Evaluating a multi-generational reintroduction program for threatened salmon using genetic parentage analysis

Abstract

Dams, utilized for hydroelectric or flood control purposes, obstruct organism dispersal and have contributed to the decline of many migratory fish populations. For threatened Chinook salmon (Oncorhynchus tshawytscha) from the Willamette River Basin in Oregon, human-assisted reintroductions are being used to facilitate dispersal to historical habitats located above dams. However, little is known about the reproductive outcomes of reintroduced individuals or the efficacy of reintroductions towards the goal of population demographic viability. Using genetic parentage assignments to 3-, 4-, and 5-year-old adult recruits, we estimated the fitness of hatchery and wild Chinook salmon reintroduced above Foster Dam on the South Santiam River, a tributary of the Willamette River. Our parentage assignments indicated that the fitness of reintroduced salmon was highly variable, with individuals producing a range of 0–40 adult progeny. We also detected a possible trend towards reduced fitness in mate pairs composed of hatchery versus wild salmon. For each of three brood years (2007, 2008, 2009), adult offspring recruitment achieved or exceeded population replacement. We observed the highest cohort replacement rate in 2009, the first year that managers aimed to release wild salmon solely above the dam. Taken together, our results suggest that human-assisted reintroductions of mature adult salmon to historical spawning habitats are a promising method of restoring natural production in populations affected by dams. Moreover, the continued used of wild fish in reintroduction operations may improve population productivity and the prospect of recovery within the South Santiam River.