I used to salmon fish many years ago and the instructions from the ghillies were always to bury and dead/dying salmon after they had laid, this also applied to the Jacks after fertilisation so I reckon they'd best be left alone
I saw an interesting documentary recently. Don't quote me, but the gist of it was that if conditions are not right, salmon will not lay their eggs and then, when they spawn again the next year this will cause them to die. Some chap was going around squeezing the old eggs out of the salmon so that they would reproduce again because they were beginning to die out in that particular area. WEIRD.
As I understand it, salmon spend all available reserves of bdoy fat and energy in order to reach their spawning ground, and they die of exhaustion having completed their reproductive tasks, so any salmon in this state is likely to be bereft of any worthwile meat, and as previous correspondents have advised, they could be diseased, so all in all, it's probably best to leave them alone.