I've seen third-world fishermen leave their catch in the bottoms of their boats all day long, without any kind of ice or refrigeration. While their constitutions may make them capable of eating fish that has sat in the sun all day, mine don't. Fish spoil very quickly, and the result can be worse than simply a bad meal.
Why Good Fish Go Bad
You'll find bacteria throughout all of nature and especially on the skin and in the slime of living fish, where rather than doing harm, they help protect the fish from disease and many parasites. You can also find bacteria in the intestinal systems of fish (and in human intestines as well). None of these bacteria affect the edibility of the fish. However, very shortly after a fish dies, these bacteria begin to attack the fish flesh, breaking it down into compounds that can be assimilated back into nature and the marine food chain.
Slow the Spoilage Process
Prior to the industrial age, man used salt to chemically retard or prevent spoilage. We are lucky to have a more modern solution - ice. Ideally, the best way to preserve the fish we catch would be to clean and vacuum-bag the meat, then flash freeze it in a matter of seconds. That remains out of the realm of possibility for most recreational anglers with small boats. But well-insulated fish boxes, big bags of ice from the local mini-mart and even ice-making machines like the Eskimo 600 can do the job admirably.
How the Pros Do It
I've had the privilege of tuna fishing with the legendary Murray brothers, who invented many of the fishing methods in use today. And while several of their fish-preservation methods address particular problems with keeping tuna in prime condition, I've discovered that the same steps can, in many instances (where the fisheries laws allow), be applied to most any fish.
Gaff the fish carefully. Aiming for the head or gill area prevents damaging the meat as well as contaminating it with any bacteria on the gaff as it enters the fish.
Stun the fish. Using your fish bat of choice, whack the fish into unconsciousness. This not only makes the next step considerably more humane, but it also helps prevent bruising the meat if the fish flops around on deck or bangs around in the fish box.
Bleed the fish. Insert a knife blade under the gill plate alongside the body and cut outward, severing the gill arches to allow the fish's heart to pump out most of the blood. Some people also make a cut through the artery in the tail. Bleeding allows the fish to cool faster and removes much of the dark, bitter meat along the fish's spine. The Murrays used to hang the fish (attached to the boat with a tail rope) partially out the tuna door so it bled into the sea.
Gut the fish. State or federal regulations sometimes require certain species to be landed whole - with head and fins intact - but unless otherwise specified, you can still gut your catch. Be sure to check your local rules, but if permissible, gut your fish as soon as possible. Gutting actually allows the chilling process to proceed much faster, as the cold can circulate inside the body cavity as well as outside. Remove all of the intestinal tract as well as the gills.
Bury the fish in ice. A fish starts to spoil the very moment it expires. Chilling it represents the single-best way to retard that spoilage. Cold-water fish spoil quickly at temperatures found in your refrigerator (about 42 degrees Fahrenheit).
You need to cool the fish as much as possible without actually freezing the flesh. If you have gutted your fish, fill the abdominal cavity with ice as you place the fish in the box. And sure, burying the fish in ice cubes in the kill box will do the job, but let me share some better methods.
Ice cubes have sharp points and can bruise the meat, making crushed ice a better choice. Crushed ice also makes the storage colder. The more ice you use, the faster your catch will chill. As you stand in the gas station wondering how much ice you need, figure on about 1 pound of ice for every 2 pounds of fish. Of course, in Florida or other tropical places, or if you have a particularly long trip back from the fishing grounds, you'll need more.
The best way: Carry enough ice so you can add seawater to it in the fish box. Ice with water transfers the cold better than just ice, and ice-brine slurry is colder still. (Remember how salt water freezes at a lower temperature than fresh water?) The salt in the brine also helps prevent bacteria from proliferating in the fish box as the liquid bathes the fish.
I love eating fish and am very fussy about the quality of my oceanic table fare. If you care for your fish using the methods here, you will find the quality of your dinners improving dramatically. Oh, by the way: We all want to be conscientious and responsible anglers, and that means taking only the fish we want for our short-term personal use.