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

Abstract

Survival 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.

Limitations of Active Removal to Manage Predatory Fish Populations

“…), and ultimately these years had the lowest estimated overall survival for hatchery fall‐run Chinook Salmon in the San Joaquin River in over two decades (Buchanan et al. 2018b). If major fluctuations in survival resulted from hostile environmental conditions during the drought, this may have precluded the detection of a predator manipulation effect.…”

“…The two study years had the lowest Chinook Salmon survival rates in the Sacramento–San Joaquin Delta from a multiyear acoustic telemetry study (Buchanan et al. 2018b). These low survival rates are part of a longer‐term declining trend in Chinook Salmon survival in the San Joaquin River (Buchanan et al.…”

“…These low survival rates are part of a longer‐term declining trend in Chinook Salmon survival in the San Joaquin River (Buchanan et al. 2018b), likely in part caused by the recent proliferation of submerged aquatic vegetation in this region and the resulting changes in the fish community (Nobriga et al. ; Conrad et al.…”

“…The survival of juveniles migrating to the ocean through the San Joaquin River drainage is the lowest of any system on the west coast of North America and represents a critical survival bottleneck (Buchanan et al. 2018b). The proximate cause of mortality for most juvenile salmon migrating through the system is hypothesized to be predation by nonnative fish species (Grossman et al.…”

Behavioral Response of Juvenile Chinook Salmon to Surgical Implantation of Micro‐acoustic Transmitters

“…; Buchanan et al. ). Taggable‐sized fish were placed ventral‐side up in a surgery cradle, with a buffered, 30‐mg/L solution of MS‐222 continuously running over the gills to maintain sedation.…”