When you've decided to harvest a fish for food, it makes good sense to kill it quickly. Humanitarian concerns aside, fish that die lingering deaths just don't taste as good. Stress hormones (mainly cortisol) and lactate are released into the bloodstream when a fish is flopping around on deck or in the cooler. These compounds have a bitter, even astringent, taste that can definitely affect the flavor of the fillet on your plate.
Of course, these chemicals are produced during the fight as well, so a fish comes aboard with them flowing through its tissues. To ensure your fish are going to be as delicious as possible, you've got to get them out of the fish's meat. There are two ways to do that: Bleed the fish, or kill it with supercooled water.
Many anglers routinely bleed fish that are known for having a stronger flavor or darker meat, such as bluefish and mackerel. But even fish that are generally regarded as being mild and white-fleshed will still taste better after bleeding. Grouper and sheepshead are not particularly fishy-flavored, but they will be even sweeter if you get the blood out of the meat.
The first method is direct bleeding. The idea is to sever a major blood vessel while the fish is still alive. The beating heart will do the work of exsanguinating the fish. This works only if the fish is still in swim-away condition, so it should be done as soon as the fish is in hand.
Bleeding fish out is messy, so think about where you want that mess to go. On the deck is an option, but you better have a washdown hose at the ready, or dip up a couple bucketfuls of seawater to rinse it off. Don't let it sit; dried blood is one of the worst messes to clean up. You can bleed the fish over the gunwale or stern, but you'd better have a damn good grip on it.
The best option is often to bleed the fish into the livewell. Your overflow and raw water pump will take care of clearing the water quickly. Yes, you'll be pumping a lot of blood into the water, and that blood may attract sharks or other pelagic predators. That can be good or bad — it's all about perspective.
If you can't take the blood, then slush 'em. Fill your cooler or fishbox about two-thirds with ice, then add clean seawater just until it's slushy enough you can easily put your hand in to the bottom. Don't leave it there — that water is really cold.
When you add a fish to the cooler, immerse it all the way into the slush. Suddenly surrounded by water 30 to 60 degrees colder than it was in a few second before will quickly put the fish into shock. The involuntary body response is similar to hypothermia in humans. Most of the blood will be drawn to the internal organs and away from the skin and muscles. The fish dies faster, with most of the blood pulled out of the parts you're going to eat.
Other methods that can be used to kill fish fast are alcohol on the gills (a spray bottle of vodka works well) or thumping the head with a billy club. However, both of these leave lots of blood in the meat. If you use the alcohol or club to knock the fish silly and then quickly bleed it, you can get the best of both worlds.