Scientific Support of Salmon and Steelhead Reintroductions in Impounded River Basins of the Pacific Northwest

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Salmon and steelhead populations in the Pacific Northwest were severely affected by hydropower development that occurred during the first half of the 20th century.  Impassable dams were constructed on many rivers throughout the region which prevented returning adult salmonids from accessing important habitats where spawning and rearing historically occurred.  In the past two decades resource managers have begun to focus on efforts to reintroduce salmon and steelhead to many of these watersheds.  In most cases, reintroduction involves the collection, transport, and release of juvenile and/or adult fish.  Research plays an important role in fish reintroduction efforts by describing how fish move within an impounded watershed, estimating survival of fish after release, during migration, or as they pass a dam or dams, and by monitoring interactions between fish of different species or origins.  The goal of our research is to provide unbiased scientific results that can be used to determine the success of ongoing reintroduction efforts and to improve future efforts.

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Shoreline trap

Shoreline trap in Lookout Point Reservoir, Oregon. Credit: Amy Hansen, USGS. (Public domain.)

In the past twenty years Pacific salmon (Oncorhynchus spp.) populations have been re-established in many watersheds where they had been absent for decades due to dam construction and hydropower development that occurred in the first half of the 20th century.  In most cases, efforts to reintroduce anadromous fish upstream of impassable dams rely on developing new passage alternatives or capturing fish and moving them around a dam or dams.  These are common bottlenecks that can limit reintroduction efforts because fish collection can be challenging, and trap-and-haul procedures can have negative effects on the population of fish that is successfully collected and transported.  Basin-specific research is often required to provide resource managers with information that can be used to improve these efforts.  We have evaluated downstream and upstream fish passage issues in multiple basins, examining performance of downstream fish passage devices and collectors, and monitoring movement patterns of adult salmonids after upstream release.

Upstream releases of adult salmonids may consist of fish of both natural-origin (NOR) and hatchery-origin (HOR) and these two groups of fish frequently behave differently after outplanting.  The potential for HOR salmon and steelhead to negatively affect NOR congeners in a given watershed is often of ecological concern.  However, in many places HOR fish are necessary for reintroduction efforts because NOR populations no longer exist or are too small to lead to a high likelihood for success.  Thus, HOR salmonids are commonly outplanted as a first step in many reintroduction efforts.  Our research has shown that HOR salmonids behave differently than NOR fish after release and provide less contribution to the spawning population.  Recent results from these studies are being used to enhance reintroduction efforts by altering release site locations for fish of differing origins.

Reintroduction science has become an important element in efforts to re-establish salmon and steelhead populations to areas affected by the presence of impassable dams.  Lessons learned from evaluations of fish reintroductions are likely to increase the success of future efforts in places like the Upper Columbia River, Washington and the Shasta River, California where new, large-scale reintroductions are being planned.

Sockeye salmon

Sockeye salmon preparing to spawn upstream of Cle Elum Dam, Washington. Credit: Ryan Tomka, USGS. (Public domain.)