People who regularly eat fish reduce their risk of stroke, depression, and age-associated mental decline, and it may help ward off a variety of chronic degenerative diseases.  By far, however, the main benefit is cardiovascular.  A Harvard study found that the risk of death from coronary heart disease (CHD) was 36% lower in people who eat a "modest" amount of fatty fish -- those high in omega-3 fatty acids -- every week.

Farmed tilapia contains only modest amounts of omega-3s, which makes it less heart-healthy than more omega-3-rich fish such as salmon, tuna, or trout.

Farmed tilapia has been found to have high levels of omega-6 fatty acids -- probably because the fish are fed corn-based feeds high in omega-6s. Omega-6s are necessary for human health, but most people already get enough from other sources.  Some scientists think too many dietary omega-6s can lead to unhealthy fatty acid ratios, which they think can cause a number of health problems, including heart disease.  Other researchers, however, doubt that hypothesis.

Most cardiovascular and other health benefits of eating fish are due to omega-3 fatty acids.  Omega-3s are essential nutrients that our bodies need to work normally, but our bodies don't produce these very efficiently.   There are several types of omega-3s, but fish and fish oil provide two, EPA and DHA, which are particularly heart healthy and have established health benefits. These healthy fats are being added to everything from eggs to peanut butter.

Fish also can provide omega-6 fatty acids, though in much smaller amounts than omega-3s.  While omega-6s also are critical to maintaining health, most people get plenty of them from other sources.

The American Heart Association recommends at least two servings of fish per week -- particularly fatty fish that are high in omega-3 fatty acids.  Each "serving" is equivalent to about 3.5 ounces of cooked fish, or about 3/4 of a cup of flaked fish. The AHA position is very close to the USDA's recommended standard of 8 or more ounces of fish or shellfish per week for adults and somewhat less for children. Fish means only fish, either fresh or saltwater. Seafood includes fish and shellfish.

Shellfish -- including crab, lobster, scallops, and clams -- is generally quite low in fat.  And the distribution of fats is healthier than with many other proteins. Shellfish tend to have higher proportions of polyunsaturated fat and lower proportions of saturated fat than beef and chicken, for instance, although they are not as high in heart-healthy omega-3 fatty acids as most fatty fish.

Shellfish (which are not actually fish, but another group of water-dwelling animals including crustaceans and mollusks) are also a very good source of protein.

Salmon is a very rich source of omega-3 fatty acids; a 4-ounce serving contains between 1,200 and 2,400 mg of omega-3s.  Other fishes high in omega-3s (in mg per 4 ounces) include mackerel (1,350 mg to 2,100 mg), albacore tuna (1,700 mg), herring (2,300-2,400 mg), anchovies (2,300-2,400 mg), sardines (1,100-1,600 mg), and freshwater trout (1,000-1,100 mg).

It depends on the fish.  Some species, such as salmon, mackerel, herring, and trout, can be high in omega-3s whether they are farmed or wild.  But much farmed fish, especially tilapia, is fed a corn-based or other diet high in omega-6s, leading to a higher ratio of omega-6s to omega-3s than is recommended.  There are, however, some farmed fish that are fed better diets -- particularly fishmeal, with added fish oils, or algae -- leading to a more favorable ratio of omega-3s to omega-6s.

According to the American Heart Association, it's best to get omega-3 fatty acids from food.  However, if you're a heart disease patient or if for some reason you can’t get enough omega-3 fatty acids through your diet, you may want to consider fish oil supplements.  As always, consult with your doctor before beginning any new supplements.

The American Medical Association says the health benefits of eating fish "greatly outweigh" the potential risks of mercury exposure or potential contamination by dioxins, PCBs, or other contaminants.  Levels of chemicals in fish such as dioxins and PCBs are very low, comparable to the levels found in meat and dairy products.  The AMA says their potential presence "should not influence individual decisions about fish intake."

The AMA says "it is not clear that mercury exposure from typical levels of fish intake has any adverse health effects" for men and women who are not of childbearing age.

Fish sometimes carry parasites, and if the fish is eaten raw or lightly preserved -- such as sashimi or ceviche -- parasites become a concern.  The most common of these parasites are nematodes (also known as "cod worms" or "herring worms") and tapeworms.  Nematodes, though rarely a problem, can infect a human host and cause limited digestive problems for a week or so.  Tapeworms are much worse: they can live in the human digestive tract for years, growing up to a couple of yards long, causing severe pain, weight loss, and anemia.

The good news: These parasites are killed when fish is properly cooked to an internal temperature of at least 140 degrees Fahrenheit. Avoid eating raw fish unless they have been properly frozen.

It is always best to cook seafood thoroughly to avoid risks of food-borne illness.  Much sushi- and sashimi-grade fish is flash frozen to kill parasites or comes from a type of fish that has a low risk of parasitic infection. In general, it is the supplier who certifies whether fish is sushi grade.  No matter how the fish is labeled, eating raw fish is always riskier than eating cooked fish.  

If you do eat raw fish, the USDA recommends using fish frozen commercially for seven days at -10°F or 15 hours at -31°F.

The USDA recommends pregnant or breastfeeding women should eat at least 8 and up to 12 ounces of seafood per week.  The USDA says "moderate evidence indicates that intake of omega-3 fatty acids, in particular DHA” from a variety of seafood “is associated with improved infant health outcomes, such as visual and cognitive development."

However, pregnant women are strongly advised to avoid types of fish that are high in mercury, particularly shark, swordfish, tilefish, and king mackerel.  The USDA says women who are pregnant or breastfeeding can eat tuna but should limit white tuna to 6 ounces per week.

Refrigerated smoked seafood -- that's otherwise uncooked -- poses an increased risk of a bacterial infection called listeriosis for pregnant women, older people, and people with weakened immune systems.  These are usually labeled as "nova-style," "lox" "kippered," "smoked," or "jerky."  For most other people, any risk is minimal.

Smoked seafood in a cooked recipe, such as a casserole, is always OK -- as is canned or shelf-stable smoked seafood.

Nearly all fish and shellfish contain traces of mercury.  But mercury exposure is passed up through the food chain, so bigger fish accumulate more as they eat smaller fish, and older fish tend to have more mercury because they have been exposed to mercury over a longer period of time.  Large, long-lived predators such as bass, tuna, swordfish, and some sharks have some of the highest concentrations of mercury.

Mercury occurs naturally in the environment but also enters the atmosphere and water from human activities. Mercury can be emitted from fossil-fuel burning plants and automobiles into the air. Then it falls to the ground in rain and snow and dry deposits and enters streams, rivers, and lakes.

For more information on the amount of mercury in particular types of fish, see the FDA report.

Potential exposure to some contaminants such as PCBs or dioxins can be reduced by removing the skin and surface fat from the fish before cooking. Most mercury, however, is in the flesh of the fish and cannot be removed with the skin.

Seafood allergies are quite common.  They can be very specific (where you're allergic to just one species of fish, but can eat other seafood without any problems) or they can be very broad-based (where you're allergic to all types of shellfish and/or multiple species of fish).  If you are allergic to seafood, it is likely to be a life-long allergy -- and it can be extremely dangerous, and even life-threatening.

Seafood also can be a source of food poisoning or parasites, especially when eaten raw.  Less commonly, cooked seafood can also cause food poisoning due to marine toxins.  The most common are scombroid or histamine fish poisoning (from bacteria in spoiled finfish such as tuna or mackerel) or ciguatera poisoning (from toxins ingested by reef fish such as grouper, snapper and sea bass).

Despite these occasional risks, however, for people who are not allergic to seafood, properly-handled fish and shellfish are a safe and healthy part of a balanced diet.

The American Heart Association recommends that you steer clear of fried fish.  Fried foods, especially when cooked in saturated or trans fats (as is the case with much fried fish), have been linked to heart disease and stroke.  In one study of older women, one serving a week of fried fish was found to be associated with a 48% higher risk of heart failure. 

Choose baked or broiled instead.