Survival of Juvenile Fall‐Run Chinook Salmon through the San Joaquin River Delta, California, 2010–2015

Abstract

AbstractSurvival of juvenile fall‐run Chinook Salmon Oncorhynchus tshawytscha through the San Joaquin River Delta of California (hereafter, “Delta”) has been low for most estimates since 2002 and has been consistently low since 2010. From 2010 through 2015, annual estimates of the probability of surviving through the Delta (from Mossdale to Chipps Island, approximately 92 river kilometers) ranged from 0 to 0.05, based on acoustic‐telemetry data from smolt‐sized hatchery Chinook Salmon. River conditions were poor in most of these years; average daily river discharge into the Delta from the San Joaquin River was <40 m3/s in four of the six study years. In the high flow year of 2011 (average daily river discharge = 278–308 m3/s), the juvenile survival probability through the Delta was estimated at only 0.02 (SE < 0.01), suggesting increased flows alone will not be sufficient to resolve the low survival through the Delta. The low survival in this short portion of the salmon's life history makes achieving a minimal smolt‐to‐adult ratio of ≥2% nearly impossible for this fish stock. Over half of the fish surviving through the Delta during 6 years of study were salvaged at the Central Valley Project's water export facility and transported for release just upstream of Chipps Island.