Is there something they could be used for, are these perhaps the ones they use in cat food? Or can people still eat them after they've changed? Just curious.
No, if the salmon has already died then it is dangerous to consume it, if it is close to death, for instance, having it's last thrashes and gasps then it would also be wise not to consume it. If the salmon looks reasonable healthy after mating then it is still edible but you'd have to be quick because they don't survive long.
Ideally, you're always wiser to get a healthy salmon before mating.