Where We Are Today: A Quantitative Baseline for Assessing the Response of Elwha River Chinook Salmon to Dam Removal

Removal of two hydroelectric dams on the Elwha River, slated to begin in fall 2011, will provide salmonid populations access to 130 km of pristine habitat. The Elwha Fish Restoration Plan relies heavily on hatchery supplementation to rebuild a self-sustaining wild population of Elwha Chinook salmon. The success of efforts to rebuild this population will depend on the abundance and productivity of wild and hatchery-origin Chinook salmon following dam removal. Until recently, relative contributions of wild and hatchery origin fish could not be assessed because spawner origin was not detectable. Beginning with the 2004 brood year, an origin-specific comparison of the survival and age-at-return was enabled by a hatchery program that fully marks or tags hatchery releases and a smolt trap study that estimates the number of wild Chinook out-migrants. All Elwha River Chinook salmon from the 2004 brood year and later were recognizable as one of three out-migrant types: wild sub-yearling, hatchery sub-yearling, or hatchery yearling. Chinook out-migrants during this period were predominantly (84-93%) sub-yearling hatchery fish. In order to compare survival and age-at-return among out-migrant types, Chinook spawner carcasses were collected from the Elwha River during spawner surveys, brood stock collection, and operation of a floating fish weir. For the 2004 brood year, survival-to-return of wild out-migrants (0.046%) was 2 to 3 times greater than sub-yearling hatchery (0.016%) or yearling hatchery out-migrants (0.021%). Age-at-return also differed among out-migrant types. Wild out-migrants returned primarily (61%) as age-4 spawners, whereas hatchery yearling releases returned primarily (55%) as age-3 spawners. Returns of hatchery sub-yearling out-migrants were evenly distributed among age-3 and age-4 spawners. The survival-to-return and age-at-return of wild- and hatchery-origin Chinook prior to dam removal should be useful for evaluating future changes in abundance and productivity and for adaptively managing the restoration of self-sustaining wild populations in the Elwha River.