Oregon and Washington.

In a recent study, Matthew Keefer, College of Natural Resources fisheries scientist; Christopher Peery, U.S. Fish & Wildlife Service fishery biologist; and Brett High, Idaho Department of Fish and Game regional fisheries biologist, used radiotelemetry to monitor several thousand adult steelhead as they used cool water sites called "thermal refugia."

When the Columbia River temperatures were warmest, usually in August, the scientists found that more than 70 percent of the tagged fish used refugia sites, sometimes holding there for weeks to months before continuing migration upstream. This behavior relieves the stress of high water temperatures, but also places the valuable fish at risk. Once concentrated inside refugia, steelhead are vulnerable to a variety of threats, including intense harvest.

"Anglers have discovered this behavior and take advantage of the tremendous fishing opportunity created by so many fish concentrated at the refugia sites," said Keefer. "The management concern is that most of the steelhead harvested in thermal refugia are only pausing on their way to rivers in interior Idaho, eastern Oregon and eastern Washington - they are harvested far from their point of origin."

Keefer further explained that some of the steelhead harvested in the lower Columbia refugia are genetically distinct and are from populations listed as threatened under the Endangered Species Act. The group's research showed that steelhead that used thermal refugia were five to 10 percent less likely to survive to spawning areas than were fish that migrated through the lower Columbia River study area without using refugia. As a result, hundreds to thousands fewer hatchery and wild steelhead reach their home rivers, and this is concern for fisheries managers.

"The behaviors we observed underscore the critical importance of thermal refugia in the Columbia River system," said Keefer. "Using the cool water helps steelhead conserve energy needed to complete migration and spawn, but those benefits must now be weighed against the increased harvest mortality."

It is important to understand how an economically valuable species like steelhead responds to high water temperatures, he explained, especially because river warming is predicted to increase with regional climate change.

The research group recently published their results in the Canadian Journal of Fisheries and Aquatic Sciences, Vol. 66 No. 10, October 2009. The article is entitled, "Behavioral Thermoregulation and Associated Mortality Trade-offs in Migrating Adult Steelhead (Oncorhynchus mykiss): Variability Among Sympatric Populations."

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Original Publication Date: October 14, 2009

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