As fish pass designers, one of the most important considerations is the swimming ability and behaviours of the fish which we are hoping to help migrate past in-stream barriers. On a recent visit to France to look at examples of fish passes in the south of the country, we were able to observe fish moving upstream at the exit of a fish pass, through a specific viewing window incorporated into the design.
A video of some small roach swimming in the flow passing downstream into the fish pass is below and it clearly shows the fish using different forms of swimming as well as exploiting micro-currents within the water column to progress upstream. Fascinating stuff!