Iron head-
The seem to be just hitch hikers - actually they may be just attaching themselves to fish just as they would any other object (rock, log etc). They feed primarily on the algae etc on the stream bottom.

Have seen leeches on salmon on every NOrth sound river systems that I have spend much time on inlcuding Nooksack, Samish, Skagit, Stillaguamish, and Snohomish systems.

Hoh#er-
the "ring worm" marks and the scale bumps are two other critters. While the fish are edible (make sure to cook them well) they are not very appetizing looking. Not good candidates for sushi!

Tight lines
Smalma