Just as was said, the fish in his way to spawning grounds will not eat and the only method to take them is flossing. You have to convince the fish and game guy that the fish took you-re hook. If snagged anywhere else you have to let it go. Now, this works only so far up the river. Later on its migratory journey the salmon goes through a lot of changes. The mouth of it gets elongated, the meet takes a white color and is not pink like it was in the ocean. At this point isn't good anymore.
Of course a lot of people still use it, but not those who know.