How to Choose the Safest, Healthiest, and Most Sustainable Seafood

Mercury is a health concern—in excess it can cause neurological issues—but keep in mind that all fish contains traces of this metal, since it’s in our water, notes Tim Fitzgerald, director of impact at the Environmental Defense Fund’s Fishery Solutions Center. “It’s about the dose you’re getting,” he says.

One easy rule of thumb? "If a fish is bigger than you, it probably has a lot of mercury," says Fitzgerald. That's because larger fish (like swordfish and tuna) eat little fish, which drives up their mercury levels even more. Still, there are plenty of safe, low-mercury seafood picks, and as long as you're not frequently eating large portions of high-mercury fish, it's not a major issue.

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Turns out, the way fish is raised and caught can also have health implications. You would think that wild-caught would always be the way to go, but that's not the case. While there is concern about the use of chemicals, overcrowding, and disease with farmed fish, farming in closed tanks can be healthy and eco-friendly. "Farmed doesn't inherently mean bad or unsafe or dirty," says Fitzgerald. "Some farms are green and sustainable," while others are bad news.

Despite the scary headlines, we should be eating more seafood, experts say. Fish and shellfish are high-quality proteins, low in saturated fat and calories, and packed with nutrients and necessary fatty acids that help promote heart health.

How to buy: Pick skipjack (light canned tuna) or fresh albacore from the U.S. or Canada. Skip bigeye and imported yellowfin, which are also called ahi and often show up on sushi menus, as well as bluefin—it's the highest in mercury.

Sustainable? Not always; when they're caught with longlines, it can harm sea life. Look for pole-and-line caught.

Nutrition: In addition to protein, albacore and skipjack offer omega-3s, niacin, vitamins B12 and D, and selenium.

Mercury: Most tuna has moderate to high levels of mercury.

How often to eat: Skipjack and fresh albacore, as often as you want. Ahi and bluefin, once a month.

How to buy: Almost all U.S. catfish is farmed in ponds and is eco-friendly.

Sustainable? Yes.

Nutrition: Catfish is a good source of vitamins B12 and D.

Mercury: Low.

How often to eat: As often as you want.

How to buy: Look for tilapia farmed in the U.S. in closed tanks, where pollution is better monitored (making it an eco-friendly choice).

Sustainable? Tilapia farmed in the U.S. and Ecuador are, but a lot of the farmed tilapia sold in the U.S. comes from Asia, where chemical use is a concern.

Nutrition: Tilapia is a super low-calorie option.

Mercury: Low.

How often to eat: As often as you want.

How to buy: The harvesting of clams (wild and farmed) does little to the environment, so all varieties are safe to buy. Most come from the U.S., though imported clams are also eco-friendly.

Sustainable? Yes.

Nutrition: Clams are high in protein and a good source of iron, magnesium, and potassium.

Mercury: Low.

How often to eat: As often as you want.

How to buy: Look for U.S. black sea bass, since it's caught by trap or handline, which is safe for the environment.

Sustainable? Most are not. Many are caught with trawls, which can cause habitat damage like unnecessary bycatch.

Nutrition: It provides protein and vitamin D.

Mercury: Low.

How often to eat: As often as you want.

How to buy: Spiny lobster from the U.S., Mexico, and Caribbean are the most eco-friendly options.

Sustainable? Usually yes, since most species are caught with pots and traps, which diminish bycatch. When additional catching gear like trawls, dip nets, and handlines are used, other sea life may be harmed.

Nutrition: Lobster is high in protein and selenium.

Mercury: Moderate.

How often to eat: Once a week.

How to buy: Avoid farmed salmon, because it's hard to know what type of environment it came from. Wild Alaskan salmon is your best bet. Opt for these species: Chinook, chum, coho, pink, and sockeye.

Sustainable? Wild-caught is, but most farmed salmon is not and has been linked to pollution.

Nutrition: Salmon is a fatty fish (the deep color is an indicator of fat content), so it's packed with healthy omega-3s. It's also a great source of vitamins B12 and D.

Mercury: Low.

How often to eat: As often as you want.

How to buy: U.S. king crab, snow crab, stone crab, southern tanner crab, blue crab, and Dungeness crab are best for the environment.

Sustainable? Mostly. Just avoid Jonah crab (which is prone to overfishing) and imported king crab.

Nutrition: It's got B12 and zinc. Prepare it yourself, since crab products can be high in sodium.

Mercury: Low.

How often to eat: As often as you want.

High levels of mercury are especially dangerous for women who are pregnant or nursing, as well as for young children. Fish that are low in mercury are safe to eat regularly while you're expecting, but avoid these varieties, according to the FDA and the American College of Obstetricians and Gynecologists:

• King mackerel
• Marlin
• Orange roughy
• Shark
• Swordfish
• Tilefish
• Bigeye tuna (often labeled ahi tuna)

The deal is too good to be true. That's a tip-off that the fish is mislabeled, says Fitzgerald. Wild-caught fish is almost always more expensive than farmed fish. While it's possible for wild-caught salmon, for example, to be sold at a discount, it's highly unlikely that it will cost less than farmed salmon. This holds true for fresh and frozen varieties all year.

You don't know where it's from. Look for the country of origin on the packaging—seafood from the U.S. tends to be safer. This label is mandatory on fresh and frozen fish in grocery stores but not in small fish markets. But if retailers add anything to the fish (like breading), they no longer have to list the country, so you won't know where it's from.

The label lists preservatives. You don't want other ingredients, says Fitzgerald. Preservatives like sodium tripolyphosphate can affect seafood. "Scallops are the poster child for this," says Fitzgerald. "If they're in a tray and there's a milky liquid, that almost always means they've been soaked in preservatives, which adds water weight you're paying for."

Want to check a fish not in our chart? Download the Seafood Watch app (free; iTunes), developed by the Monterey Bay Aquarium. You can search for any kind of seafood to find answers about how and where it was raised.