Health Canada's been saying it for years. Now the Journal of the American Medical Association has added its voice to the chorus: eating fish is good for you, even if it has a little mercury in it.

Salmon is high in omega-3 fatty acids.

A study in the journal's Oct. 18, 2006, edition found that the benefits of eating fish regularly far outweigh the risks. Researchers at the Harvard School of Public Health found that eating moderate amounts of fish - about 80 grams of farmed salmon or 160 grams of mackerel a week - reduced the risk of death from coronary heart disease by 36 per cent. The researchers also found that eating that amount of fish - or taking a fish oil supplement - reduced the risk of death from any cause by 17 per cent.

Fish is high in omega-3 fatty acids, especially species such as salmon and mackerel, which are fattier than leaner white fish like cod and haddock. Those fatty acids have been shown to be beneficial to cardiovascular health, although the tendency of contaminants to collect in fatty deposits has been a concern.

The researchers said when it comes to farmed salmon, the cardiovascular benefits outweigh cancer risks from contaminants such as PCBs or dioxins by a factor of at least 300 to one. But they added that some locally caught sport fish from contaminated inland waters should be avoided.

In the past, Health Canada advised consumers to limit their consumption of swordfish, shark or fresh and frozen tuna to one meal per week. Those fish are among the top predators in the oceans and their bodies tend to contain higher concentrations of contaminants such as mercury than other fish.

For young children and women of childbearing age, Health Canada recommended a limit of one meal per month of swordfish, shark or fresh and frozen tuna.

On Mar. 26, 2007, Health Canada released revised guidelines for the consumption of certain species of fish. The agency added marlin, orange roughy and escolar to its list of fish that could contain higher levels of mercury. It also revised consumption amounts to better reflect changes in the Canada Food Guide.

For most people, Health Canada advises that consumption of the above species of fish be limited to 150 grams per week. For women who are or may become pregnant or who are breastfeeding, the limit is 150 grams per month. For children between the ages of five and 11, the limit should be 125 grams per month. And for young children — between the ages of one and four — the limit should be 75 grams per month.

Health Canada advisory on canned tuna

On Feb. 19, 2007, Health Canada issued new guidelines for the consumption of canned albacore tuna by women and children. The decision came after a CBC News investigation found higher than expected levels of mercury in canned albacore tuna bought at supermarkets in Toronto, Winnipeg and Vancouver.

Albacore — or white — tuna typically contains more mercury than "light" tuna, because it's generally from a larger, older fish that has accumulated more mercury.

Health Canada says its standards for mercury in fish are among the toughest in the world, but — as a precaution — it's advising:

• Women who are or who may become pregnant, or who are breastfeeding should limit their weekly consumption of canned albacore tuna to no more than four Food Guide servings. One Food Guide serving is 75g, 2½ oz, 125 ml, or ½ cup.
• Children between one and four years should limit their weekly consumption of canned albacore tuna to no more than one Food Guide serving. Children between five and 11 years old should limit their weekly consumption of canned albacore tuna to no more than two Food Guide servings.
• Health Canada has not issued any advisories about the consumption of "light" tuna.

Why is mercury a concern?

Mercury is a naturally occurring element that's found in soil and rocks as well as bodies of water. We absorb trace amounts of it from dental fillings, air and water pollution and from the food we eat. Fish tend to be our largest source of mercury.

A high level of mercury damages our nervous systems and can inhibit brain development in young children. It's not clear what the long-term effects of extremely low levels of mercury are.

Why is mercury a problem in fish?

Mercury binds tightly to the proteins in fish tissue. Most fish will contain trace amounts of mercury, depending on the level of mercury in their environment and their place in the food chain. The bigger the fish and the higher up the food chain it is, the more mercury it will tend to contain. Large predatory fish species tend to have higher levels than non-predatory fish or species further down the food chain.

Health Canada guidelines for mercury in fish call for a concentration of no more than 0.5 parts per million - about half the limit considered safe in the United States. The Canadian Food Inspection Agency regularly tests fish sold commercially. Most fish come in well under the limit, with the exception of swordfish, shark and fresh or frozen tuna, which is why officials recommend limits on the consumption of those species.

Is mercury a problem in freshwater fish?

It can be, depending on the body of water the fish calls home. The CFIA tests fish caught by commercial freshwater fishermen and if it finds high levels, it will shut down or severely restrict the fishery.

Recreational fishermen should check with their provincial or territorial governments for information on safe consumption standards and guidelines.

One of Canada's top salmon-fishing rivers is the Credit River, which flows into Lake Ontario in Mississauga, just west of Toronto. The Ontario Ministry of Natural Resources advises people not to eat any Chinook salmon caught in the river that are more than 55 centimetres (22 inches) long. The ministry recommends no more than four meals a month of Chinook salmon smaller than that. For Coho salmon caught in the river, the ministry advises not to eat anything longer than about 35 cm (14 inches).

What about other contaminants?

Fish have also been known to contain trace amounts of carcinogens such as dioxins and PCBs. But the Journal of the American Medical Association study found that fish were responsible for only about nine per cent of dioxins and PCBs in the food supply. More than 90 per cent came from other foods such as meats, vegetables and dairy products.

The study found that the cardiovascular benefits of eating fish were far greater than the cancer risks.

What part of the fish can be safely consumed?

Contaminants are concentrated in the fattest part of the fish - under the skin and in the organs, especially near the belly. Your best bet is to stick to the skinless, boneless dorsal fillets of fish such as salmon, trout and carp.

Do not eat the organs of any fish no matter where you caught it. Organs can be high in both heavy metals and pesticides.

It's generally safe to consume the whole side fillet of lean species such as walleye, pike, bass, perch, and panfish from inland locations.

How can I protect myself?

Eat species of fish that are known to be less contaminated.

Avoid larger fish. The older a fish is, the longer it's been exposed to potential contaminants in the water. Smaller fish haven't had as much time to absorb contaminants. They also tend to taste better than larger fish.

Species such as perch, sunfish and crappie tend to contain lower levels of contaminants than larger predatory fish such as walleye, pike, salmon or trout.

If you clean and cook your fish properly, you will lower your risk of exposure to contaminants. Before you cook your fish, trim off the fatty areas and throw away the flesh around the belly area. Allow fat to drip away during cooking.