Mmmmmm, partially agree. They feel something that tells them not to do whatever they're doing. I don't think a fish getting a hook in it's mouth is equivalent to me getting a crowbar rammed through my jaw. I think they get some type of chemical released that says don't do what you're doing. Probably not much different than the chemical that says, time to spawn, or time to relate to wood, etc. Do they know something is prodding them in the mouth? no doubt. Do they wince in pain the whole time it's there? no way. They know it's there, they know it needs to get out, but I think that's about the extent of it.
-Brian