In an April 2017 post, I related factors likely important to late-fall-run Chinook salmon in the reach of the Sacramento River downstream of Shasta Reservoir. In this post, I update that assessment with 2015 and 2016 escapement estimates and coded-wire-tag return data from Coleman Hatchery smolt releases from brood years 2008-2013.
Late-fall-run Chinook salmon escapement reached new lows in 2015 and 2016 (Figure 1). The adult returns in these two years were the product of spawning in brood years 2012 and 2013 and of early rearing conditions in winter of 2013 and 2014 (critical drought years).
The low escapement in 2015 and 2016 is also reflected in the spawner-recruit relationship (Figure 2). There is a continuing significant positive spawner-to-recruit relationship and even stronger effect of water-year type, with poorer recruitment from dry-year winter rearing conditions.
The low 2016 escapement is likely in part a consequence of a very poor return from brood year 2013 hatchery smolts (Figure 3). Of the approximately one million smolts tagged and released in 2014 at the Coleman hatchery near Redding, less than a tenth of a percent survived to be counted in fisheries and escapement surveys. A good survival rate would be 1 to 3 percent, as occurred for brood year 2010 (2013 run).
Hatchery and wild smolts from brood year 2013 had poor flow conditions in early winter of 2014 (Figure 4), while brood year 2010 had the best flow conditions. There were no flow pulses to help the smolts move the 200 miles down to the Bay-Delta in January 2014.
The fact that few late-fall smolts showed up in south Delta salvage in 2014 (Figure 5), compared to higher salvage in 2013 (Figure 6), is compelling evidence that smolt survival to the Delta was very poor in 2014.
Conclusion: To sustain the late–fall-run salmon population, higher winter flows and flow pulses are warranted in the lower Sacramento River in drier years.