All fish are broken down into two very broad categories – fish and shellfish.
In the most basic terms, fish have fins, backbones, and gills. Shellfish have shells. (Pretty basic!). Fish can further be broken down into categorizes based on the water in which they live, their skeletal structure, and their fat content.
Freshwater or Saltwater Fish
Fish without shells fall into two categories – freshwater or saltwater. Because saltwater provides more buoyancy than fresh water, saltwater fish (cod, flounder, tuna) can afford to have thicker bones. Freshwater fish (catfish, perch, trout) can’t be weighted with a heavy skeletal framework. Instead, their structure is based on hundreds of tiny bones. And, these tiny bones can be frustrating to many diners.
Skeletal structure categorization
Flatfish swim horizontally along the bottom of the sea, are shaped like an oval platter, the top side being dark and the bottom white (or counter shading – a form of camouflaging). Both eyes are on the side of the body that faces upward. They have a backbone that runs through the center of the fish. There are four fillets, two upper and two lower. These include flounder, sole, and halibut.
Roundfish have a rounder body, with eyes on both sides of their head. They have a backbone along the upper edge, with two fillets on either side of the backbone. These types include trout, bass, perch, and salmon.
There are also cartilaginous fish such as ray, skate, shark, and monkfish, which have no bones, but instead have only cartilage.
Fat content categorization
Fish can also be categorized by their fat content – lean, moderate, and high-fat (or oily). Often recipes and chefs will pair a fish with a cooking technique based on the fat content of the flesh. Oily fish are generally prepared by dry-heat techniques like grilling or broiling. Moderate fat fish work well with any technique, with the possible exception of deep-fat frying. And, lean-fish are often poached, sautéed, pan-fried, or deep-fried.
The oil in lean-fish is concentrated in the liver, rather than throughout the flesh. Their fat content is generally less than 2 ½ percent, and the flesh is mild and lightly colored. Lean fish include black sea bass, brook trout, cod, croaker, flounder, haddock, hake, halibut, Pollack, ocean perch, red snapper, rockfish, smelt, sturgeon, and tilefish.
Moderate-fat fish usually have less than 6 percent fat and include barracuda, bluefish, striped bass, swordfish, tuna, and whiting. The wider distribution of fat in moderate and high fat fish gives their flesh a darker color, firmer texture, and more distinctive flavor.
The fat content of high-fat fish can reach as high as 30 % (as with eel), but the average is closer to 12%. These include herring, mackerel, rainbow trout, salmon, and yellowtail.
Whole fish come either ungutted or drawn (meaning its guts and sometimes its gills have been removed, but its head, tail, and fins are still intact). A fish that’s dressed has, in addition to being drawn, had the scales removed; the head and tail may be removed from a dressed fish as well. Whole-dressed usually refers to a whole fish; pan-dressed refers to a fish with head, tail, and fins removed.
Fresh fish should immediately be refrigerated, tightly wrapped, and used within a day to 2 days at the most. Never store ungutted fish, as the entrails decay much more rapidly than the flesh. Make sure your whole fish is nestled in crushed ice at the fish counter. Take a look around and check whether the surrounding area is clean. You shouldn’t see any bits of fish parts, liquids or blood pooling lying about. Note the smell. A fishy smell indicates that you need to walk away. A faint salty or seaweed aroma is perfectly acceptable. And don’t be afraid to ask the monger for a sniff of the fish if you are unsure about its quality. Trust your instincts, good fish looks good, smells like the sea, and has firm unmarred flesh.
If you buy fresh whole fish at the store, you’ll want to look for:
- Eyes: should be bright, clear, and full. Avoid cloudy or sunken eyes as this generally indicates the fish is stale.
- Skin: Look for shiny, brightly colored skin; it shouldn’t look dry or have scales coming away. It shouldn’t have spots of pink (which indicate bruising) or brown spots (which indicate spoilage). Look for firm flesh that clings tightly to the bones and springs back when pressed with your finger. You want the fish to have a moist, shiny appearance.
- Odor: It should have a fresh, mild, clean, and briny aroma, like the sea. Very strong odors are an indication that the fish is aging or was improperly handled.
- Gills: should be red to bright pink, and free of slime or residue.
Fillets are a boneless, lengthwise cut from the sides of a fish. The skin may or may not be removed before cooking.
Fish steaks are cross-section cuts from large, dressed fish, such as tuna, salmon, or halibut. They’re usually 5/8 to 1-inch thick, and contain a section of the backbone. The skin is generally not removed.
Fish fillets and steaks should also have a fresh, not fishy, odor, firm texture, and moist appearance. Fillets should have an even color tone and not have any dark or dry spots.
When purchasing raw frozen fish, make sure it’s solidly frozen with no signs of freezer burn (white frost on its edges). It should be tightly wrapped in an undamaged, moisture and vapor-proof material, and should have no odor. Any white, dark, icy, or dry spots indicate damage through drying or deterioration. Avoid fish that’s suspected of having been thawed and refrozen, as this reduces the overall quality of texture and flavor. If you see signs of damage or age, the fish will most likely have lost so much moisture that its taste and texture will be markedly inferior.
The home refrigerator is not generally cold enough to store fish. Fish keeps almost twice as well at 32° or 33° (its temperature when buried in ice) as it does at 40°F (the temps of most home ‘fridges). You can fill a baking pan or the vegetable bin with ice and bury your fish in there. It will maintain the quality longer, but honestly, who’s going to do that at home? So, instead, I recommend using the fish the same day it is purchased, or using a frozen fish.
If you purchased a fresh fish and your plans change and you can’t cook it that evening, freeze it. Home freezing of fish is very practical, as long as you understand that it’s short-term. For the most part, frozen fish doesn’t keep as well as frozen veggies or frozen meat. Home frozen fish should be used in a month or two. To freeze, wrap the cleaned and rinsed fish tightly in plastic wrap, then wrap again. Place in your freezer at a temperature of 0° or lower.
Pre-frozen fish can be stored, tightly wrapped, up to 6 months (because this fish was wrapped and frozen more quickly, and it had less time being exposed to the air, than the home-frozen fish, it can be stored longer). Thaw the frozen fish in the refrigerator 24 hours before cooking. Quick-thawing can be accomplished by placing the wrapped, frozen fish in cold water to cover, changing the water often, and, allowing 1 hour to thaw. Never refreeze fish. Cook it as soon as possible.
Canned fish, such as tuna, salmon, and sardines, will generally keep for about a year stored at 65° or less.
Shellfish are divided into categories based on their skeletal structure as well:
- Univalves or single shelled mollusks like abalones and urchins
- Bivalves or two shells joined by a hinge such as clams, mussels, oysters, and scallops
- Crustaceans have jointed exterior skeletons or shells such as lobsters, shrimp, and crayfish
- Cephalopods have tentacles attached to their head like squid and octopus. (And, yes in the culinary world, these are often categorized in the shellfish category despite the lack of a shell.)
It’s pretty easy to tell if lobsters, crab and other living seafood are fresh. Usually if they’re alive they’re moving about actively. Their tank water should be clean. Don’t buy anything that’s listless or dead.
It can be a bit trickier to tell if clams, mussels and shellfish are fresh. The shells should be tightly shut. Or, if they’re slightly open (no more than a 1/8th of an inch), they should quickly snap shut when you touch or tap them. If they’re dead or aged, they should be avoided.
Shrimp should have a shiny, wet appearance with tight scales and, as with all other fish, no odor. Fresh shrimp is highly perishable, and generally shrimp is flash-frozen on the boat in order to preserve freshness. Shrimp may come previously de-veined, and this is the type of shrimp that’s recommended for the beginner cook. Buying the shrimp whole and cleaning them yourself is more economical, but it’s a huge time-saver to buy pre-cleaned shrimp. There are both saltwater and freshwater species of shrimp.
Shucked shellfish refers to the removal of a mollusk or fish from the shell; this term also refers to the item’s market form sold as meat only, along with natural juices. Molluscs such as oysters, clams, and mussels may be available shucked; scallops are nearly always sold shucked.