The Relationship Between Environmental Variables and Migration Behavior of Adult Salmon Returning to the Stanislaus River, CA

Pacific salmon have suffered dramatic declines in both abundance and diversity across the full extent of their range, from southern Japan to southern California, and multiple salmon evolutionarily significant units (ESUs) have been listed under the Federal Endangered Species Act. Chinook salmon (Oncorhynchus tshawytscha) in the Sacramento-San Joaquin Delta are no exception; two Chinook salmon ESUs (Central Valley spring-run and Sacramento River winter-run) are currently protected by federal legislation, and the Central Valley fall/late fall-run Chinook ESU is designated as a species of special concern. Using over seven years of fish passage data recorded using a portable weir and Vaki RiverWatcher digital infrared fish counting system, this study examines the effects of environmental variables on the migration timing of fall-run Chinook salmon adults returning through the San Joaquin River to its Stanislaus River tributary. We consider environmental variables such as water flow, water temperature, dissolved oxygen, and turbidity. The San Joaquin River is highly managed for diverse purposes including agricultural irrigation, flood control, municipal water use, recreation, and multiple fisheries—the results of this study will provide valuable information for managing salmon populations in California’s Central Valley, and for understanding the broader impacts of environmental variation on Chinook salmon migration behavior.