What happens to Steelhead when they reach a mature age that makes them so sensitive to having the protective slime removed from being touched by anglers?

When a smolt gets the Adipose clipped it must get some of the slime removed from handling. Not to mention having the adipose clipped off and raw flesh exposed to the elements.

I believe what I have read from Biologists about the removal of protective slime being harmful to the fish but am curious as why it does not seem to affect the smolts?