Well in all my years of fishing I have not seen anyhting that says that they do feel pain from the hook.
On many different occassion I have caught fish that have several hooks in their mouths and it doesn't seem to effect their attitude nor willingness to eat. Even fish that have swallowed a hook deep in their stomaches don't seem to be affected by it as far as normal behavior.
When a fish fights the line, it fights because you are making it go a direction it doesn't want to go. Allow a fish to simply take line without reistance and they act normally swimming in normal directions and pattern without anything that indicates they are in pain or distress.
So in short, I don't think they do feel any pain from the hook. The only thing I do to be humane is make sure I kill them as quickly as possible if I am going to keep them to eat. No need to let them suffer and die a slow death.