Fish like tuna, salmon, trout, luderick especially, but any fish will have its table qualities significantly enhanced by bleeding when you kill it. You've got to do it while its alive to allow the fish to die by lack of blood pressure. Its less relevant for many types of fish if you're going to eat them immediately, but it'll significantly improve the table qualities of any fish you freeze or refrigerate for later.
If you brain the fish, you foul the flesh because the fish can't bleed out fully.
I've never caught tuna or 'real' salmon, but I usually kill trout on the spot and cook them on the fire, or sometimes later that day. Maybe I'm not getting the full flavour of my trout, but the idea of bleeding to death doesn't appeal to me, so I don't think I'll bother (and yeah, to be honest, I'm also too lazy to bleed it to death ).
Argh, trout! I must go fishing!
jessb: I think you're absolutely spot on. It's a completely obvious thing which people seem to miss.
Things like this, and stories like Metal_Jazz's (sorry to single you out, you were just most recent) are pretty traumatic to hear about. Do we really get much out of dwelling on the gory details?
Dipcdame: I can't recall talking about cats on this site for ages, and in this case I'm not even really talking about cats, I'm more interested in other issues. The 'we hate cats' record will be played by many people, it will never stop, because some people really do get upset when countless animals are tortured to death because of human ignorance and stupidity. I'd rather see you get over people getting upset about animal torture than people getting over the torture, so in your own words, get over it.