YouTube - How to kill a fish quickly
I had a bad first try, but I didn't miss on the second thrust.
I've seen fish revive way too often after being struck on the head. How can you know if they're merely stunned or not? Drive a knife in their skull and you know they're dead. For Pickerel/walleye I break their neck using my palm on their chin... by the time the nose touches the spine the neck is surely broken.