Abstract
Anthropogenic structures in rivers are major threats for fish migration and effective mitigation is
imperative given the worldwide expansion of such structures. Fish behaviour is strongly influenced by
hydrodynamics, but little is known on the relation between hydraulics and fish fine scale-movement.
We combined 3D Computational fluid dynamics modelling (CFD) with 2D and 3D fish positioning to
investigate the relation between hydrodynamics and the downstream movement of Atlantic salmon
smolts (Salmo salar). We show that fish use fine-scale flow velocity and turbulence as navigation cues
of fine-scale movement behaviour. Tri-dimensional swimming speed and swimming direction can be
explained by adjustments of fish to flow motion, which were linked to fish swimming mode. Fish
diverge from the flow by swimming at speeds within or higher than their prolonged speeds (0.38-0.73
m s-1). Flow direction played a pivotal role on fish swimming performance, with high upstream and
downwards velocities impacting swimming the most. Turbulence was also influential, by benefiting
swimming performance at low TKE (< 0.03 m2 s-2) or constraining it at higher levels. We show that
fish behaviour is affected by interactions of several hydraulic variables that should be considered
jointly.
imperative given the worldwide expansion of such structures. Fish behaviour is strongly influenced by
hydrodynamics, but little is known on the relation between hydraulics and fish fine scale-movement.
We combined 3D Computational fluid dynamics modelling (CFD) with 2D and 3D fish positioning to
investigate the relation between hydrodynamics and the downstream movement of Atlantic salmon
smolts (Salmo salar). We show that fish use fine-scale flow velocity and turbulence as navigation cues
of fine-scale movement behaviour. Tri-dimensional swimming speed and swimming direction can be
explained by adjustments of fish to flow motion, which were linked to fish swimming mode. Fish
diverge from the flow by swimming at speeds within or higher than their prolonged speeds (0.38-0.73
m s-1). Flow direction played a pivotal role on fish swimming performance, with high upstream and
downwards velocities impacting swimming the most. Turbulence was also influential, by benefiting
swimming performance at low TKE (< 0.03 m2 s-2) or constraining it at higher levels. We show that
fish behaviour is affected by interactions of several hydraulic variables that should be considered
jointly.
