Spring means the fishmonger at my local farmers market is back in business, and that means easy access to super-fresh local fish for weekend meals. While the best way to enjoy fish is to eat it immediately after buying it, there will inevitably come a time when you have the opportunity to buy fish on, say, Saturday, and you won't be able to use it until Sunday or Monday. Which is exactly what happened to me this last weekend with a few beautiful Boston mackerel I picked up on Saturday morning.
You may think it's enough to just toss your catch on a refrigerator shelf next to yesterday's leftovers, but this actually isn't ideal. Even the freshest fish will noticeably degrade after spending a night at 38°F (3°C), which is about the temperature that most home refrigerators maintain. True, eating fish that's been stored at that temperature overnight won't kill you. But it's not great if you're planning a raw or near-raw preparation, like ceviche, for which you'll want your fish as pristine as possible.
Fortunately, you can easily store fish in your refrigerator at lower temperatures by using ice or ice packs. By laying your fish fillets or cleaned whole fish on top of ice, you can decrease the temperature to about 32°F (0°C), which will help keep fish fresh for up to two or three days. This is why fishmongers always have plenty of crushed ice on hand. (It's also why many of them display their fish on aluminum sheet pans; for the same reason that such pans are really good at defrosting frozen foods, they also keep fish cold by channeling heat energy from the fish to the ice on which the pan rests.)
There are a few additional things to keep in mind as you go about storing your fish. First and foremost, be sure your hands are clean whenever you handle raw fish. Next—and just as important as keeping the fish cold, if not more so—keep your fish flesh dry. The bacteria responsible for fish spoilage thrive in moisture, so the wetter the flesh, the quicker it will degrade.
As a corollary to that rule, you'll also want to keep fish fillets stored in a single layer; stacking or piling the fish pieces together maximizes their exposure to each other's moisture, producing an especially bacteria-friendly environment. Finally, cover your fish well while storing: The air in your refrigerator is extremely dry, and, though you don't want any excess moisture on the surface of your fish, you also don't want it to dry out.
The best way we've found to store fish at home is to rinse the fish fillets and dry them thoroughly with paper towels, then place them in a single layer in a zip-top bag. Press out all the air, lay the bag on top of a plate or tray lined with ice or ice packs, and set more ice or ice packs on top of the zip-top bag. (If you don't have ice packs that neatly fit a tray or plate, you can always ask your fishmonger for a couple of bags of crushed ice along with your purchase, which they'll happily provide.) Then place the entire setup on the bottom shelf of the fridge, all the way in the back.
If you're concerned about all the plastic this process involves, know that you can easily eliminate much of that waste by placing the fish on top of an aluminum sheet tray; nestling that pan in a layer of ice placed in a perforated pan; setting that pan in a third, larger container; and covering the whole shebang up. This method has two drawbacks, namely, the lack of ice on top, and the need for a set of pans that, while common in professional kitchens, aren't often seen in homes. If you purchase fresh fish often, you may want to buy a set of pans for this purpose, making sure they're sized to fit into a reasonable volume of space in your refrigerator.
Whichever method you choose, as long as the fish flesh remains dry and cold and there's no chance of it submerging in ice melt, it'll keep far better than it would if you just threw it into the fridge. So, the next time you spot a nice-looking piece of arctic char or sea bass or swordfish at your fish market, pick it up, even if you know you won't get around to using it for a day or so. You can still look forward to a citrusy aguachile or a crispy pan-seared fillet for dinner.