Rheotaxis behavior of brown trout and grayling in the perspective of natural velocity variations

Rheotactic behavior of fish, which is physiologically associated to the animal’s lateral line sensory system, has a strong influence upon fish behaviour in the wild. However, criteria for flows that inform the rheotaxis of native fish species in Central Europe are lacking. Thus, it was the aim of the presented study to investigate rheotaxis of brown trout (Salmo trutta) and grayling (Thymallus thymallus) using a novel experimental setting. Sediment Impact Assessment Flumes (SIAFs) allowed us to determine the orientation of fish at different velocities within the flumes. Because the flumes could be discharged from clockwise and counterclockwise, the threshold criterion for rheotaxis was marked to the active 180° turn of single- or group of fish when the direction of the velocity was changed. A strong rheotactic behaviour was documented above 8 cm s−1, a partial reduction at lower current velocity, with a clear drop off below 2 cm s−1. The findings of this experimental design were related to the natural flow velocity variations in selected riffle-pool reaches during low flow conditions of European rivers. It could be shown, that the flow velocities in pools were beyond the threshold criteria even for small brooks.