Investigating physical drivers of straying behavior in California Central Valley fall-run Chinook salmon (Oncorhynchus tshawytscha) through ecohydraulic analysis

Abstract:

Pacific salmon (Oncorhynchus spp.) have a complex life cycle and many life stages are subject to altered geomorphic and hydrologic conditions in regulated rivers throughout their range. “Straying” is a behavioral phenomenon in which spawning adult salmon migrate to non-natal freshwater habitats, and stray rates have been shown to occur far beyond anticipated baseline levels in regulated watersheds with high rates of hatchery production. In these systems, the physical conditions present at tributary junctions may have important implications for upstream navigation. We seek to address the following questions: 1) Can we identify patterns of migratory habitat selection within a large river confluence that are driven by a) flow conditions, b) hydraulic flow features, or c) fine-scale physical habitat attributes? 2) Are there consistent behavioral responses to physical habitat conditions at a large river confluence? This study examines the confluence of the Feather (FR) and Yuba (YR) Rivers in northeastern California to investigate physical migratory cues and preferred migratory microhabitat (1m²-scale) for CA Central Valley fall-run Chinook salmon. Adult escapement data on the YR from 2004 to 2011 show that straying of adult FR hatchery-origin salmon increased with higher flow and lower temperature in the YR, suggesting the presence of hydraulic and thermal influences on navigational cues. To characterize migratory behavior, we utilized duel-frequency identification sonar (DIDSON) to observe adult salmon passing the confluence in 2019. 12 DIDSON monitoring sites in the immediate FR/YR confluence area representing varying microhabitats were sampled under two different 4-day flow conditions during the fall-run Chinook migratory period in September and October (mean FR:YR flow ratios were 8.66 and 4.02, respectively). Individual adult salmon detections are geo-referenced and compared to measured temperatures as well as modeled depths and velocities using the 2D hydrodynamic model Tuflow HPC. A habitat selection model is generated to characterize microhabitat preference as a function of flow-driven hydraulics and physical habitat conditions at the confluence. The results have important implications for anadromous fish conservation planning and flow management in regulated river systems.