I think the most humane thing to do for a fish that has swallowed a non-retrievable hook is take a blunt object and hit the fish ontop of the head firmly. You have to do this multiple times, or the fish will not be completed "executed."
You probably need to cull fish anyway, so this is a good situation to do so.
Don't let the fish swim around in complete frustration and panic. The best thing to do is put it out of it's misery.
