Local hydraulics influence habitat selection and swimming behavior in adult California Central Valley Chinook salmon at a large river confluence

Abstract

Migratory habitat selection in adult anadromous salmonids occurs in response to a combination of physical, chemical, and biological cues. Migratory behavioral responses to localized hydraulics are not well understood and hydraulic flow features can be particularly complex at confluence junctions. In some cases, hydraulics may play a partial role in migratory routing, with implications for population structure where wild- and hatchery-origin fish hybridize. This study investigated two questions about such confluences: (1) Can patterns in migratory microhabitat selection or migratory swimming behavior in adult Chinook salmon be attributed to micro-scale hydraulic conditions driven by discharge magnitude and ratio at a confluence? (2) What is the relative influence of selectivity for hydraulic conditions compared to temperature and/or turbidity in micro-scale habitat selection or migratory swimming behavior at a confluence? The fall 2019 migration of California Central Valley Chinook salmon (Oncorhynchus tshawytscha) at the confluence of the Feather and Yuba Rivers in northeastern California served as a testbed. Using two dual-frequency identification sonars, 12 monitoring sites representing distinct physical microhabitats upstream of, within, and downstream of the confluence were repeatedly sampled during two four-day flow periods (mean flow ratios between the Feather and Yuba Rivers were 8.66 and 4.02, respectively). Temperature magnitudes and ratios flipped between these sampling periods. We used a multiple regression analysis using the F test for significance and a corrected Akaike information criteria (AICc) analysis to identify predictors of both detection rate (# individuals/m3/min) and percent occurrence of directed, milling, and backtracking swimming behaviors. A combination of conveyance (m2/s), temperature, and turbidity was found to perform best in predicting detection rate (p < 0.001). No suitable model was found to predict directed behavior. Milling was best predicted by a combination of all hydraulic variables (p < 0.001) and although temperature alone was found to best predict backtracking (p < 0.01), we identified a candidate model including conveyance and temperature as predictors (ΔAICc = 3.66, p = 0.02) which aided in the interpretation of our results. This study provides evidence that channel hydraulics play an active role in the sum of navigation cues that are utilized by migrating adult salmon en route to spawning grounds and should be considered in future investigations of homing and straying patterns in anadromous salmonids.