I always stun and bleed the fish I keep. In the boat, we use a five gallon pail to contain the blood. I slit a couple of gill rakers and put the fish, head down, in the bucket to bleed out. This set up is probably too small for fish over ten pounds. Making the slit under the gills sounds like a good idea, I'll have to try it.

A fish processor told me to always hang fish head down, also. He said that when you hang them head up, all the blood and bodily fluids pool in the flesh near the tail. He used cotton cord looped around the tail to hang fish in his cooler. In the morning there would be a puddle of fluids under each fish.