When the dam was built, they didn’t know they were cutting off the fish from it’s only spawn point in the world (it was discovered later).
Still, there’s an argument to be made for building a bypass or fish ladder in every single new and existing dam regardless of what the current expected impact is.
Studying fish habits when building dams is an ancient practice.
My guess: a political, hasty, rush to success to curry favor and clout drove bad decisions regarding a massive government project. Is that not the way of public bureaucracy, universally?
https://en.wikipedia.org/wiki/Fish_ladder