Abstract
The Central Valley fall-run Chinook salmon (Oncorhynchus tshawytscha) is the dominant population complex supporting the California and Southern Oregon commercial salmon fishery. The stock is largely dominated by hatchery productionand has shown high variability in adult returns, suggesting that hatchery practices are critical to thelong-term sustainability of the fishery. We compiled information from numerous sources to synthesizetrends in the number, location, size, and timingof fall-run Chinook salmon released from the five Central Valley hatcheries between 1946 and 2012. Approximately 2 billion fish were released duringthis period, nearly half of which were released from the single federally operated hatchery. Juveniles have been planted off-site in the estuary with increasing frequency since the early 1980s, particularly by state-operated hatcheries. Approximately 78% of all releases occurred between January and June, including ~25% in April and ~20% in May. Release timing and size trends differed among hatcheries,and were correlated. For example, the Coleman and Nimbus hatcheries tended to release small fish (<5 g, on average) early in the year, while the Feather, Mokelumne, and Merced hatcheries tendedto release larger fish (>10 g, on average) later in the year. Moreover, sizes-at-release (by month) haveincreased since the 1980s, leading to the emergence of a new life-history type that now comprises nearly all of the estuary releases: springtime releases of large ocean-ready “advanced smolts.” We collapsed release timing and size data into a single index of life-history diversity and our results indicate a reduction in juvenile life-history diversity, with decreased variability in release number, timing, and size through time. Together, these results indicate a reduction in the diversity of life-history types represented in the fall-run Chinook salmon hatchery releases, which may be a factor that contributes to the decreased stability of the Central Valley fall-run Chinook salmon stock complex.
Highlights
Habitat loss and large-scale harvest of valuable animals and plants often requires extensive supplementation of artificially-propagated individuals in order to sustain the services they provide to humans (Laikre et al 2010)
To examine temporal trends in release variability, we examined the coefficient of variation (CV) for release number, size, and timing
Using the data compiled from the California Department of Fish and Wildlife (CDFW) annual reports and CDFW and U.S Fish and Wildlife Service (USFWS) databases, we calculated the number of fall-run Chinook produced in Central Valley hatcheries across the years 1946 to 2012
Summary
Habitat loss and large-scale harvest of valuable animals and plants often requires extensive supplementation of artificially-propagated individuals in order to sustain the services they provide to humans (Laikre et al 2010). An example is the post-World War II release of hundreds of billions of hatchery-reared Pacific salmonids across much of their Pacific Rim distribution (Mahnken et al 1998). In the face of major habitat destruction and population declines (Lichatowich 2001), constraints on habitat restoration efforts (Bond and Lake 2003), and human nutritional preferences (FAO 2014), salmonid hatcheries satisfy important societal demands (NRC 1996) while providing a potentially valuable conservation tool (Naish et al 2008). Despite the benefits of fish hatcheries, the longterm sustainability of such technological fixes has been questioned (e.g., Fraser 2008). The debate over the value of fish hatcheries for Pacific salmon is contentious (Ruckelshaus et al 2002), with arguments in both their defense (e.g., Brannon et al 2004) and opposition (e.g., Myers et al 2004)
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