So what are the best choices for both the heart and the brain?
Salmon and oysters top the list as high in heart-healthy omega-3 fatty acids and very low in mercury, and there are numerous other low-mercury choices, too.
Yet the government has no consumer-friendly list of its own mercury testing results to help people of different ages choose seafood.
In fact, the good news about low-mercury choices has been far overshadowed by a battle over which fish the Food and Drug Administration should warn people most at risk from mercury, pregnant women and young children, to avoid. That controversy made headlines again recently as the FDA grappled with whether certain types of ever-popular tuna should be on the do-not-eat list for those people.
The potential backlash effect, even mercury critics acknowledge, is that many Americans could be scared away from fish in general — a bad choice.
"It's really unfortunate," especially for middle-aged people who are most in need of fish and least at risk from mercury, says Dr. William S. Harris of the American Heart Association.
His organization recommends that most people eat a variety of fish rich in omega-3s at least twice a week, even more for those diagnosed with heart disease.
"The message should be: `Eat more fish for your health while minimizing your mercury intake,"' adds Ned Groth, a scientist with Consumers Union, a nonprofit group that is pushing the FDA to publicize low-mercury choices.
Mercury pollution washes into waterways and builds up in fish. The bigger the fish, the more mercury it contains.
Over time, the metal can accumulate in fish-eaters' bodies, too. High enough levels can damage the growing brains of fetuses and young children. About 8 percent of women of childbearing age have enough mercury in their blood to put a fetus at risk.
Far less is known about the potential dangers of mercury-containing seafood in other people. Consumer advocates say about 3 million people are extreme seafood lovers, eating so much of it per week that, depending on what varieties they choose, they might be at risk, too.
Still, exposure by fetuses and young children are clearly the biggest concern. The FDA's scientific advisers recently urged the government to stress low-mercury choices for women of childbearing age and youngsters, so the FDA is rewriting its seafood recommendations. The new list is due out next spring.
For now, a review of FDA's mercury measurements in 39 seafood varieties shows:
Salmon, oysters, whitefish, sea bass, freshwater trout and sardines contain both high levels of heart-healthy omega-3s and low mercury levels, below 0.13 parts per million.
Other low-mercury choices include perch, king crab, flounder, sole, pollock, catfish, croaker, scallops, crawfish, shrimp, clams and tilapia. They contain less omega-3s, but servings can add up.
Tuna is controversial, because different varieties contain different amounts of both mercury and heart-healthy fats. Canned light tuna contains a small amount of omega-3, about as much as shrimp, and fairly low 0.13 ppm mercury. But fresh tuna steaks and the more expensive canned white or albacore tuna contain three times as much mercury, and almost as much omega-3 as salmon.
That puts albacore in the medium-mercury range. So many consumer groups recommend that pregnant women and children stick to modest amounts of the lower-mercury light tuna — about 9 ounces a week for women and 3 ounces for youngsters.
Also in the medium-mercury range are saltwater trout, bluefish, lobster, halibut, haddock, snapper and crabs. Grouper and orange roughy are at the high end of this group. FDA's advisers said women of childbearing age probably should limit these fish to a serving a week.
The FDA advises women of childbearing age to avoid shark, swordfish, king mackerel and tilefish, which contain the most mercury of species tested to date.
Go easy on fried and breaded fish like fish sticks; heart-harming grease outweighs the omega-3s.
Check local mercury advisories if you're eating fish from local lakes and ponds, which can be much more polluted than the sources of commercial fish.
For most men and postmenopausal women, mercury concern plummets and the main message is to eat a variety of fish and more of it, the heart association says.
"If they're eating the same fish day after day, that's probably not wise," said Harris, a researcher at the Mid-America Heart Institute in Kansas City, Mo. "It's probably good to mix it up."