Since sizzling summer days are prime season for tuna fishing all around Florida, for species including blackfin, yellowfin and skipjacks, let's talk about protecting your investment in a day of offshore fishing in terms of proper tuna prepping and fish care.
Yes, you can safely enjoy sashimi or seared tuna steaks from fish that were fighting on the end of your line a few minutes before. But the delightful flesh will taste even better--shortly and later--if you check the urge for instant gratification.
Usually, you catch tunas in schools and it's pure pandamonium in the cockpit until the action subsides. Once it does, someone--mate or crew or both--should make a slurry of chipped ice and seawater, preferably in a fish box below the deck or in a double-insulated cooler or insulated fish bag. The ratio should be about 2/1 parts ice to seawater with about five pounds of ice per 12 pounds of fish. Here's the official recommendation from the United Nation's Food and Agriculture Organization.
Seawater freezes at a lower temperature than fresh water, so the slurry chills the flesh to optimal levels, and makes the ice last longer.
Next, grab a knife, gut the fish, and slit their gills to "bleed them." Then place the fish belly down in slurry of ice chips and seawater. The slurry will rapidly become bright red. Let the fish bleed out in the slurry for at least half an hour, so that the chemical elements that can make it taste strong and "fishy" pass from the animal's circulatory system, and so that the flesh is chilled through. Cold meat is easier to cut, and melts in your mouth. Make sure to keep it out of the sun.
Rinse the filet(s) in clean seawater--fresh water will water-log it while salt water will preserve its texture. Then slice the chilled, clean filets into neat sashimi slices, using a very sharp knife, or cut into sandwich-sized squares if you've got a grill or burner onboard. If you're down for topside sashimi, mix Wasabi, ginger and soy in a small bowl, and make sure none of the crew mistakes your fingers for fish. You will start a feeding frenzy!
Obviously, leave what you don't eat in the slurry, adding ice and seawater occasionally throughout the day, and adding both as the box fills up with fish. Remember that as the ice melts the water will lose salt content, so keep adding seawater so that you're fish don't get washed out.
Care must continue when the fish leaves the fishbox. Whether you're running your own boat, riding along on friend's vessel, or hiring a charter, make sure you have a cooler to keep the fish cold while transporting the fish from the dock to where you're staying, and to where you're planning on eating it.
Tuna will keep for several days in the fridge if you keep it covered. It's best to make an estimate of what you can eat within 72 hours and freeze the rest immediately. A vacuum sealer is a wise investment. It prevents freezer burn and preserves moisture in the fish.