For whatever it's worth, I think it's a coho, although nothing would surprise me. Like many, I used process of elimination to arrive at my guess.

My thoughts:
- Spots on tail not big enough to be a pink.
- Tail wrist looks too narrow for a chinook.
- Tail does not appear sufficiently forked to denote as chum.
- I have handled very few sockeye, but I seem to recall they have very small scales, and the scales on this fish look medium to large.

Whatever it is, it's a beautiful specimen (at least on the tail end).