The American River fall-run Chinook salmon are often referred to as a hatchery run. They are confined to the lower 20 miles of river below Folsom-Nimbus dams and are supplemented by Nimbus Hatchery smolt releases. Adult escapement (run size) is estimated from hatchery counts (Figure 1) and in-river spawning surveys (Figure 2). The run peaked with 100,000+ spawners from 2000-2004, after six wet years (1995-2000) and the initiation of large-scale releases of hatchery smolts to the Bay beginning in 1995 (Figure 3). After the initial success of Bay releases, the total numbers of smolts released dropped from the 8-12 million range to 4-5 million around the year 2000.
Since 2010, more releases have been shifted back to the river. The shift seems appropriate in wetter years like 2010, 2011, 2017, and 2019, but not in drier years like 2012, 2013, 2016, and 2018.1 Adult returns from dry year releases have been 2-to-7 times higher for Bay releases than for river releases. In 2020, a dry year, releases to the river occurred in early May, when downstream water temperatures were above the 68°F/20°C stress limit for juvenile salmon (Figure 4).
Unless winter-spring flows and water temperatures in the American River and Delta are improved,2 and problems with water temperatures during the fall spawning season are fixed,3 wild and hatchery production from the American River will continue to suffer. Until these issues are resolved, continued releases of American River hatchery smolts to the Bay remain necessary to sustain the salmon run.
For more on the American River hatchery program, see http://goldenstatesalmon.org/2020-salmon-update/ and https://www.facebook.com/NimbusHatchery/videos/932316603863844/ .