Plenty of dams do have fish ladders, although Wikipedia suggests that they're not perfect: https://en.wikipedia.org/wiki/Fish_ladder#Effectiveness

They frequently suffer from misaligned incentives, with damns mostly being judged by the existence of a fish ladder, not its effectiveness. And while a stronger current helps fish find the way, that's water that's "lost" for the dam operator.

You can do it - but for the big dams it’s too long/tall.

Arguably you could instead of damming a river divert a portion of it upstream into a new valley for a lake, leaving the old river alone.

There's this super over-engineered fully automatic fish elevator to go up a 20m tall weir [1] (pdf and German warning). Interestingly, there is no safe way for the fish to go back down, yet.

[1] https://www.bkw.ch/fileadmin/bt3_news/MyConvento/2021/10/06/...

Most of the damns up here in the PNW already have fish ladders. They help but they only partly mitigate the issue.

There's a pretty crazy company developing this system that actually shoots fish up the damn like those pneumatic canisters bank drive throughs use (or used?). I just remember seeing a video about it but don't know anything more.

The other problem is the fish need to go downriver too. The dam extracts energy from the water flow and slows down the river - making it difficult for newly hatched salmon to even get to the ocean in the first place.

The dams simply need to be removed, starting with the ones that offer minimal value in other dimensions (electricity, water supply, navigable waters, recreation, etc...) until eventually the vast majority are gone and only a few are left to protect important infrastructure that cannot feasibly be moved.