Do you know of inedible fish? I'm not aware of any in the UK. The blow fish comes to mind for tropical waters - but the Japanese eat them after v. careful preparation to remove the poisonous parts. The prickly looking & venomous French rascasse /scorpion fish is commonly used in Bouillabaisse.
I guess the other concern is not to target rare/endangered fish species (although it seem like all/most fish stocks are under severe pressure from massive commercial overfishing worldwide now
) or under size fish - sea species often have Minimum Landing Sizes these days. Or dangerous species.