Movement behavior of juvenile salmonids in boulder-dominated channels: effects of boulder dispersion, flow, and species

Instream boulder placement is a widely adopted technique for restoring degraded streams to enhance instream complexity, aquatic habitat quality, fish passage, and overall ecological value. However, biological responses to these structures have been evaluated less frequently and with lower consistency as compared to physical/hydraulic habitat assessments. This experimental study examines the impact of instream boulder placement on the behavior of juvenile Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss), focusing on their movements upstream through a flume. The experimental setup featured seven boulder arrangements, including rock-ramp (areal densities from 0.0% to 8.3%) and cluster formations (V- and I-weirs), under two flow rates. Top-down and underwater cameras recordings were analyzed using AI-driven object-tracking algorithms document the movement behavior of the fish. We found that juvenile salmonids in fast flowing water have prolonged resting periods in favorable microhabitats created behind boulders. Atlantic salmon moved upstream most readily with the I-weirs, while rainbow trout did so with the V-upstream weirs. The results revealed strong predictive relationships between dimensionless ground speed and both Froude number and dimensionless swimming speed. Moreover, this study identified strong nonlinear relationships between habitat-scale hydrodynamic complexity metrics and key fish behavioral responses, including resting time, ground speed, passage time, and passage efficiency. The findings of this study enhance our understanding of fish behavior in complex hydraulic environments and offer empirically-based design guidance for more effective, species-specific fish passage and habitat restoration efforts. The study also demonstrates the utility of experiments using live fish in flumes to test fish passage structure designs.