Seafood is an important part of a healthful diet. It contains heart-protecting omega-3 fatty acids. It's also a good source of protein and is usually low in saturated fat.

But some seafood can contain significant amounts of mercury, a potentially harmful heavy metal that's present in the ocean due to both natural causes and human activities and accumulates in fish as they feed. Studies of fish-eating populations have linked low-level mercury exposure in pregnant women and young children with subtle impairments in neurological and behavioral functioning, such as hearing, eye-hand coordination and learning ability.

Swordfish, shark, tilefish and king mackerel are very high in mercury. Other species -- including Chilean bass, halibut, American lobster and Spanish mackerel -- occasionally contain as much mercury as those most-contaminated fish.

Then there's tuna. On average, albacore -- also known as white tuna -- contains higher mercury levels than solid-light tuna, which in turn contains more than chunk-light, according to limited data from the Food and Drug Administration. Thus, canned light tuna has long been recommended as the safer choice.

Yet our recent analysis of FDA data has shown that canned light tuna sometimes harbors mercury at albacore-like levels. Government test results we scrutinized indicated that 6 percent of the light-tuna samples contained at least as much of the metal, in some cases more than twice as much, as the average in white tuna.

The FDA has not warned consumers about those occasionally higher mercury levels because it believes they don't pose any significant threat.

But our fish-safety experts note that because some cans of canned light tuna are much higher in mercury than average, there's enough uncertainty about the safety of even brief exposure of a fetus to such higher levels that a more cautious approach is warranted. Based on the FDA data and the mercury-exposure levels that the Environmental Protection Agency deems acceptable, we think it's prudent for pregnant women to avoid canned tuna entirely. As for other at-risk consumers:

Young children (up to about 45 pounds) can safely eat about one-half to one 6-ounce can of chunk-light tuna per week, or up to one-third of a can of solid-light or white tuna, depending on their weight.

Older children (45 to 130 pounds) should eat no more than one to three cans of chunk-light tuna per week, depending on their weight. (Government regulators assume that the heavier the child, the more mercury can be safely consumed.) Or they could have one-third to one can of solid-light or white tuna per week.

Women of childbearing age who aren't pregnant are advised to eat no more than about three chunk-light cans per week, or one can of solid-light or white tuna. That's because mercury can linger in the body after you stop eating fish.

Men and older women, meanwhile, can confidently eat the same weekly intake that's considered safe for women of childbearing age who are not pregnant. They can quite likely eat more without harm, our experts say, but the exact amounts are not known.

For worry-free seafood eating, consider wild salmon, shrimp and tilapia. These species have such consistently low mercury levels that everyone, including pregnant women and young children, can safely eat them. Hake, sardines, crawfish and pollack (often used in fish sticks) are also OK for women of childbearing age, although young children may want to also have limited intake of these species.

Do you want the benefits of fish without actually eating it? You can get protein from reasonably low-fat sources such as skinless white chicken, lean beef or pork, tofu and beans. And fish-oil supplements appear to be a safe, reliable way to get omega-3 fatty acids. Our 2003 tests of those pills found no problems with quality or worrisome amounts of any heavy metals we assessed -- including mercury.