Do you mean adipose, or one of the ventral fins? Sometimes, do to unexplained acts, a wild fish can lose it's adipose. If it's one of the other fins, well that's a different case (but could be an accident as well by a predator or swimming through an obstruction).
I know there is a good run of resident coho's throughout puget sound. I only fish around Vashon and around all the inlets of Gig Harbor/Key Peninsula. But I know I can catch a coho or SRC almost all year around. They don't get to substantial size as I'll catch over on coastal rivers, but they're still nice fish nonetheless. Luckily, I have access to alot of private beaches, so I get some prime spots most bankers don't see.
Perks of my job. :hehe:
But, depends on which fin was gone, and how the body looked around it. Was there any damage, or was it clean and well healed? I know I've seen fish brought in that had all fins attached and the fishchecker found tags in heads (making it a hatchery). Hard to say reversal though. I guess if you took scale samples and sent to a lab, they could tell you if this fish was a resident or hatchery.
I'm no scientist though. I just love to fish.