Couple other features that seal the deal as chum...

1) Sockeye have no dark markings in the mouth or gumline. The mouth is white or perhaps a faint washed out gray. Look carefully at the fine linear black marks on both the upper and lower gumlines. (Yeah, I know, I should have taken a better open mouth close-up.)

2) And finally as others have already noted is how they cut. Sockeye have the most vibrant candy red flesh you'll ever see on a salmon. This fish clearly did not.