In a post last year I remarked on the progressive management of the Mokelumne River hatchery. The hatchery is a mitigation hatchery operated by the California Department of Fish and Wildlife in partnership with the East Bay Municipal Utility District. Its activities over the past several decades have led to the recovery of Mokelumne River salmon.

The hatchery’s 6 million smolt releases continue to survive well under present management practices (Figures 1 and 2). Escapement of adults to the river and hatchery below Camanche Dam have numbered from 10,000 to 20,000 in recent years,¹ a remarkable improvement given that they were produced during the 2012-2016 drought period.

The hatchery is still learning under its adaptive management program. The program has a diversity of release strategies that provide valuable information about the program and what works and what does not. Table 1 shows a summary of their smolt releases from 2016.

Table 1.  Summary of Mokelumne Hatchery smolt releases in spring 2016.  Source:  https://www.rmpc.org

Returns from the 2015 spawn year (brood year 2015) from code-wire-tagged smolt groups released in below-normal/dry water year 2016 ranged from near zero to two percent (Figure 3). The half-million smolts released to the river had near zero returns to fisheries and river/hatchery, while the half-million releases to Half Moon Bay on the coast south of San Francisco had a good return of 2.2 percent. Returns from the remaining 5.5 million smolts released at Sherman Island in the west Delta ranged from 0.1 to 1.4 percent.

The wide-ranging survival rates provide clear guidance for future hatchery smolt-release management.

• April releases do not fare as well as May releases, suggesting that the larger size of May smolts provides a survival advantage.
• River releases in a below normal or dry year like 2016 have poor survival, especially with below average freshwater flows in the lower San Joaquin River (Figure 4) and low Delta outflow (Figure 5).
• Releases at Sherman Island in the west Delta in early June had poor survival compared to May releases, coincident with rapidly falling flows (Figures 4 and 5) and rising water temperatures (Figure 6).

2017 and 2018 release strategies were similar, but weighted more heavily toward Bay and ocean releases.

• Over one million smolts were released on the coast and at the Golden Gate in 2017; 730,000 in 2018.
• Most of the 4 million smolts released at Sherman Island in 2017 were released in May.
• Over 4 million smolts were released at Sherman Island in 2018 from mid-April to early June over a broad range of conditions.
• Nearly 500,000 smolts were released in the Mokelumne River in 2017 and 2018. The 2017 smolts were released in June of a wet cool year, while the 2018 releases were in May of a drier, warmer year.²

If and when information on the tag returns from 2017 and 2018 releases becomes available, it may be possible to move toward a still more effective release strategy that increases smolt survival and contributions to fisheries and river escapement.

Figure 1. Escapement estimates to the Mokelumne Hatchery 1964-2018

Figure 2. Escapement estimates to the Mokelumne River 1952-2018.

Figure 3. Percent return of 2016 Mokelumne Hatchery tagged smolt releases. Source: https://www.rmpc.org

Figure 4. Inflow of San Joaquin River to the Delta in May 2016 along with long-term median flow.

Figure 5. Delta outflow in spring 2016.

Figure 6. Water temperature in lower San Joaquin River at Jersey Point in spring 2016.

In a January update on late-fall-run salmon, I noted record low runs of late-fall-run Chinook salmon to the upper Sacramento River in 2015 and 2016. That trend continued in 2017 (Figure 1), as shown in recent data published by the California Department of Fish and Wildlife.  Spawning runs from 2015-2017, the product of runs from the 2013-2015 drought, were three lowest since year 2000.

As it was for spawning run 2013, the rate of return for 2014 spawning run of tagged late-fall-run hatchery smolts was very low (Figure 2).  I concluded in the prior post that low river flows and lack of flow pulses caused the poor return rate.  There were flow pulses in winter 2014-15 (Figure 3).  However, only the later season release group responded strongly favorably to a flow pulse (Figure 4).  This  suggests that later winter releases of fish from the hatchery, in combination with flow pulses, provides higher rates of returns for hatchery fish.  Prescribed flow pulses from Shasta Reservoir in dry winters like 2014-15 (Figure 5) appear to improve survival of late-fall-run hatchery smolts.

Figure 1. Spawner-recruit relationship for late-fall–run salmon. Number is log10-3.5 transformed escapement (recruits) for the fall of that year. For example, year “16” represents escapement for late fall 2016, which includes spawners from early winter 2017. Spawners represent escapement from three years earlier (brood year). In the example, spawners for year 16 were the progeny of escapement in 2013. Colors represent winter rearing condition two years earlier. In the example, red “16” represents dry winter 2014. Green represents normal years two years earlier. Blue represents wet years two years earlier.

Figure 2. Hatchery smolt survival for brood years 2008-2013 based on coded-wire-tag returns.

Figure 3. Winter 2014-15 Sacramento River flows downstream of Battle Creek along with 50-yr average.

Figure 4. Return rates by date of release for late-fall-run tagged hatchery smolt release groups from winter 2014-15. Source: https://www.rmpc.org

Figure 5. Winter 2014-15 Sacramento River flows downstream of Shasta Dam along with 50-yr average. Note short term reservoir water releases in December and February prescribed to support upper river hatchery smolt outmigration.