Abstract
Fall-run Chinook salmon (Oncorhynchus tshawytscha) from the Sacramento–San Joaquin River system form the backbone of California’s salmon fishery and are heavily subsidized through hatchery production. Identifying temporal trends in the relative contribution of hatchery- versus wild-spawned salmon is vital for assessing the status and resiliency of wild salmon populations. Here, we reconstructed the proportion of hatchery fish on natural spawning grounds in the Feather River, a major tributary to the Sacramento River, using strontium isotope (87Sr/86Sr) ratios of otoliths collected during carcass surveys from 2002 to 2010. Our results show that prior to the 2007–2008 salmon stock collapse, 55%–67% of in-river spawners were of hatchery origin; however, hatchery contributions increased drastically (89%) in 2010 following the collapse. Data from a recent hatchery marking program corroborate our results, showing that hatchery fish continued to dominate (∼90%) in 2011–2012. Though the rebound in abundance of salmon in the Feather River suggests recovery of the stock postcollapse, our otolith chemistry data document a persistent decline of wild spawners, likely leading to the erosion of locally adapted Feather River salmon populations.
Highlights
The Sacramento–San Joaquin River system in California’s Central Valley (CV) is the foundation of California’s water supply, providing water for approximately 35 million residents and supporting a multibillion dollar agriculture industry, and is home to the southernmost spawning runs of Chinook salmon (Oncorhynchus tshawytscha) in the Northern Hemisphere (Fisher 1994; Yoshiyama et al 1998; Moyle 2002; Williams 2006)
Hatcheries were built along CV tributaries to mitigate for dam construction and habitat loss, and many salmon populations in the CV are heavily subsidized by hatchery production (HSRG 2012, 2014; Palmer-Zwahlen and Kormos 2015)
Of the CWT retrieved from fish at Feather River Hatchery (FRH) from 2002 through 2007, 15.58% were 2-year-olds, 56.07% were 3-year-olds, 28.23% were 4-year-olds, and 0.13% were 5-year-olds (Mesick et al 2009)
Summary
The Sacramento–San Joaquin River system in California’s Central Valley (CV) is the foundation of California’s water supply, providing water for approximately 35 million residents and supporting a multibillion dollar agriculture industry, and is home to the southernmost spawning runs of Chinook salmon (Oncorhynchus tshawytscha) in the Northern Hemisphere (Fisher 1994; Yoshiyama et al 1998; Moyle 2002; Williams 2006). Chinook salmon populations have persisted in California’s highly variable Mediterranean climate by exhibiting a diverse portfolio, expressed as distinct run types (spring, fall, late-fall, winter) and plastic life history strategies (Yoshiyama et al 1998; Hilborn et al 2003; Williams 2006), which buffers population abundance against stochastic environmental variability. Habitat loss and degradation, water diversions, fish harvest, and the construction of dams, which blocked large areas (>80%) of spawning habitat and rearing grounds, have resulted in population decline threatening the long-term survival of salmon in the CV (Yoshiyama et al 2000, 2001). Spring- and winter-run Chinook salmon are listed as threatened and endangered, respectively, under the federal Endangered Species Act (NMFS 1999, 2005), while fall–late-fall-run salmon are considered species of concern and are targeted for harvest in the ocean fishery. Fall-run Chinook salmon from the Sacramento–San Joaquin River system form the backbone of California’s ocean salmon fishery, contributing sub-
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