h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The methods given in the preceeding "Gutting Fish" are crude and generally wasteful. Firstly breaking a fishes neck is a difficult and inefficient way to try killing fish. It will just stress the poor thing further, and stress increases lactate and enzyme levels which accelerate spoilage.
The quickest and most humane way to kill fish is to destroy the brain. This can be done in smaller fish (<10kg) by striking firmly with a suitable hard object right behind a line between the eyes, often a slight soft depression indicates the spot. On larger fish, an ice pick or similar pointed tool is best inserted at about a 45degree angle to the backbone, at the same spot just behind the eyes. This will effectively kill the fish anmd also minimize it's thrashing(bruises aren't good eating).
Almost all fish should be immediately bled apon landing, simply cutting a gill arch suffices.
Picking up most fish by the tail will also diminish the quality, as will any rough handling.
Quite simply; stun or kill, bleed, gut and cool as rapidly as you can.