Optimum fish passage and guidance designs are based in the hydrogeomorphology of natural rivers

Understanding hydrodynamic cues used by outmigrating juvenile salmon (emigrants) to guide fine‐scale swim path selection is critical to successful fish guidance and passage at man‐made structures. We show how these cues can be inferred from channel features and complex flow fields of natural rivers through which emigrants pass. We then describe a new cue, ‘total hydraulic strain’, integrating properties of flow acceleration and turbulence through the spatial gradients in velocity to create a single flow field distortion metric amenable to the analysis of fish movement at the scale of large man‐made structures. We explain how total hydraulic strain, together with the magnitude of velocity, provide sufficient information for any fish to distinguish between the two categories of channel features with their mechanosensory system. We demonstrate that total hydraulic strain, velocity magnitude and hydrostatic pressure can be integrated into rule‐sets (the Strain–Velocity–Pressure (SVP) Hypothesis) to explain emigrant swim path selection near dams. To confirm the reasonableness of the SVP Hypothesis, we describe how its separate elements can be detected by different components of the fish mechanosensory system. We evaluate the SVP Hypothesis by (1) using it to explain the traces made by acoustically tagged emigrants overlaid on coincident total hydraulic strain and velocity magnitude fields, (2) using it to explain different passage efficiencies of competing bypass designs and (3) testing it via stepwise discriminant analysis to infer the relationship between hydrodynamic pattern and emigrant orientation. We conclude the SVP Hypothesis is a reasonable and useful approximation of the strategy used by emigrants to select their swim path through complex flow fields sufficient to serve as the basis of guidance and bypass system design. Published in 2008 by John Wiley & Sons, Ltd.