The worst mistreatment of a fish comes by some local northeastern fishermen towards Spiny Dogfish. Apparently in some places, they travel in huge schools,follow boats, attack gamefish, and monopolize hooks (that is, they are junkfish) somehow, people take this as justification to treat them horribly... fishermen kill every Dogfish they catch, hundreds of them. Then there are the guys who club them with baseball bats, cut them up alive, or cut off their tail, fins, even their snout... and then throw them back, so that they suffer to death or get torn apart by other Dogfish.
And now, of course, Spiny Dogfish are threatened.
Strangely enough, if Dogfish are such a problem... I have been saltwater fishing since I was 5 (11 years) and I have only ever seen ONE landed.