Spatial variation of juvenile Chinook Salmon (Oncorhynchus tshawytscha) survival in a modified California river

Spatial variation in habitat structure is a well-recognized characteristic of lotic ecosystems. The resulting patchwork mosaic provides opportunities for growth and survival as well as mortality risks for fish migrating through these systems. Dam construction within a watershed has numerous downstream tail water effects that alter creation and evolution of habitat features used by anadromous fishes that spawn and rear in these reaches. California Central Valley Pacific salmon (Oncorhynchus spp) are largely restricted to areas below impassable dams and these populations are undergoing widespread declines. High mortality of juvenile salmon in these reaches is thought to be a limitation to recovery. We conducted a radio telemetry study to evaluate spatial variation in juvenile Chinook Salmon (O. tschawytscha) survival and survival-environment relationships in three reaches of a Central Valley tail water. Additionally, we used mobile surveys to identify the location of last known detections to elucidate areas of disproportionate mortality. Our results indicated variation in flow and salmon size had greater explanatory power than year effects. However, relationships between flow, salmon size and survival were reach-specific indicating drivers of survival are spatially heterogeneous. Significant clumping of last known detections in the two lower reaches of the river suggests that mortality may be more frequent in these locations. Our results suggest that there is substantial spatial variation in migration mortality and both environmental conditions (flow) and salmon characteristics (size) have a strong influence on observed mortality. This information should be used to target actions that can support Chinook Salmon recovery.