The adipose fin does have a function, when a fish makes a fast acceleration, in speed or change of direction, that little fin is doing a job. That being said, you need to look at the costs to benefits of any "marking" strategy. How else can you externally mark a fry/smolt so you can easily identify it as an adult? They have tried some branding techniques but the adipose fin clip is pretty easy. Sure there is an associated mortality throughout their lifespan, but frankly, so what? As far as the handling and slime removal thing goes, when you are marking fry/fingerlings/smolts your are handling growing critters with, presumibly, healthy biological defense mechanisms. Returning spawners biological systems as they enter the river are placing priority on producing eggs/sperm and reserving energy for travel and spawning which is a big draw on their anti-inflamatory response. If you remove their slime they have little energy to replace it. See Sparky's Law!