Here are the best sustainable fish choices that are healthy for you—and the planet. Plus, 5 to watch out for.

You probably already know that you're supposed to be eating fish twice a week. Fish are a lean, healthy source of protein-and the oily kinds, such as salmon, tuna, sardines, etc., deliver those heart- and brain-healthy omega-3 fats you've probably also heard you should be getting in your diet.

But then there's also this concern about the environment-and choosing seafood that's sustainable. So, if you're like me, you often stand at the fish counter a little perplexed: what's good for me and the planet? We did some research to find the healthiest fish to eat when it comes to sustainability, mercury content and nutritional benefits.

5 of the Healthiest Fish to Eat

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1. Atlantic Mackerel

This species is a fast-growing fish, meaning it can repopulate easily and handle higher amounts of fishing. The gear used to catch Atlantic mackerel is efficient and not likely to cause major habitat destruction, another reason this guy is an ocean-friendly choice. This strong-flavored fish is high in heart-healthy omega-3s, a good source of protein—delivering 20 grams in a 3-ounce fillet—and pairs well with bold seasonings. Check out our recipe for Korean Grilled Mackerel flavored with rich Korean chili paste and fresh ginger.

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2. Wild-Caught Alaskan Salmon (including canned)

Wild-caught salmon from Alaska is low in contaminants, including mercury and lead, and comes from well-managed fisheries. To give you an idea of how well managed Alaska's salmon fishery is, consider this: biologists are posted at river mouths to count how many wild fish return to spawn. If the numbers begin to dwindle, the fishery is closed before it reaches its limits, as was done recently with some Chinook fisheries. This close monitoring, along with strict quotas and careful management of water quality, means Alaska's wild-caught salmon are both healthier (they pack more than 1,500 mg of omega-3s per serving and carry few contaminants) and more sustainable than just about any other salmon fishery.

Buying salmon in a can also makes a more affordable way to get this healthy seafood in your diet. Canned salmon is not just a great source of omega-3 fats, it is one of the best sources of nondairy calcium. A 3-ounce serving has 18% of your daily needs. Canned wild salmon is typically sockeye or pink from Alaska, but you'll want to check the label to make sure. Try our Quick Lentil Salmon Salad for an easy and nutritious lunch on-the-go.

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3. Sardines, Pacific (wild-caught)

The tiny, inexpensive sardine is making it onto many lists of superfoods and for good reason. It packs nearly 1,200 mg of omega-3 fats per serving and is one of the very, very few foods that's naturally high in vitamin D. Many fish in the herring family are commonly called sardines. It's also one of few foods naturally high in calcium, packing 33% of your daily needs per serving.

Quick to reproduce, Pacific sardines have rebounded from both overfishing and a natural collapse in the 1940s. Get a taste for sardines in our delicious Lemon-Garlic Sardine Fettuccine that even sardine skeptics might enjoy.

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4. Rainbow Trout (and some types of Lake)

Rainbow trout (also referred to as steelhead trout), is one of the best fish to eat when it's farmed in the U.S. or indoor recirculating tanks, according to Monterey Bay Aquarium Seafood Watch. Trout ranks just under canned pink salmon when it comes to omega-3 content and is a good source of potassium, selenium and vitamin B6 while offering more than a day's worth of vitamin B12.

Lake trout is a great alternative when it's sourced from the right places. Seafood Watch advises buying lake trout caught in Lake Superior's Minnesota waters.

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5. Herring

Herring is a Nordic Diet staple and for good reason—it has a higher omega-3 content than sardines, trout and mackerel. It's also an excellent source of vitamin D and selenium. You'll typically find herring that has been canned, cured or smoked on restaurant menus, but it can also be eaten fresh.

Seafood Watch recommends buying U.S. Atlantic herring caught with purse seines or California herring caught with set gill nets. If you haven't befriended your local fishmonger yet, they will help you figure out the sourcing methods of your desired fish.

5 Fish to Avoid

A number of environmental organizations have also advocated taking many fish off the menu. The large fish listed below are just five examples EatingWell chose to highlight: popular fish that are both depleted and, in many cases, carry higher levels of mercury and PCBs. The Environmental Defense Fund (EDF) has also posted health advisories on some of these fish.

1. Bluefin Tuna

The World Wildlife Fund put the bluefin tuna on its list of endangered species, and Seafood Watch warns their populations are depleted and overfished. Bluefin have high levels of mercury and can be high in PCBs, so EDF recommends eating no more than 1 serving per month of this fish.

2. Orange Roughy

This fish lives a long life but is slow to reproduce, making it vulnerable to overfishing. As Seafood Watch puts it: "Orange roughy lives 100 years or more-so the fillet in your freezer might be from a fish older than your grandmother!" This also means it has high levels of mercury, causing EDF to issue a health advisory.

3. Salmon (Atlantic, farmed in pens)

Most farmed salmon are raised in tightly packed, open-net pens often rife with parasites and diseases that threaten the wild salmon trying to swim by to their ancestral spawning waters. Open-net farmed salmon are often given antibiotics to combat diseases, and their food and waste pollutes the ocean. Freshwater-farmed salmon have earned a Best Choice status from Seafood Watch and some open-net systems are rated as Good Alternatives (see more salmon recommendations from Seafood Watch). There is hope that consumer pressure will encourage more farms to continue to adopt better practices.

4. Mahi-Mahi (Costa Rica, Guatemala & Peru)

Imported, longline mahi-mahi, or dolphinfish, is rated as one of the least eco-friendly fish by the Environmental Defense Fund. There is concern about bycatch, including sea turtles, seabirds and sharks, getting tangled in the fishing gear when mahi-mahi is fished. However, mahi-mahi caught in the U.S. and Ecuador with troll lines is ranked under Good Alternative by Seafood Watch and is the better choice if you're hankering for this particular fish.

5. Halibut (Atlantic, wild)