As the old saying goes, "There are a lot of fish in the sea." And that's not to mention the fish found in oceans, gulfs, lakes, rivers, reservoirs, streams, ponds and bayous. Hundreds of different types of fish, both wild and farmed, are eaten around the world. These denizens of both fresh and saltwater cook quickly and are healthful and delicious.

Types of Fish
Fish are available in an amazing range of sizes and shapes. Think of giant tuna weighing hundreds of pounds, and then of tiny anchovies, which can be eaten in one bite. Some fish, known collectively as flatfish, are flat and swim horizontally. The most common examples are flounder, fluke, halibut and sole. Most fish, however, are round bodied and swim vertically.

Nearly all fish are sold already filleted because today's cooks do not want to contend with bones. If you are dealing with a whole fish, you can easily fillet it yourself, or you can ask the fishmonger to fillet it for you. But cooking a fish whole has its advantages, too: Its flesh is especially moist since it is protected from the heat by the skin.

Many fish are lean and, when cooked, have a delicate taste and a soft and flaky texture. Sole, flounder, bass, snapper, skate, cod, perch and whitefish are good examples. Other fish, including swordfish, halibut, tuna, monkfish and mahimahi, have firm, springy flesh. Some fish that are oily and have an especially strong taste are mackerel, bluefish, salmon and sardines. Generally, fish take little time to cook, so you can often count on a quick preparation.

Choosing Fish
Buy fish from a specialized market or supermarket fish counter with good turnover. Fish and shellfish should be displayed on crushed ice or in refrigerated cases with thermometers that clearly display a temperature of 33°F or lower.

It is best to go to the market with no fixed idea of the fish you want and to purchase what looks freshest and most appealing. Most fish are caught year-round, but some have a season, which factors into availability.

Use all your senses to choose fish. In a market in which the fish is packaged, pick it up and inspect it. If it has an off odor, pass it by. Touch the fish if you can; it should feel firm, not flabby. The eyes of a whole fish should be clear, the scales intact and the tail moist. Besides whole fish, you can also purchase steaks, cross-sectional cuts containing a small section of the backbone, or fillets, boneless portions cut from the sides of the fish. Both steaks and fillets should have solid flesh with no gaps.

Shellfish should look wholesome and clean. All bivalves, like clams, mussels and oysters in the shell, must be alive. If they are, their shells will close tightly when you touch them. (The exceptions are cockles and soft-shelled clams, whose shells are always partially open.) Live crabs and lobsters should show signs of life either in their tanks or on ice.

Frozen fish is fine if it has been professionally flash frozen (home freezers are not cold enough to keep fish frozen properly for long). Avoid any that look dry, indicating freezer burn, or that come in packages containing liquid that has frozen, a sign of defrosting and refreezing—and of damage to the fish's texture. Defrost frozen fish slowly in the refrigerator.

Serve 1⁄4 to 1⁄2 pound of fish fillets or steaks per person and about twice that amount when whole.

Storing Fish
Unwrap purchased fish or shellfish, place it on a plate or in a bowl, and then rewrap and refrigerate. Live shellfish, such as oysters, mussels, clams and lobsters, must be kept alive in the refrigerator with their wrappings open so they can breathe.

For the best taste, cook and eat fish or shellfish the day you buy it.