Controls on the entrainment of juvenile Chinook salmon into large water diversions and estimates of population-level loss

The potential impact on Chinook salmon populations from entrainment of juveniles at the Central Valley Project and State Water Project export pumps has been estimated by examining the number of individuals salvaged at the export facilities in the south Delta. That use of available data has been codified into regulations controlling water-export operations in the 2019 Biological Opinion that addresses impacts from the operations. However, without knowing the number of salmon available to be salvaged, the population-level impact – the percent of the total salmon populations — cannot be determined. Furthermore, there is considerable uncertainty about the individual runs of salmon that appear at the facilities, only certain ones enjoy protected status. In this study, 14 years of coded wire-tag release and salvage data for winter-run, fall-run, and late-fall-run Chinook were analyzed to estimate the population-level impacts from entrainment and from environmental drivers affecting fish that are salvaged.  Coded wire-tag data is optimal for this analysis because the identify of runs and the number of fish released are known, therefore population-level impacts of losses at the export facilities can be estimated. The analysis revealed that average entrainment losses for Sacramento River-origin Chinook salmon, comprise a small fraction of total population mortality, less than 1%, during outmigration. Impacts on winter-run Chinook up to 5% and late-fall-run Chinook up to 8% occurred, but only at the highest water-export levels, whereas fall-run Chinook losses were always less than 1%. San Joaquin River-origin fall-run salmon experienced much higher impacts, up to 17.5%, relative to Sacramento River-origin fish. Salvage increased with water export volumes for fish released in both Sacramento and San Joaquin rivers, but the relationship was not exponential as would be expected if fish were being drawn to the facilities from throughout the Delta. Study results indicate that entrainment losses represent a small fraction of Chinook salmon mortality in the Delta and that the highest loss rates only occur with the highest water-export rates. Manipulating export volumes, especially at export rates below 6000 cfs, is unlikely to be an effective management strategy that can provide substantial survival benefits for Chinook salmon out-migrating through the Delta.

Zeug SC, Cavallo B. 2014. Controls on the entrainment of juvenile Chinook Salmon (Oncorhynchus tshawytscha) into large water diversions and estimates of population-level loss. PLoS One 9(7):e101479.

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