Contaminants to watch out for
Mercury and polychlorinated biphenyls (PCBs) are the primary contaminants to be concerned about in seafood, says Tim Fitzgerald, a marine scientist in the oceans program at the Environmental Defense Fund.

"Most mercury that enters the food chain is coming from the atmosphere," says Fitzgerald. "It then gets into the rivers, lakes, seas and ocean bays, where it is converted by bacteria into a form of mercury that is toxic to humans." But it is possible to eat seafood with low levels of mercury without health risk, he says. "Your body flushes it out on a fairly short timescale."

PCBs are a man-made class of chemicals that were designed to withstand high pressure and heat, so while production of PCBs was banned in the late 1970s, the resistant chemicals remain in the environment. Once consumed, PCBs also stay in your body for a long time. "Depending on the mixture, PCBs can take five to 20 years to break down by half," says Fitzgerald.

What is sustainable seafood?
Seafood is harvested two ways: from a farm and from the wild. When determining whether seafood has been sustainably harvested, you have to break it down into these two categories.

Buy wild-caught seafood that has an abundant population. "Numerically, you want to go for seafood species that have a shorter lifespan and reproduce in big numbers," says Sheila Bowman, the Seafood Watch senior outreach manager at Monterey Bay Aquarium. Also look for seafood that was caught using gear that did not damage the ocean floor and did not catch unintended species of fish, called bycatch.

"A net that drags along the bottom of the ocean does more damage than a net that sits up off the bottom," says Bowman. She also recommends looking for fish that were caught with a fishing pole, rather than a net. This is a premium product called "pole-caught," as in pole-caught tuna, and is usually labeled as such.

To find out whether a fish has been sustainably caught, Bowman suggests consulting a seafood guide like that published by the Monterey Bay Aquarium (see below). Your fishmonger can also help with your selection. If you want to learn what seafood is considered safe in your region, follow advisories from your local fish and wildlife department or state health department.

Seafood farms with open-net systems within an ocean can transfer pollutants such as disease and parasites to wild populations. Farmed fish can also escape and impact wild species. "If you move your fish farm out of the natural environment into a pond or tank, you can manage the waste products," says Bowman. If you do choose farmed seafood, do your best to learn if the farm it's coming from is a responsible one.

Top picks for your dinner table
So what type of seafood should you select for your dinner table, and how often can you eat it without risking your health? The short answer, according to Fitzgerald, is two servings of seafood a week, with at least one of the servings being a fatty-type fish, such as wild-caught salmon or small mackerel, so you'll get your omega-3s. "Eat fish that are small and low on the food chain so they haven't accumulated a lot of contaminants," says Fitzgerald.

But he says there are also some mid-size fish that are safe to eat in moderation, such as halibut. "Once a week is probably OK for halibut," says Fitzgerald. "The rule of thumb is if the fish, when whole, is about the same size as you, or bigger, you probably don't want to eat a lot of it."

The top eight seafoods recommended by the Monterey Bay Aquarium, taking into consideration both health and sustainability, are troll- or pole-caught albacore tuna from the U.S. or British Columbia, Canada; farmed mussels; farmed oysters; wild-caught Pacific sardines; wild-caught Oregon pink shrimp; farmed rainbow trout; wild-caught Alaska salmon; and wild-caught British Columbia spot prawns. A second-tier selection, with a focus on sustainability but with fewer health benefits, includes farmed arctic char; farmed bay scallops; farmed U.S. crayfish; wild-caught California, Oregon or Washington Dungeness crab; wild-caught U.S. Atlantic longfin squid; and longline-caught Alaska Pacific cod.