Click on Home or the logo above to navigate to the main California Fisheries Blog webpage.

Merced River Salmon

The Merced River salmon population trends follow a similar pattern to those of other Central Valley rivers (Figure 1). Droughts (76-77, 87-92, 07-09, and 13-15) drive the population down. The basic response appears as a two year lag, reflecting the fact that primary mortality comes in the first year of life while living in rivers and migrating to the ocean. Lack of lag in some years likely reflects poor river conditions in late summer and fall when high mortality of adults may occur during their spawning run. The population increases in normal-wet year sequences (82-86, 95-00, and 10-12). The recent better drought performance with good runs in 2016 and 2017 (not shown) likely reflects the practice of trucking most of the Merced Hatchery smolts to the Bay in spring since 2010.

High trucking survival, especially in dry years, is indicative of the real problem facing Merced, San Joaquin, and Sacramento River salmon: poor river habitat conditions downstream of the hatcheries and upper river spawning grounds. One only has to look at water temperatures and flows in the lower San Joaquin River in winter-spring to see that survival conditions are poor in spring, especially in drier years.

With 2017 being a wet year, Merced Hatchery fall run smolts were released in spring at the hatchery outlet instead of being trucked to Bay pens. Approximately 1,250,000 smolts were released in three groups: 4/24, 5/3, and 5/18 (Figure 2). The problem with these releases even in a wet year like 2017 is warm water in the San Joaquin River below the mouth of the Merced River (Figure 3). In dry years like 2015, water temperature are are even higher and occur earlier in spring, with lethal temperatures (>770F) occurring by late April (Figure 4). This is the reason why the hatchery trucks smolts to the west Delta in dry years.

Looking at the most recent tag return data (Figure 5), it appears that trucking to the Bay or west Delta is the best course of action even in wet years like 2011. Because smolts were released at the hatchery in wet year 2017, a poor return would be expected in 2019. A good return is expected in 2018 because smolts were trucked in 2016. Based on these data, trucking would be the best choice in all years.

Merced hatchery smolts are expected to be released later this spring. DFW should truck these smolts to the west Delta. This is particularly important because since 2011, spring Delta exports have been higher than they generally were over the previous three decades. During the Vernalis Adaptive Management Program (VAMP) from 1999 through 2010, April 15 – May 15 exports were restricted to 1500 cfs. Higher spring export levels since the end of VAMP are a real threat to Merced and other Central Valley salmon populations (Figure 6).

Figure 1. Merced River salmon run (escapement to river) 1975-2016. Source: CDFW GrandTab.

Figure 2. Summary of salmon salvage at south Delta pumping plants in winter-spring 2017. Note three Merced hatchery smolt release groups reached the south Delta salvage facilities in early May and continued to be salvaged into mid June. Only 20% of Merced hatchery smolts were tagged, so many of the non-marked smolts at that time were likely also hatchery smolts. Source: http://www.cbr.washington.edu/sacramento/tmp/deltasalvagelength_1521657439_409.html

Figure 3. Water temperature in the San Joaquin River below the mouth of the Merced River at Crows Landing in spring 2017. Red line is high stress level that would lead to poor growth and survival, and high rates of predation. Green line is upper end of optimal growth potential. Yellow line is moderate stress level.

Figure 4. Water temperature in the San Joaquin River below the mouth of the Merced River at Crows Landing in spring 2015, a critically dry year. Red line is high stress level that would lead to poor growth and survival, and high rates of predation. Green line is upper end of optimal growth potential.

Figure 5. Adult hatchery salmon return percentage from tagged smolts released from 2007 to 2013 at the hatchery or trucked to the west Delta. Data Source: http://www.rmis.org/ .

Figure 6. May 2017 salmon salvage at south Delta export facilities with export cfs. Data source: CDFW.