|See Also||Clinical Nutrition|
Fish is a broad term for any of the 20,000 species of cold-blooded aquatic vertebrates worldwide that contain fins, gills, and a streamlined body and live in fresh or salt water.
Fish are categorized as being either freshwater, saltwater or shellfish depending on where they live. They can be further divided based on their fat composition into:
- Lean fish, which contain less than 5% fat (e.g.: haddock, barramundi, pickerel, monkfish, turbot, grouper, cod, sole, perch, shellfish)
- Semi-fatty fish, which contain 5%-10% fat (e.g.: sea bass, swordfish, marlin, shark, tilapia, snapper)
- Fatty fish, which contain more than 10% fat (e.g.: salmon, tuna, sardines, herring, mackerel, lake trout)
- Fish are an excellent source of high-grade protein, poly-unsaturated fatty acids (omega 3’s) and minerals - especially iodine and potassium.
- Each type of fish varies somewhat with respect to its nutritive value. 
- The amount of unsaturated fat contained in fish and shellfish varies depending on the species and the season.
- Following are the nutritional benefits offered by commonly consumed fish and shellfish: 
- Sardines, mackerel, anchovies, salmon and herring are the highest in omega 3 fatty acids.
- Halibut, salmon and mackerel are fatty fish that are good sources of vitamin A and vitamin D and omega 3 fatty acids.
- Herring, oysters and sardines contain zinc and vanadium and omega 3 fatty acids
- Whitefish such as flounder, cod and haddock are very low in total fat and a good source of vitamin B12, selenium, zinc, phosphorus and iodine.
- Sardines are rich sources of the nucleic acids DNA/RNA.
- Shellfish are low in fatty acids but relatively high in dietary cholesterol that may not be harmful if consumed in moderation.
- Shrimp are one type of shellfish that is very high in cholesterol. There is 195 mg of cholesterol per 31/2 ounce serving
- Fish fat (which takes the form of oils) contains polyunsaturated fatty acids called omega 3’s which have anti-clotting properties and can be protective against heart disease and high blood pressure.
- Fish oils can help lower blood cholesterol levels when they replace saturated fats in the diet.
- Omega 3’s can help control inflammatory responses in the body that can lead to conditions such as arthritis and psoriasis.
- Studies have shown that consumption omega 3 fatty acids and cold-water fish may offer some protection against the development of diabetes.
- An average daily intake of one ounce (24 grams) of fish was associated with a significantly lower incidence of glucose intolerance. 
Fish that live in polluted waters can cause poisoning, as the flesh of some fish easily absorbs toxic substances, such as DDT (dichlorodiphenyl trichloroethane), PCB (polychlorinated biphenyl) and mercury.
The level of mercury is a key concern when it comes to consuming fish because:
- Once mercury enters a waterway, naturally occurring bacteria absorb it and convert it to a form called methyl mercury. This transition is particularly significant for humans, who absorb methyl mercury easily and are especially vulnerable to its effects. Since mercury is odourless, invisible and accumulates in the meat of the fish, it is not easy to detect and can't be avoided by trimming off the skin or other parts.
- Once in the human body, mercury acts as a neurotoxin, interfering with the brain and nervous system. In adults, mercury poisoning can adversely affect fertility and blood pressure regulation and can cause memory loss, tremors, vision loss and numbness of the fingers and toes. A growing body of evidence suggests that exposure to mercury may also lead to heart disease.
- Mercury exposure is especially harmful during fetal and childhood development and should be avoided by women who are pregnant or breast feeding and all children. Even in low doses, mercury may affect a child's development, delaying walking and talking, shortening attention span and causing learning disabilities. Less frequent, high dose prenatal and infant exposures to mercury can cause mental retardation, cerebral palsy, deafness and blindness.
- Following are some guidelines with respect to mercury levels in fish:
- It is best to check local advisories regarding the water safety in nearby lakes, rivers and costal areas if you are fishing in those waters.
- Ensure you know the source of the fish you are purchasing and consuming
- Carefully choose fish that are low in mercury.
- Shark, swordfish, king mackerel, and tilefish contain the highest levels of mercury.
- Canned light tuna, salmon, pollock, and catfish contains lower levels of mercury depending on where they are caught.
Parasites in fish are a natural occurrence and fairly common. Parasites normally only pose a health risk if fish is consumed raw as in sushi or gravlax.
- Raw fish should be frozen to an internal temperature of −20°C (−4°F) for at least 7 days to kill parasites
Many fish consume algae and other organisms that contain biotoxins which can be harmful to humans. Biotoxins accumulated in fish and shellfish include:
- okadaic acid
- ciguatoxin (deadly to humans)
- domoic acid (deadly to humans)
These toxins are primarily found in shellfish (except for ciguatoxine) and when consumed can cause symptoms such as diarrhea, dizziness and a temporary feeling of claustrophobia.
Cod Liver Oil
It is advisable to avoid consuming cod liver oil as it may contain toxic levels of vitamins A and D and other environmental contaminants that often concentrate in this fish’s liver. 
- The skin should be glistening, pearly, tight and adhere to the flesh.
- The flesh should be firm and elastic; it should not be marked, retain finger impressions or separate easily from the bones.
- The scales should cling to the skin, and be shiny and intact.
- The belly should be neither swollen, nor dull, and should have a sweet and pleasant odor (a strong fish smell indicates lack of freshness). 
Frozen fish: The flesh of frozen fish should have a fresh, firm and glistening appearance without any evidence of drying or freezer burn.
- It should frozen solid and enclosed in watertight packaging without any frost or ice crystals on the inside.
- A defrosted fish will have a slightly different flavor and texture from a fresh fish. It should not be refrozen without being cooked beforehand. 
Salt-cured fish: The flesh should have a good color and pleasant smell, and not be dried out.
Storing, Cooking and Eating Fish
- All fish should be consumed within 2 days of being thawed or purchased (if fresh).
- Wipe the fish well with a damp cloth, wrap in waxed paper and place in an airtight container in the fridge.
- Fresh fish and shellfish should not remain at room temperature for more than two hours because of the risk of bacterial infection.
- Fish and shellfish are best when cooked at lower temperatures (330 - 325˚F) in order to preserve flavour, juices and nutrients.
Global consumption of seafood has doubled since the 1970’s and roughly 130 million tons of seafood is harvested every year. As a result a number of fish have been over-fished and are now facing extinction. This will have a profound affect on our oceans and ecosystem. It has been estimated 90% of all large, predatory fish are already gone from the world's oceans and we are now fishing the last 10% of species such as swordfish, tuna and sharks. The marine species are not able to reproduce quickly enough to sustain this demand and a recent scientific study predicted a world-wide fisheries collapse by 2048. 
The 2006 IUCN Red List outlines 1,173 fish species that are threatened with extinction including: Atlantic cod, Devil's Hole pupfish, Coelacanths, and Great white sharks. This list can be accessed at: http://www.iucnredlist.org/ 
The solution to over-fishing to consume seafood in a more sustainable manner. This can be done by making a concerted effort to purchase fish only from sustainable fisheries which: 
- Utilize stocks that are healthy and abundant.
- Do not threaten populations or impede the ecological role of any marine life.
- Minimize bycatch, where possible.
- Are managed to sustain long-term productivity of all impacted species.
- Are conducted such that impacts on marine habitats are minimized and the ecological and functional roles of these habitats are maintained.
- Do not seriously reduce ecosystem services provided by any species, or result in harmful changes such as trophic cascades, phase shifts, or reduction of genetic diversity.
Seafood Watch provides a pocket guide of sustainable seafood by region which can be found at: http://www.montereybayaquarium.org/cr/cr_seafoodwatch/sfw_recommendations.aspx?c=ln
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- The IUCN Red List of Threatened Species http://www.iucnredlist.org Retrieved 12 March 2012
- Seafood Watch “Developing Seafood Watch Recommendations” http://www.montereybayaquarium.org/cr/cr_seafoodwatch/content/media/MBA_SeafoodWatch_RecommendationProcess.pdf Retrieved 12 March 2012