The issue with fish ladders is not that they kill fishes, actively. The issue is that fish just don't really go back up fish ladders at dam, with the rates estimated as low as 3%. https://en.wikipedia.org/wiki/Fish_ladder#Effectiveness
Another issue with dams and ladders for that matter is that they significantly lengthen the journey time. Additionally protected marine mammals take advantage of the backlog of these blockages and consume significantly more fish as they have no quick escape.
