On a recent trip to France, our Technical Director visited a few weirs on some rivers in Brittany, mainly in search of Allis shad. While there, he looked at several fish passes, which have been built predominantly to improve fish passage for salmon.
One of the fish passes, at the bottom of the Blavet river is a pool and slot pass, with large head-drops between the pools and very high energy densities in the pools of the pass. Despite this, the pass apparently still passes some shad upstream and huge numbers of sea lamprey were seen in the area around the fish pass and even moving up through the fish pass. The video can be seen HERE.