This isn’t exactly true. Fish ladders and weirs shouldn’t be grouped together like this. Many hatcheries have a weir salmon cannot cross and a ladder as the alternative path the fish take by feeling the flow of water across the ladder and going upstream. The ladders lead to hatcheries where the fish reproduce. And new tiny fish are efficiently raised in protected tanks and later released to go back downstream. In other words, the weir and ladder are a combination to make the hatchery work, and not substitutes for each other. Also ladders can work very well. There are many badly designed ones but the good ones basically let every fish move upstream.