You are here: Home / Fish




By Medifit Education








fish, limbless aquatic vertebrate animal with fins and internal gills. There are three living classes of fish: the primitive jawless fishes, or Agnatha; the cartilaginous (sharklike) fishes, or Chondrichthyes; and the bony fishes, or Osteichthyes. These groups, although quite different from one another anatomically, have certain common features related to their common evolutionary origins or to their aquatic way of life. Fish were the earliest vertebrates and presumably evolved from a group of aquatic lower chordates (see Chordata); the terrestrial vertebrates evolved from fishes.

There are over 20,000 living species of fish. They range in size from the .31-in. (7.9-mm) Paedocypris that lives in tropical swamps in Sumatra to the 45-ft (14-m) whale shark. Many are brightly colored, and many have shapes and patterns that serve as camouflage. They are found in all marine, fresh, and brackish waters throughout the world and at all depths. Members of different species of fish tolerate water temperatures ranging from freezing to over 100°F (38°C). Most are confined either to saltwater or to freshwater, but some are physiologically adapted to moving from one to the other. A number of fishes that are born in freshwater spend their adult lives in the ocean, returning to their birthplace to spawn; the reverse of this migration occurs in some fishes born in the ocean. Many fishes stay in tightly organized groups, called schools; others are solitary and congregate only for feeding and spawning. Fish may be carnivorous, herbivorous, or omnivorous. Some fish are scavengers on lake or ocean bottoms. Fish are a major source of human food as well as of oil, fertilizer, and feed for domestic animals (see fishing).

A number of aquatic invertebrate animals and groups have common names that include the term fish (for example, crayfish and shellfish), but these do not resemble and are not related to true fishes. Furthermore, there are members of the terrestrial vertebrate classes, such as whales and sea snakes, that have adopted an aquatic way of life; these may superficially resemble fishes and are sometimes erroneously called fishes, but they are air-breathers, and their anatomical structure reveals their relationship to land animals.



How do we know a fish when we see one? Not as simple a question as you might think!

Fishes are animals that live and swim in the water (although you might see a fish like a mudskipper or walking catfish crawling on land), are cold-blooded (except for tunas and marlins and mako sharks that are warmer than the water), breathe using gills (usually, but lungfish and some others have lungs), have backbones (but not always of bone, such as in sharks, which are cartilage), have a scaly skin (except for eels, which are scale-less), and have various fins instead of limbs (except for a few that do actually have limbs, like lungfish and coelacanth).  So, we usually know a fish when we see one, but there are lots of exceptions to our fishy definitions.  Most people will recognize a “typical” fish like a goldfish, bass, bluegill, snapper, or grouper because of experience with aquariums, going fishing, or enjoying fish for dinner.  And most people know that lampreys, sharks, rays, eels, seahorses, and other strange-looking aquatic creatures are fishes, while shellfish, cuttlefish, starfish, crayfish, and jellyfish (despite their names) are not fishes. But some fish species are weird enough, and look enough like salamanders or other animals, that it is not always easy to be sure that one is looking at a fish.

There are more than 32,000 described species of fishes, more than all the amphibians, reptiles, birds, and mammals combined. Fishes are an important food resource worldwide, and fishing pressure has caused many fish stocks to crash or be at risk. Both commercial fishermen and sport fishermen exploit coastal marine fish throughout the world, and fish farming is becoming more common, particularly for high-priced food fish such as salmon. Fishes are also popular as pets, with the aquarium trade in live fishes caught from the wild and being raised in captivity growing ever more popular. Fishes have had a strong role in human activities across many cultures, serving as deities, subjects of art and sculpture, legend and story, and more recently as the main characters in books and movies.



Lampreys and hagfish (superclass Cyclostomata) are a group of jawless fishes at the base of the vertebrate tree of life, whose adults are characterized by a toothed, funnel-like sucking mouth and horny (not bony) teeth. Lampreys are best known for species like the marine lamprey, which bore into the flesh of other fish to rasp their flesh and suck their blood, although most species of lamprey are not parasitic. Hagfishes are also delightfully interesting creatures, capable of producing copious amounts of thick mucous, able to tie themselves in knots, and often found burrowed into the bodies of large fish that may still be alive. Biologists debate whether lampreys and hagfishes are closely related at the root of the vertebrates, or whether lampreys are more closely allied with other vertebrates than are hagfishes.

Cartilaginous fishes (class Chondrichthyes) are the chimeras, sharks, skates and rays. They have skeletons made of calcified cartilage rather than bone. Cartilage is tough and flexible, and can be just as hard and strong as bone, providing enough structural support to enable many sharks and rays to grow to very large sizes (the whale shark is the largest fish). This group includes the largest, fiercest, and most famous marine predators alive today. Most cartilaginous fishes live in marine habitats all their lives, but a few species of sharks and rays live in fresh water during all or part of their lives. All cartilaginous fishes are carnivorous and most species feed on live prey. There are some species that feed on the remains of dead animals and still others that are filter feeders. The class Chondrichthyes is further divided into subgroups, with Holocephali containing chimeras, and Elasmobranchii containing sharks (Selachii) as well as skates and rays (Batoidea).

RAY-finned fishes (Actinopterygii) are the most diverse of the major groups of fishes, containing more than 25,000 species such as gars, bowfin, eels, salmon, trout, catfish, piranhas, lanternfish, cods, anglerfish, tarpon, basses, cichlids, butterflyfish, wrasses, parrotfish, and many others. Ray-finned fishes share a set of basic characteristics, including a skeleton made up of true bone (although cartilage is also present in many places), an upper jaw that consists of two bones (the maxilla and premaxilla), and fins that are supported by a set of bony spines and rays covered with a thin layer of skin. The skull of ray-finned fishes is extremely diverse and highly adaptable. It contains a large number of different mechanisms for enhancing bite force and jaw protrusion, resulting in a wide range of feeding adaptations and ecological roles for the actinopterygian fishes.


LOBE-finned fishes (class Sarcopterygii) are a very special group of bony fishes with limb-like fins that are fleshy at the base and bones connected in series that look and function much like limb bones. The living sarcopterygians include lungfishes (which have both lungs and limb-like fins) and coealacanths, both of which are living representatives of diverse fossil groups.   Lobe-finned fishes hold special interest to evolutionary biologists because members of this group gave rise to the first four-legged land vertebrates (tetrapods). In fact, all amphibians, reptiles, birds, and mammals are included as descendants in the tree of life of sarcopterygians—from that point of view, we are all just fancy terrestrial lobe-finned fishes!


If you’ve ever swum through supplement research, you’ve surely dipped a toe in the ocean of information surrounding fish oil. This little supplement has made waves, but should you dive headfirst into the fish fry? In a word, yes. In a bad joke, fish oil is off the hook!

It might sound silly, but there’s nothing fishy about this incredible supplement. Care to know more? Then cast your line and get ready for school. I’ll teach you the ins and outs of this popular dietary supplement.



Fish oil contains essential fatty acids (EFAs) that have been shown to offer myriad benefits. Since they’re the most widely researched and important, we’ll focus here on two essential Omega 3 fatty acids: EPA and DHA.

These fatty acids are termed “essential” because we need them for proper function, but our bodies cannot produce them. Thus, we must obtain them through food or supplements.

While there are a number of EFA supplements, such as flax and other oils, fish oil is one of the finest sources available.



Fish oil supplements provide a wide variety of health benefits. Thus, they’re incredibly useful for the general population and can help everyone from athletes to sedentary individuals.

Bodybuilders and physique athletes find fish oil intriguing due to potential body composition benefits, but almost anyone looking to support overall health may be interested in the fine fats from fish oil.



As mentioned, fish oil’s primary, active components are EPA and DHA. In the body, EPA and DHA can be stored in the phospholipid bilayer of cells for future use. If you consume enough EPA or DHA, you can significantly alter the composition of the fats that make up the bilayer of your cells. This can have a profound impact on processes that utilize those stored lipids (fats).

Of particular relevance is the anti-oxidant and acute, exercise-related anti-inflammatory nature of EPA and DHA. Since high-intensity exercise and weight training cause increases in inflammation and oxidation, reducing this response may be beneficial in improving exercise recovery. While it’s emerging, the research has yet to fully examine this angle of EPA and DHA supplementation.

Perhaps more interesting for people looking to build muscle, EPA and DHA supplementation has been suggested to support muscle protein synthesis and limit muscle protein degradation. This can mean less muscle breakdown and more muscle growth.



The essential formula for gaining muscle is that the rate of lean tissue gain, or protein balance, is the rate of synthesis minus the rate of degradation. Since EPA and DHA can increase synthesis and decrease degradation, they can light both ends of the candle and help push the muscle-building balance in your favor!

Additionally, EPA and DHA have been suggested to support insulin function and increase glucose and fatty acid uptake into muscle cells. This may help partition nutrients toward muscle and away from fat, improving overall body composition and providing more fuel to the muscle during workouts.

EPA and DHA have also been suggested to increase cardiac output and stroke volume, which may help support healthy blood flow and possibly exercise performance, though this has not been examined directly.



Different fish oil supplements have different concentrations of EPA and DHA. In order to gain the body composition and muscle metabolic benefits of these two fatty acids, make sure you consume around 2-4g of EPA & DHA combined every day.

There is some evidence that taking fish oil with meals may enhance the anabolic response to a meal. Additionally, taking fish oil with a meal may improve the fish oil’s uptake, though this is personal speculation.

Fish oil—EPA in particular—has been shown to work synergistically with HMB for reducing muscle protein breakdown. Since EPA can enhance muscle protein synthesis, it may also be useful to take with leucine, BCAAs or whey protein, which also support muscle protein synthesis.

Of course, you can also stack fish oil with your usual foundational supplements (such as a multivitamin).



As a rule, I recommend purchasing supplements from labels which can provide lab analysis of their products. This ensures that what’s on the label is actually in the bottle.

Since most of the benefits from fish oil are from the EPA and DHA content, look for supplements which contain a high concentration of these fatty acids.

Fish oil is tolerated well by most people, though there are a few side effects that some people may experience. These include excessive bleeding if you take a very high dose of fish oil—far beyond what is recommended here. This is due fish oil’s ability to break down blood clots in high concentrations.

A small percentage of people are allergic to fish oil and should not consume it.

Perhaps the most common side effect is gastrointestinal distress from fish oil consumption. Many people also experience unpleasant belching while taking fish oil capsules. If this is the case, try using a product that has enteric-coated capsules, which should reduce the incidence of gnarly fish oil burps.

As always, talk with your physician before starting any new supplement.


Fish are an awesome source of essential fats, especially EPA and DHA. Unless you love daily deep-sea diving, a fish oil supplement is one of the best ways to get the multiple muscle-building and health benefits of essential fishy fats.





Fish is a low-fat high quality protein. Fish is filled with omega-3 fatty acids and vitamins such as D and B2 (riboflavin). Fish is rich in calcium and phosphorus and a great source of minerals, such as iron, zinc, iodine, magnesium, and potassium. The American Heart Association recommends eating fish at least two times per week as part of a healthy diet. Fish is packed with protein, vitamins, and nutrients that can lower blood pressure and help reduce the risk of a heart attack or stroke.



Eating fish is an important source of omega-3 fatty acids. These essential nutrients keep our heart and brain healthy. Two omega-3 fatty acids found in fish are EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). Our bodies don’t produce omega-3 fatty acids so we must get them through the food we eat. Omega-3 fatty acids are found in every kind of fish, but are especially high in fatty fish. Some good choices are salmon, trout, sardines, herring, canned mackerel, canned light tuna, and oysters.



  • Help maintain a healthy heart by lowering blood pressure and reducing the risk of sudden death, heart attack, abnormal heart rhythms, and strokes.
  • Aid healthy brain function and infant development of vision and nerves during pregnancy.
  • May decrease the risk of depression, ADHD, Alzheimer’s disease, dementia, and diabetes.
  • May prevent inflammation and reduce the risk of arthritis.



Fish is an important cultural icon in Washington State that defines a recreational as well as a spiritual way of life in the Pacific Northwest. Fish is not only an important source of nutrition, the act of catching, preparing, and eating fish are important cultural and family practices as well. To Native American Indian Tribes of Washington, fish, especially salmon, are an integral part of their lives, and serve as a symbol of their prosperity, culture, and heritage.




In my previous post, I discussed why the mercury content of fish is not worthy of concern for most fish species (see this post).  My point isn’t just that we don’t need to worry about eating too much fish; we should really be eating way more of it!

It completely frustrates me that it is generally recommended for pregnant women to limit seafood consumption to two 6oz servings per week in order to avoid excess mercury exposure.  Many women take this a step further and avoid all seafood while pregnant.  Some even avoid seafood while lactating.  Not only is the mercury exposure from seafood a complete non-issue (with the exceptions of the few fish that are higher in mercury than selenium), but by limiting seafood during pregnancy, women are missing out on the best food source of DHA, an extremely essential nutrient for their health and the health of their growing baby.  In fact, a maternal diet rich in DHA has been shown to improve a baby’s IQ by 10 points.  The recommendation should probably be for a minimum of three 6oz servings of oily cold-water fish per week for these women, if not a diet that is heavily based on fish as a protein source (although, there still is a legitimate rationale for avoiding sushi).  I personally wonder how different my two pregnancies would have been if I had known this back then.



Fish and shellfish are rish in long chain omega-3 fatty acids DHA and EPA.  These are the anti-inflammatory omega-3 fats that are readily used by our body.  The shorter chain omega-3 ALA, which is in flax, chia and walnuts, is actually not easily used by our body because it must first be converted into DHA or EPA, which is a very inefficient process.  A 3.5oz serving of wild-caught salmon (fresh or canned; any species), sardines, albacore tuna, trout or mackerel has over 500mg of DHA + EPA.  Fish which have moderate amounts of DHA+EPA (150mg-500mg per 3.5oz serving) include haddock, cod, hake, halibut, shrimp, sole, flounder, perch, bass, oysters, crab and farmed salmon).1  Why not just get your DHA and EPA from fish oil supplements?  These polyunsaturated fats are very easily oxidized in response to heat or light and are not very shelf stable, especially once isolated.  Consuming oxidized omega-3 fats is not helpful to your health (contributes to inflammation as opposed to reducing it).  Eating fresh, frozen or canned whole fish protects the omega-3 fats from oxidation plus provides all the necessary cofactors for optimal absorption and use by the body.

The protein in fish and shellfish is very easy to digest and research shows that the amino acids in fish are more bioavailable (your body can absorb and use them more readily) than beef, pork or chicken 2,3.  Fish and shellfish also have a balanced quantity of all of the essential amino acids, giving them very high Amino Acid.

Fish is also rich in two very important minerals which can be challenging to get in sufficient quantities from other foods:  iodine and selenium.  Iodine (which is also rich in algae and seaweed) is vital for normal thyroid function but is also extremely important for proper immune system function, wound healing, and fertility.  Table salt is enriched with iodine due to rampant dietary iodine deficiency (goiters were very common before the advent iodized salt).  Since paleo diets tend to be lower in salt (and many people switch to sea salt, which is not iodized), it is very important to include food sources of this essential mineral.  Selenium is required for a class of enzymes called selenoenzymes which are part of the body’s natural protection against oxidants.  Selenoenzymes are particularly important for protecting the brain against oxidative damage, but selenium deficiencies are also linked to thyroid disease, cardiovascular disease and cancer.

Fish is also a food source of Vitamin D (which can also be found in organ meats).  Vitamin D is a steroid hormone that controls expression of more than 200 genes and the proteins those genes regulate.  Vitamin D is essential for mineral metabolism (it regulates absorption and transport of calcium, phosphorous and magnesium) and for bone mineralization and growth.  Vitamin D is also crucial for regulating several key components of your immune system, including formation of important anti-oxidants.  Very importantly, Vitamin D has recently been shown to decrease inflammation and may be critical in controlling auto-immune and inflammatory diseases.  Vitamin D is also involved in the biosynthesis of neurotrophic factors, regulating release of such important hormones as serotonin (required not only for mental health but also for healthy digestion!).  Vitamin D helps control cell growth, so it is essential for healing.  Vitamin D also activates areas of the brain responsible for biorhythms.  Scientists continue to discover new ways in which Vitamin D is essential for human health; for example, Vitamin D may prevent cancer.  As we spend less and less time outdoors (our bodies synthesize Vitamin D in response to sun exposure), dietary vitamin D becomes more and more important.

fish fry


Oily cold water, wild-caught fish will have the highest omega-3 and Vitamin D content.  However, even fresh water white fish are an excellent source of protein.  The only fish that are worth limiting in your diet are farmed tilapia and farmed catfish as these fish tend to have higher omega-6 content 1 (they still have that great easily digested protein though!).  Yes, fish can be expensive.  Canned fish (especially sardines and salmon) are great inexpensive options.  Pickled herring and smoked kipper are often less expensive as well.  I can usually find frozen wild-caught pink salmon fillets on sale for $4 per pound at my local grocery store (which means it’s cheaper than ground beef!).  The take home message here is that fish is good for you, so eat it as often as you want.



By Medifit Education