Swimming behavior of emigrating Chinook Salmon smolts

Abstract

Swimming behavior of Chinook Salmon (Oncorhynchus tshawytscha) smolts affects transit time, route selection and survival in complex aquatic ecosystems. Behavior quantified at the river reach and junction scale is of particular importance for route selection and predator avoidance, though few studies have developed field-based approaches for quantifying swimming behavior of juvenile migratory fishes at this fine spatial scale. Two-dimensional acoustic fish telemetry at a river junction was combined with a three-dimensional hydrodynamic model to estimate in situ emigration swimming behavior of federally-threatened juvenile Chinook salmon smolts. Fish velocity over ground was estimated from telemetry, while the hydrodynamic model supplied simultaneous, colocated water velocities, with swimming velocity defined by the vector difference of the two velocities. Resulting swimming speeds were centered around 2 body lengths/second, and included distinct behaviors of positive rheotaxis, negative rheotaxis, lateral swimming, and passive transport. Lateral movement increased during the day, and positive rheotaxis increased in response to local hydrodynamic velocities. Swim velocity estimates were sensitive to the combination of vertical shear in water velocities and vertical distribution of fish.

Route use of emigrating steelhead in a heavily modified river delta

“…The latter may occur if salmonid smolts tend to favor one side of the channel under high-flow conditions either because of refuge from high current speed or because off-channel rearing habitat becomes available during high flows. The cross-channel fish distribution at upstream locations may persist to downstream river junctions if shoreline conditions are favorable or if high flows deter the lateral movement required to cross the midchannel current (Hance et al 2020;Holleman et al 2022).…”

“…Finally, larger fish were expected to be more likely to remain in the main-stem San Joaquin River route at both junctions because their larger size was expected to enable them to maneuver in faster water flow and to require fewer evasive actions in response to predators and thus be less likely to enter the interior Delta involuntarily (Swanson et al 2004). Time of day was included to account for differences in visual cues and predator evasion actions that might affect fish routing (Holleman et al 2022).…”

“…Covariates were measured at either the (upstream) entrance or the (downstream) exit of the junction, as described below. Although emigrants entered the junctions from upstream, several investigations have observed juvenile salmonids exhibiting behaviors such as holding, foraging, and lateral movement in these junctions, suggesting that some migrants experience conditions at the downstream edge of the junction before their final route fate is determined, whether by active choice or by passive dispersal in response to flow patterns (Anchor QEA 2022;Dodrill et al 2022;Holleman et al 2022). Additionally, neither junction is in a purely riverine region with unidirectional flow, so both downstream and upstream forces are important components of the hydraulic conditions within the junction.…”

“…There is a growing body of literature that relates shortterm fish routing behavior to localized environmental conditions represented by river flow, current speed, and other hydrodynamic features that coincide with (Dodrill et al 2022;Holleman et al 2022) and occur prior to (Goodwin et al 2023) the behavior under investigation. Other factors shown to be important include time of day (TOD; Swanson et al 2004;Anchor QEA 2022), fish cross-sectional location (Hance et al 2020), and fish size (Swanson et al 2004).…”

“…Previous studies on junctions in the south Delta have characterized short-term behavioral actions in response to changing localized hydrodynamic conditions, including reverse flows from tidal fluctuations and water pumping operations (Anchor QEA 2022;Dodrill et al 2022;Holleman et al 2022). However, in a hydrologically complex region like the Delta, fish may experience a given river junction multiple times if they encounter reverse flows, and their preliminary route of entry may differ from…”