Fishbreath. although that is a very sad occurance I am sure you are aware that is extremely rare. There are people who would like you to believe it's common but it's not.

Anyway I strongly recommend not keeping the fish for 2 reasons. The heafty penalty. and because even a dead fish is not a complete waste. There has been a ton of research on the nutrients in streams. All of the studies basically say they there is a lack of dead fish in our rivers. Here in the northwest the salmon is both the top and bottom rung of the food chain. dead salmon decompose and are eaten by everything from insects to crayfish to otters and eagles. Some of these in turn become food for salmon smolts.
It's sad that this fish wont be in the spawning population but it is far from a complete loss.