"Eat fish," the advice so frequently offered by health experts, sounds simple. In practice, it isn't. When you step up to the seafood counter, you come with an array of priorities that don't always dovetail: promoting heart health, sidestepping contaminants like mercury, and avoiding the purchase of fish produced in an unsustainable way. A fish that scores high marks in one category may fall short in another. Add in the enormous variety of fish available, and you're in for confusion. "Fish is so inherently different from any other food," says Tim Fitzgerald, a scientist at the Environmental Defense Fund. "There are hundreds of different species caught or farmed in different ways. For someone who doesn't think about it for eight or 10 hours a day, it's daunting."
Here's a breakdown of the three major considerations:
Heart health. The American Heart Association recommends eating fish that contain omega-3 fatty acids, which are linked to improvement in some measures of heart disease risk. The AHA suggests mackerel, lake trout, herring, sardines, albacore tuna, and salmon as good sources of omega-3s—but fried or breaded fish does not count. (Fish is not the only source of omega-3s; plant-based sources, like walnuts and tofu, are also recommended.)
Some researchers say we should be careful to balance the ratio of omega-3s to omega-6s; omega-6s are another class of fatty acids that may help promote inflammation. Alice Lichtenstein, past chair of the AHA's nutrition committee and a nutritional biochemist at Tufts University's Friedman School of Nutrition Science and Policy, says she's not particularly concerned about fish choices based on the omega-3/omega-6 ratio. "I don't think there are enough data to show we should be concerned," she says. "What's important is that we should all start eating fish."
Contaminants. But there's more to consider than your heart. Mercury and other contaminants, including PCBs, can build up to elevated levels in big fishes that are high on the food chain—including species like that albacore tuna that is recommended as an omega-3 source. These contaminants are largely a concern for pregnant women and—because half of pregnancies are unintentional and mercury lingers in the body for a few months—women of childbearing age, says Emily Oken, a physician and assistant professor in the department of ambulatory care and prevention at Harvard Medical School. It's still unclear, she says, whether contaminants are a significant issue for full-grown adults who aren't or won't become pregnant.
The Environmental Defense Fund has a guide to the maximum number of servings of various fish that can be eaten each month. It also has pocket guides for both regular fish and sushi to help you pick the safest fish.
Sustainability. In addition to concerns about your health, there are concerns about the ocean's. Sustainable fisheries produce species that are "abundant, well-managed, and caught or farmed in environmentally friendly ways," says the Monterey Bay Aquarium's Seafood Watch program, which publishes regularly updated pocket guides for several regions of the country. (You can also get them for your iPhone.) Such guides are necessary because there are few reliable rules of thumb for choosing the most sustainable fish, says Geoff Shester, senior science manager of the aquarium's Sustainable Seafood Initiative. Some fishing techniques are fine, but others can accidentally snag other species, including other fish and seabirds, or dredge up the ocean floor. And some farms can pollute the environment or allow farmed fish species to escape into the wild and upset the ecological balance; others are contained systems that may actually improve the environment. For example, some shellfish are more sustainable when farmed, but other seafood varieties, like salmon, are always best for Mother Earth when they've been caught in the wild.
And that's where one of the recommendations for omega-3 consumption runs smack up against ecological concerns. Farmed salmon "is one of the best values in fish that are rich in omega-3s," says Fitzgerald. But because it's inherently difficult to farm in a sustainable way, he says, it's on the "avoid" list. (The EDF's and Monterey Bay Aquarium's lists are identical, combining their expertise on contaminants and sustainability.)