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2.3.1: Fish Classifications and Composition

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    • Marshall Welsh & William R. Thibodeaux
    • Finch Henry Job Corps Center & Nicholls State University

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    Fish Classification:

    Fish are classified in several ways. To distinguish major groups of animals that live in waters they broader category may be broken down in two major ways: finfish vs. shellfish, vertebrates vs. invertebrates, or fresh vs saltwater.

    Simply put, finfish have fins and shellfish do not (and not all shellfish have shells!) So often times, the presence of a backbone is used to classify these creatures. Fish that have them are called vertebrates and those that lack a backbone are called invertebrates. The last of which, fresh vs saltwater is used in describing finfish and is described below.


    Found in the fresh waters of rivers, lakes, and streams as well as the salt water of oceans and seas. The most popular finfish in North America include tuna, cod, pollock, and salmon.

    Fish and Shellfish Classification_finfish.png


    Freshwater fish are found within the worlds less salty waterways such as streams, rivers, and ponds. Freshwater fish tend to be less "fishy" in flavor. Top freshwater fish are listed below: 

    1.) Catfish - known for a mild taste with a slightly denser texture making it less flaky and more versatile to cook.

    2.) Largemouth Bass - white flaky meat and extremely popular due to its mild flavor.

    3.) Trout - Referred to as "chicken of fish" because of its unoffensive and mild flavors.

    4.) Salmon-  most notable for its orange/red color and high omega-3 fatty acid content. Mild flavor with rich texture.

    5.) Walleye- Soft and flaky white fish that is easy to de-bone.

    6.) Pike- Known to be more "fishy" than other freshwater fish and harder to de-bone.

    Freshwater fish can either be considered lean or fatty which is described in more detail below.


    Saltwater fish are found in oceans or in the areas of water that mix with fresh and salt water. Some fish, such as salmon, are considered anadromous, meaning they live their lives in both fresh and salt water.

    Top saltwater fish are listed below:

    1.) Salmon (see above)

    2.) Tuna: commonly canned, fresh high grade tuna has a dark red color and may be consumed raw.

    3.) Anchovies: small fish that are often used as a pizza topping in the U.S.

    4.) Sardines: Small fish, typically sold in the U.S. in cans are a staple in Mediterranean cuisine.

    5.) Snapper: Rosey colored skin distinguishes this one from the others. This is a white fish that has a firm texture and sweet/nutty flavor.

    6.) Swordfish: Large fish, that may have high levels of methylmercury. The flsh can have an orangy tint to it due to their diet but the color of the flesh may vary.

    7.) Grouper: Two main types, red and black. Red is most frequently found in the marketplace for its value and flavor. The flavor is mild and the red grouper is sometimes described as sweet.

    8.) Cod: Appreciated for its mild flavor and flaky white flesh cod is used to make cod liver oil as it is a source of omega-3's and fat soluble vitamins A, D, and E.

    9.) Hake: Similar to cod (and sometimes classified as being in the same family) with a mild flavor and flaky skin. 

    Fatty Fish (Oily Fish)

       Fatty fish, also referred to as oily fish, are fin fish that have a higher amount of fat than other more leaner fish species. The fat in the fish is typically found along the sides, closest to the skin, or deep inside the fish around organs. The fat found in fish is mostly unsaturated in nature which are described to be a healthy fat and one that is required for overall good health. See image below for amounts of omega 3 fatty acids that are found in some common fish species. Common fatty fish include: salmon, mackerel, lake trout, butterfish, whitefish, and herring.

    Seafood Chapter (1).png

    Lean Fish

    A fish that contains < 2.5% fat is considered to be lean. Due to the lower fat content of these fish, they tend to be lower in calories as well. Examples of lean fish include: cod, pike, haddock, flounder, sole, whiting, red snapper, and halibut. 


    Structure and Muscle Composition (Finfish)

    Whether the fish comes from fresh or salt water finfish are composed similar to that of edible land animals with muscle, connective tissue, and fat. However, unlike land animals the structure of muscle and composition of fat make fish much more delicate which requires different cooking techniques and care. Naturally occurring oils and unstable fleshy tissue make spoilage a real concern and fish will spoil at a faster rate than red meats or poultry. However, fish tend to be lower in saturated fats, higher in polyunsaturated fats, and are good sources of protein. Fish is a high protein, low-fat food that is rich in vitamin D and great source of other vitamins and minerals such as phosphorus, potassium, zinc, iodine, and magnesium.

    The muscle structure of fish differs from land animals in that the muscles are arranged in sheets rather than in elongated bundles of fiber-like structures. In fish, these sheets are arranged in myotomes that are separated by a thin later of connective tissue. The connective tissue is made of collagen that is not as strong as the collagen found in land meats and will gel at a lower temperature. This characteristic is what allows fish to cook at lower temperatures and frankly- fall apart or "flake" while cooking.

    Fish get their color from the protein myoglobin (that should sound familiar!) but the over-all color of the fish is greatly dependent on the daily activity of the fish. Fish that swim long distances rely predominately upon "slow twitch fibers" which function best in the presence of oxygen. In contrast, fish that do lots of quick movements have more "fast twitch fibers". These muscle fibers work in the absence of oxygen and are; therefore less pigmented and appear white. 

    More myoglobin (unfortunately) leads to greater rancidity.

    pexels-рыба-моей-мечты-10514187.jpg Thanks to salmons' distinct color, the contrast between the color of the muscle and connective tissue between the myotomes is highly visible. In this photograph, you can see the distinct shape that the myotomes have that look almost wave-like and the thin sheets of white connective tissues (made of mostly collagen) that is holding the thin sheets together. 








    Photo from Pexels



    The term shellfish is a commercial rather than scientific classification and are considered invertebrates which also includes crustaceans and mollusks. Vertebrates are distinguished from invertebrates by the presence of a skeletal structure or in other words a backbone.

    Shellfish are often more expensive and less widely available to the general public. Special cooking techniques are often required which makes cooking these types of fish more complicated for the average at home chef.

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    Structure and Muscle Composition (Shellfish)

    Mollusks are shellfish characterized by soft, unsegmented bodies with no internal skeleton. Most mollusks have hard outer shells. Single-shelled mollusks such as abalone are known as univalves. Those with two shells, such as clams, oysters and mussels, are known as bivalves. Squids and octopuses, which are known as cephalopods, do not have a hard outer shell. Rather, they have a single thin internal shell called a pen or cuttlebone.

    Crustaceans are also shellfish. They have a hard outer skeleton or shell and jointed appendages. Crustaceans include lobsters, crabs and shrimp.

    The flesh of fish and shellfish consists primarily of water, protein, fat and minerals. Fish flesh is composed of short muscle fibers separated by delicate sheets of connective tissue. Fish, as well as most shellfish, are naturally tender, so the purpose of cooking is to firm proteins and enhance flavor. The absence of the oxygen-carrying protein myoglobin makes fish flesh very light or white in color. (The orange color of salmon and some trout comes from pigments found in their food.) Compared to meats, fish do not contain large amounts of intermuscular fat. However, the amount of fat a fish does contain affects the way it responds to cooking. Fish containing a relatively large amount of fat, such as salmon and mackerel, are known as fatty or oily fish. Fish such as cod and haddock contain very little fat and are referred to as lean fish. Shellfish are also very lean.



    Univalves are mollusks with a single shell in which the soft-bodied animal resides. They are actually marine snails with a single foot, used to attach the creature to fixed objects such as rocks.

    Conch are found in warm waters off the Florida Keys and in the Caribbean. Beachcombers prize the beautiful peachy-pink shell of the queen conch. Conch meat is lean, smooth and very firm with a sweet-smoky flavor and chewy texture. It can be sliced and pounded to tenderize it, eaten raw with lime juice or slow-cooked whol

    Snails. Although snails (more politely known by their French name, escargots) are univalve land animals, they share many characteristics with their marine cousins. They can be poached in court bouillon; removed from their shells and boiled; or baked briefly with seasoned butter, or sauce. They should be firm but tender; overcooking makes snails tough and chewy. The most popular varieties are the large white Burgundy snail and the small garden variety called petit gris.

    Fresh snails are available from snail ranches through specialty suppliers. The great majority of snails, however, are available canned; most canned snails are the product from France or Taiwan.



    Bivalves are mollusks with two bilateral shells attached by a central hinge.

    Clams are harvested along both the East and West Coasts, with Atlantic clams being more significant commercially. Atlantic Coast clams include hard-shell, soft-shell, and surf clams. Clams are available all year, either live in the shell or fresh - shucked (meat removed from the shell). Canned clams, whether minced, chopped, or whole, are also available.

    Atlantic hard-shell clams or quahogs have hard, blue-gray shells. Their chewy meat is not as sweet as other clam meat. Quahogs have different names, depending upon their size. Littlenecks are generally under 2 inches (5 centimeters) across the shell and usually are served on the half shell or steamed. They are the most expensive clams.

    Soft-shell clams, also known as Ipswich, steamer ,and long-necked clams, have thin, brittle shells that do not completely close because of the clam's protruding black-tipped siphon. Their meat is tender and sweet. They are sometimes fried but are more often served steamed.

    Surf clams are deep-water clams that reach sizes of 8 inches (20 centimeters) across. They are most often cut into strips for frying or are minced, chopped, processed and canned.

    Mussels are found in waters worldwide. They are excellent steamed in wine or seasoned broth and can be fried or used in soups or pasta dishes.

    Oysters have a rough gray shell. Their soft, gray, briny flesh can be eaten raw directly from the shell. They can also be steamed or baked in the shell or shucked and fried, sautéed or added to stews or chowders. Most oysters available in the United States are commercially grown and sold either live in the shell or shucked. There are four main domestic species.

    Scallops contain an edible white adductor muscle that holds together the fan-shaped shells. Because they die quickly, they are usually shucked and cleaned on board the ship. The sea scallop and the bay scallop, both cold-water varieties, and the calico scallop, a warm-water variety, are the most important commercially. Sea scallops are the largest, with an average count of 20 to 30 per pound. Larger sea scallops are also available.

    Bay scallops average 70 to 90 per pound; calico scallops average 70 to 110 per pound. Fresh or frozen shucked, cleaned scallops are the most common market form, but live scallops in the shell and shucked scallops with roe attached (very popular in Europe) are also available. Scallops are sweet, with a tender texture. Raw scallops should be a translucent ivory color and non- symmetrically round and should feel springy. They can be steamed, broiled, grilled, fried, sauteed, or baked. When overcooked, however, scallops quickly become chewy and dry. Only extremely fresh scallops should be eaten raw.



    Cephalopods are marine mollusks with distinct heads, well-developed eyes, a number of arms that attach to the head near the mouth, and a saclike fin-bearing mantle. They do not have an outer shell; instead, there is a thin internal shell called a pen or cuttlebone.

    Octopus is generally quite tough and requires mechanical tenderization or long, moist-heat cooking to make it palatable. Most octopus is imported from Portugal, though fresh ones are available on the East Coast during the winter. Octopus is sold by the pound usually fresh, frozen, or whole. Octopus skin is gray when raw, turning purple when cooked. The interior flesh is white, lean, firm, and flavorful.

    Squid, known by their Italian name, calamari, is becoming increasingly popular in the United States. Similar to octopuses but much smaller, they are harvested along both American coasts and elsewhere around the world, (the finest are the East Coast loligo or winter squid). They range in size from an average of 8 to 10 per pound to the giant South American squid, which is sold as tenderized steaks. The squid's tentacles, mantle (body tube), and fins are edible. Squid meat is white to ivory in color, turning darker with age. It is moderately lean, slightly sweet, firm and tender, but it toughens quickly if overcooked. Squid is available either fresh or frozen and packed in blocks.



    Crustaceans are found in both fresh and salt water. They have a hard outer shell and jointed appendages, and they breathe through gills.

    Crayfish generally called crayfish in the North and ‘crawfish’ or ‘crawdad’ in the South, are freshwater creatures that look like miniature lobsters. They are harvested from the wild or aqua-farmed in Louisiana and the Pacific Northwest. They are from 3 1/2 to 7 inches (8 to 17.5 centimeters) in length when marketed and may be purchased live or precooked and frozen. The lean meat, found mostly in the tail, is sweet and tender. Crayfish can be boiled whole, and served hot or cold. The tail meat can be deep-fried, or used in soups, bisque or sauces. Crayfish are a staple of Cajun cuisine, often used in gumbo, etouffee and jambalaya. Whole crayfish become brilliant red when cooked, and used as a garnish

    Crabs are found along the North American coast in great numbers and are shipped throughout the world in fresh, frozen and canned forms. Crab meat varies in flavor and texture and can be used in a range of prepared dishes, from chowders to curries to casseroles. Crabs purchased live should last up to five days; dead crabs should not be used.

    King crabs are very large crabs (usually around 10 pounds [4.4 kilograms] caught in the very cold waters of the northern Pacific. Their meat is very sweet and snow-white. King crabs are always sold frozen, usually in the shell. In-shell forms include sections or clusters legs and claws or split legs. The meat is also available in "fancy" packs of whole leg and body meat, or shredded and minced pieces.

    Lobsters have brown to blue -black outer shells and firm, white meat with a rich, sweet flavor. Lobster shells turn red when cooked. They are usually poached, steamed, simmered, baked or grilled, and can be served hot or cold. Picked meat can be used in prepared dishes, soups or sautés. Lobsters must be kept alive until just before cooking. Dead lobsters should not be eaten. The Maine, also known as American or clawed lobster, and the spiny lobster are the most commonly marketed species.

    Maine lobsters have edible meat in both their tails and claws; they are considered superior in flavor to all other lobsters. They come from the cold waters along the northeast coast and are most often sold live. Maine lobsters may be purchased by weight (for example, 1 1/4 pounds [525 grams], 1 1/2 pounds [650 grams] or 2 pounds [900 grams] each), or as 'chix' (that is, a lobster weighing less than 1 pound [450 grams]). Maine lobsters may also be purchased as ‘culls’ (lobsters with only one claw) or ‘bullets’ (lobsters with no claws). They are available frozen or as cooked, picked meat

    Shrimp are found worldwide and are widely popular. Gulf whites, pinks, browns, and black tigers are just a few of the dozens of shrimp varieties used in food service operations. Although fresh, head-on shrimp are available, the most common form is raw, head-off (also called green headless) shrimp with the shell on. Most shrimp are de-headed and frozen at sea to preserve freshness.

    Shrimp are available in many forms: raw, peeled and deveined; cooked, peeled and deveined; and individually quick-frozen, as well as in a variety of processed, breaded or canned products. Shrimp are graded by size, which can range from 400 per pound to 8 per pound (extra-colossal), and are sold in counts per pound. For example, shrimp marketed as "21- 26 count" means that there is an average of 21 to 26 shrimp per pound; shrimp marketed as "U-10" means that there are fewer than 10 shrimp per pound.

    Prawn is often used interchangeably with the word shrimp in English speaking countries. Although it is perhaps more accurate to refer to fresh-water species as prawns, and saltwater species as shrimp, in commercial practice, prawn refers to any large shrimp. Equally confusing, scampi is the Italian name for the Dublin Bay prawn (which is actually a species of miniature lobster), but in the United States scampi refers to shrimp sautéed in garlic butter



    Fish and shellfish are low in calories, fat and sodium, and are high in protein and vitamins A, Band D. Fish and shellfish are also high in minerals, especially calcium (particularly in canned fish with edible bones), phosphorus, and potassium and iron (especially mollusks). Fish are high in a group of polyunsaturated fatty acids called omega-3, which may help combat high blood cholesterol levels and aid in preventing some heart disease. Shellfish are not as high in cholesterol as was once thought. Crustaceans are higher in cholesterol than mollusks, but both have considerably lower levels than red meat or eggs.

    The cooking methods used for fish and shellfish also contribute to their healthfulness. The most commonly used cooking methods are broiling, grilling, poaching and steaming-add little or no fat.




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