I wrote about this 25 years ago. I've been following it. Tidal barrages across rivers, like the Annapolis one, have clearly killed fish. They kind of slice and dice them. In-stream holds a lot more potential. It's tough though, because it's really hard to study and monitor. Picking the Bay of Fundy, for example, you have the waters moving at three or four knots.
I think we're going to find a way to develop tidal that will have a relatively small impact, given the overall biomass, on fish.