Coming from the pacific northwest and being a salmon fisherman, you really have to be concerned about the blood inside of the salmon fouling the taste of the meat.
I'm sure of a humane way to kill anything, and I'm almost positive the way we bleed out fish uphere has to be one of the most inhumane ways of killing a fish.
However, its a source of food and to get the best taste from your catch you want to bleed out a fish immediatly after it has been caught and you decide to keep it.
to bleed out the fish you do not want to kill it first, the whole process of bleeding out a fish is done by the fishes heart.
you can bleed out (without killing it immediatly) a fish two diffrent ways,
insert a filet knife BEHIND the gills and slice the main artery of the fish that runs along the spine. letting the heart pump all of the blood out of the body.
or you can cut a vertical slice right behind one of the gill plates deep enought until blood begins to come out of the wound, again letting the heart pump out all the blood.
Don't think bleeding out a fish is a humane way of killing the animal its far from it. It just happens to be a widley used practice to preserve the flavor of the meat.
the most humane way of killing a fish that I can think of is to play catch and release then let it get eatin by something else.
My suggestion to you would be as follows.
Focus less on the humane style of killing the fish, and more on preserving the fishes flavor.
That way when its time for dinner the fish tastes so good there won't be anything left of it and the little guy didn't get killed for nothing.
catch what you use, not what you can.