Its a must for all creatures with blood that we are going to eat including all fish as far as I know and critical for warm blooded fish like shad, yellowtail and tuna.
The reason is stress hormones, histamines etc. which can build in the blood due to the suffering before death and this can make the meat tough, tasteless or pap on the one hand or dangerous to some to eat on the otherhand. Someone who has allergy/histamine issues can end up in hospital from eating a yellowtail that has been flapping on the beach for a few hours dying of asphyxiation. We hang the cow to drain the blood so the steak is nice and tender.. Same thing. I kiss my fish goodbye, tell it that its life is not in vain, let it relax and give up, some don't haha, and then break the neck if I can and hit the artery running between the gills. The blood should spray out like an ER show. Leave the fish head down till the blood has drained which takes a minute or 3 and then remove the gills and guts if they come out attached. If the fish are on the bite I gut later but place the fish immediately in a wet bag in the wind and shade and gut them when I get chance before I leave. If the fish dies so you can eat it, one must respect it or at least its flesh.
Shame, my leeries all go back to swim another day, but each to their own. Just please return breeding stock so the rest of us can enjoy their sport fishery.