Age Structure and Hatchery Fraction of Elwha River Chinook Salmon: 2017 Carcass Survey Report

Monitoring the recolonization of Pacific salmon and steelhead following the removal of two dams is a critical component of the Elwha Restoration Project. During the fall of 2017, we collected adult Chinook salmon (Oncorhynchus tshawytscha) carcasses from the Elwha River in order to evaluate the proportion of hatchery fish, the age distribution of returning adults, and the ratio of fish that exhibited stream vs ocean type life history strategies. Surveys were conducted in the upper watershed (river km 61.5) downstream to where the river enters into the Strait of Juan de Fuca, including three tributaries. Of the carcasses sampled from the river and its tributaries (N = 520), the majority (88 %) were located upstream of the former Elwha Dam site. We also sampled fish (N = 566) throughout the season at the WDFW hatchery in the lower Elwha River. Carcasses were sampled for physical measurements, hatchery marks, scales and genetics. We sampled 850 non-jack carcasses during the sampling season, representing 33.67 % of the estimated escapement above the Elwha SONAR site. Over 96% of the fish sampled were marked hatchery fish. Age-3 was the dominant age class (60%), and age-2 fish (jacks) accounted for less 6% of our total sample. We sampled one age-2 natural origin fish and ten age-3 natural origin fish, 20 age-4 natural origin fish and two age-5 natural origin fish whose parents had access to habitat upstream of the former Elwha dam site following its removal in 2012. However, we have not observed a reduction in hatchery mark rate for the age classes that might have been produced by spawners upstream of the Elwha Dam site, and thus have no evidence that recolonization of newly accessible habitat has boosted natural production of Chinook salmon. Natural origin fish returning to the river as age-4 or age-5 adults to date were exposed to extreme environmental conditions associated with dam removal. All of the Chinook that migrated to the ocean as yearlings were hatchery origin, and so we did not observe any stream-type life histories among unmarked fish. We estimated that Chinook that spawned naturally in the Elwha could have deposited over 2.6 million eggs in 2017. Finally, an analysis of spawner to spawner productivity indicated that naturally spawning fish from the five most recent complete cohorts (brood years 2008 - 2012) did not replace themselves (average productivity = 0.28), whereas the combined productivity of natural plus hatchery spawners exceeded replacement in four of the last five complete cohorts (average productivity = 2.2).