[Old Man Dave]

You can stab a flatfish between the eyes, you can smack it with a 4lb lump hammer, you can gut it and/or fillet it alive, but in my experience non of these will lead to its quick demise. The only way to kill a flatfish is to completely sever the spinal chord just behind the head. For this you need a sharp knife and a very steady hand, especially if you are trying to do this on the surface while treading water.

Almost all flatfishers that I know of don't kill their catch at sea. They just spike them and thread them on a stringer. Gruesome but practical. Unless it's over 20lb (you should be so lucky) they don't even wriggle very much.

If you were on SCUBA and could work on the bottom or a freediver who could reach a nearby rock or climb into a boat then you could maybe do some humane finishing off. However for most people this is not the usual practice and trying to severe the spine of a slippery flatty is going lead to injury to you not the fish.

Most round fish, bass, mullet, pollack etc are easily dispatched using a knife or sharp stringer but much as I'd like to say the same for flats I can't.

If your conscience really troubles you maybe you gotta try to finish them off but personally I'm okay with the pragmatic approach.

Dave