All fish are broken down into two very broad categories fish and shellfish. In the most basic terms, fish are equipped with fins, backbones and gills, while shellfish have shells of one form or another. Fish without shells are separated into two groups freshwater fish and saltwater fish. Because salt water provides more buoyancy than fresh water, salt-water fish such as cod, flounder and tuna can afford to have thicker bones. Freshwater fish like catfish, perch and trout can't be weighted with a heavy skeletal framework. Instead, their structure is based on hundreds of minuscule bones, a source of frustration to many diners. Additionally, fish are separated into two more categories: flatfish and roundfish. Flatfish, which swim horizontally along the bottom of the sea, are shaped like an oval platter, the top side being dark and the bottom white. Both eyes are on the side of the body facing upward. Roundfish have a rounder body, with eyes on both sides of the head. Further, fish are divided into three categories based on their fat content lean, moderate-fat and high-fat. The oil in lean fish is concentrated in the liver, rather than being distributed through the flesh. Their fat content is less than 2½ percent and the flesh is mild and lightly colored. Fish in the lean category include , brook trout, cod, drum flounder, haddock, hake, halibut, pol-lack, red snapper, rockfish and tilefishModerate-fat fish usually have less than 6 percent fat and include barracuda, striped bass, swordfish, tuna and whiting. The fat content of can reach as high as 30 percent (as with eel), but the average is closer to 12 percent. Some of the more popular high-fat fish are Atlantic herring, butterfish, mackerel, smelt, sturgeon and drawn, meaning its entrails and sometimes its gills have been removed. A fish that has been dressed has, in addition to being drawn, had the scales removed. Whole-dressed usually refers to the whole fish; pan-dressed to a fish with head, tail and fins removed. Fish fillets and steaks should have a fresh odor, firm texture and moist appearance. Fillets are a boneless, lengthwise cut from the sides of a fish. They are usually single pieces, though butterfly fillets (both sides of the fish connected by the uncut strip of skin on the belly) are also available. Fish steaks are cross-sectional cuts from large, dressed fish. They're usually 5⁄8 to 1 inch thick and contain a small section of the backbone. Fresh fish should immediately be refrigerated, tightly wrapped, and used within a day 2 days at most. Never store ungutted fish, as the entrails decay much more rapidly than the flesh. When purchasing raw frozen fish, make sure it's solidly frozen. It should be tightly wrapped in an undamaged, moisture- and vaporproof material and should have no odor. Any white, dark, icy or dry spots indicate damage through drying or deterioration. Avoid fish that is suspected of having been thawed and refrozen, a process that reduces the overall quality of both texture and flavor. Frozen fish should be stored in a moisture- and vaporproof wrapping in the freezer for up to 6 months. Thaw in the refrigerator 24 hours (for a 1-pound package) before cooking. Quick-thawing can be accomplished by placing the wrapped, frozen fish in cold water, allowing 1 hour to thaw a 1-pound package. Never refreeze fish. Canned fish, such as tuna, salmon and sardines, will generally keep for about a year stored at 65°F or less. However, since the consumer doesn't know under what conditions canned goods have been stored in warehouses, the best idea is to buy only what will be used within a few months. Fish are an excellent source of protein, B complex vitamins and minerals including calcium, iron, potassium and phosphorus. Both saltwater and freshwater fish are low in sodium content and, compared to meat, also low in calories. Cooking fish: Fish can be cooked in myriad ways including baking, broiling, frying, grilling and steaming. A general rule for cooking fish is to measure it at its thickest point, then cook 8 to 10 minutes per inch (4 to 5 minutes per half inch). To test fish for doneness, use a fork to prod it at its thickest point. The fish should be opaque, its juices milky white. Undercooked fish is transluscent, its juices clear and watery; overcooked fish is dry and falls apart easily. Another test is to insert an instant-read thermometer at the thickest pointfish that's done will register 145°F. For further questions, call the free government-sponsored fish and shellfish hotline at 800-332-4010.