When designing new structures or considering remediation for existing ones, it is important to consider the swimming ability of the fish likely to pass through or over the structure.

This short video shows that small or juvenile fish operate at a very fine resolution not able to be easily measured or observed.

You will see in the video fish riding very subtle pressure waves within complex or nature-like flows that can have a different velocity and/or direction in the same location. 
It also shows fish climbing vertically when the conditions allow.