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We're repeatedly told to eat two fish meals per week. It's a good source of protein, vitamins and minerals that's low in cholesterol-raising saturated fat.

And some types of fish contain high levels of beneficial omega-3 fatty acids. Research has shown that eating fish, especially fatty fish, helps lower the risk of heart attack, stroke, arthritis, macular degeneration and Alzheimer's disease.

But worry over chemical contaminants in fish – and the ecological toll of overfishing – can make choosing which type of fish to eat a perplexing task. And if you choose fish based only on its omega-3 fat content, you could end up eating a highly contaminated meal.

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Now, a new study suggests making the right seafood choice is easier than you think. In general, the healthiest and safest seafood choices are also the most environmentally friendly ones.

Concerns about harmful chemicals in fish, especially mercury, have prompted some people to drop fish from their menu.

Mercury occurs naturally in the environment and is also released in the air and water from industrial pollution. Once it's in the water, bacteria convert it to methylmercury, which is then absorbed by fish.

Larger, longer-living predatory fish (e.g. Chilean sea bass, grouper, swordfish and some types of tuna) end up with the most toxins. As smaller fish are eaten by larger ones, contaminants are concentrated and accumulated. Cooking has little impact on mercury content.

If you regularly eat fish high in mercury, the metal can build up in the body and lead to health problems. Since it affects the nervous system, the developing fetus and young children are particularly vulnerable. If women consume too much mercury before and during pregnancy, it may increase the risk of birth defects and learning disabilities in children.

That's why many agencies advise women who are pregnant, could become pregnant, or nursing and young children to avoid eating high-mercury fish such as tuna steaks, king mackerel, shark, swordfish, marlin, orange roughy, tilefish and escolar.

Canned albacore (white) tuna should also be limited since albacore is a large species of tuna that accumulates moderate amounts of mercury. Canned light tuna usually contains skipjack, a smaller fish relatively low in mercury.

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There's also the issue of sustainability – and choosing fish that are fished or farmed responsibly. Sustainable seafood is caught in a way that doesn't harm the environment or other species of fish and is not overfished. Overfishing is the main cause of declining fish populations globally.

Balancing all this information – omega-3 fat content, mercury levels and ecological risks – may seem like a challenge.

But according to researchers from the Arizona State University, it's not. Their new findings, published in the journal, Frontiers in the Ecology and the Environment, show that healthy seafood is also sustainable seafood.

The current study analyzed 44 species of fish based on criteria for health (omega-3s), safety (mercury) and sustainability, and determined that health and ecological sustainability go hand in hand.

Fish deemed unsustainable had significantly higher levels of mercury. Long-lived fish that accumulate substantial levels of mercury are the often most overfished.

In other words, if you eat low-mercury seafood, you're also much more likely to be eating sustainable seafood.

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Omega-3 content, however, was not consistently linked to ocean-friendly seafood. Because omega-3 levels tend to be slightly lower in sustainable, low-mercury fish, the researchers suggest that people should eat more of the sustainable choices to boost omega-3 intake.

It's important to note, however, that several good sources of omega-3s such as salmon were not included in the analysis because they were missing from the database. (Salmon is low in mercury and some wild-caught types are also considered sustainable.)

Today there are a number of seafood-awareness programs to help guide you at the fish counter. The Monterey Bay Aquarium, which has a reputation for promoting safe and sustainable seafoods, has a "Super Green" list of choices that are high in omega-3s, low in mercury and caught sustainably.

The super green list includes freshwater coho salmon (U.S. and B.C.), farmed oysters, wild-caught Pacific sardines, farmed rainbow trout and wild-caught salmon (Alaska).

Farmed Arctic char, wild-caught Dungeness crab (California, Oregon, Washington), farmed mussels and wild-caught longfin squid (U.S. Atlantic) are also good choices, but provides smaller amounts of omega-3s.

SeaChoice (Seachoice.org) is a coalition of Canadian conservation organizations created in 2006 to help Canadians make sustainable seafood choices. The organizations' downloadable seafood and sushi selectors indicate best choices and ones to avoid.

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Choosing fish isn't black and white – either all fish is healthy and should be eaten often or it's all contaminated and should be avoided. It's entirely possible to reap the health benefits of fish while minimizing your intake of chemical contaminants and supporting responsible fishing at the same time.

Making smart seafood choices

The Arizona State University study ranked seafood choices as green, red or grey based on health, safety and sustainability data. Grey choices, not listed below, are those that are sustainable but high in mercury or unsustainable but low in mercury.

Green choices: Low mercury, high sustainability

Pacific herring* (B.C.)

Red king crab (Bristol Bay)

Pacific cod (Alaska/B.C.)

Tanner crab (US Bering Sea)

Atlantic pollock (Northeast Arctic/New England)

Alaskan pollock (Eastern Bering Sea)

Atlantic mackerel* (Northeast Atlantic)

American plaice (New England)

Canary rockfish (US Pacific coast)

Black rockfish (US Pacific coast)

Yellowfin sole (Bering Sea/Aleutian Islands)

European anchovy* (South Africa)

Rock sole (Bering Sea/Aleutian Islands)

Pacific Ocean Perch (Alaska/US Pacific Coast)

Ocean perch (Newfoundland)

Alaska plaice (Bering Sea/Aleutian Islands)

Flathead sole (Bering Sea/Aleutian Islands)

Skipjack tuna* (Central Western Pacific)

Arrowtooth flounder (Bering Sea/Aleutian Islands)

English sole (US Pacific coast)

*Indicate good sources of omega-3 fatty acids

Red choices: High mercury, low sustainability

Bluefin tuna (Eastern Atlantic)

Yellowtail flounder (Georges Bank)

Swordfish (Mediterranean)

Spanish mackerel (US South Atlantic)

Gag grouper (US Gulf of Mexico)

Leslie Beck, a registered dietitian, is the national director of nutrition at BodyScience Medical. She can be seen every Thursday at noon on CTV News Channel's Direct. bodysciencemedical.com

Editor's note: Arizona State University was incorrectly identified in the original version of this article. This version has been corrected.

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