Relationships between Chinook Salmon Swimming Performance and Water Quality in the San Joaquin River, California

There is currently only a limited understanding of the relationship between water quality and predation on Pacific salmon Oncorhynchus spp. smolts. We addressed the hypothesis that poor water quality will decrease a smolt's swimming performance and presumably its predator evasion capabilities. Predation is a major factor affecting salmon smolt survival throughout the San Joaquin River and the Sacramento–San Joaquin Delta of California. Prior studies have quantified predation rates, but the effect of water quality on predator evasion capability has not previously been evaluated. We quantified the swimming performance of juvenile Chinook Salmon O. tshawytscha in relation to water quality variables. The maximum swim speeds ( U max ) of 45 hatchery‐reared smolts (7.1–9.9 cm FL) were measured in controlled (laboratory) and field environments by using a mobile swim tunnel respirometer; measurements were obtained before and after the fish received a 2‐d exposure to the lower San Joaquin River while being held in flow‐through cages. To sample across a diversity of environmental conditions, we conducted trials during a 6‐week period that coincided with the peak smolt out‐migration. Regression models were constructed to evaluate relationships between swimming performance and four water quality covariates (water temperature, turbidity, dissolved oxygen, and conductivity). We found negative relationships between U max and both temperature and turbidity, and we described these relationships graphically. Our findings suggest that water quality management strategies with the potential to improve salmon smolt survival include managing temperatures and suspended sediment concentrations to optimize the swimming capacity of migrating smolts and possibly improve their ability to evade predators. Received July 12, 2016; accepted December 6, 2016 Published online February 21, 2017