Sounds Fishy.

For healthy eaters, the choice between beef and fish can seem like a no-brainer--one is linked to heart disease, and the other is linked to its prevention. Over the past decade, the pro-fish chorus has only grown louder as scientists have discovered that the omega-3 fatty acids found in many fish may ward off heart attacks, strokes and possibly cancer. And salmon, rich in omega-3s, is even promoted as a skin treatment. In light of the latest mad-cow scare, it might appear best to simply substitute fish for beef and stop worrying altogether--no threat of BSE, no heart disease, no wrinkles, no problem. But in the scientifically murky world of food safety, things are never that easy. Fish have problems of their own: their tissues can pick up pollutants from the tainted waters around them, including chemicals that may be harmful to humans, such as mercury and polychlorinated biphenyls (PCBs). And the list of risky species is growing. Last month the U.S. Food and Drug Administration warned women of childbearing age to limit their intake of tuna, America's most popular seafood. The surprising move has left a lot of fish aficionados wondering if it's time to order tofu.

But do the risks of eating fish outweigh the benefits? For most, fish can still safely be considered a health food. In November, the American Heart Association said that it's better to eat fish than to avoid it, especially since there are still plenty of seafood like cod, farmed tilapia and shellfish that remain pure. And while the fish richest in omega-3s often have the highest levels of contaminants, they are still safe, to a point. There's little solid evidence that low levels of contaminants like mercury in fish can harm healthy adults. The only groups that might need to worry are young children and expectant women who might be exposed to mercury--and they can still eat most fish in moderation.

Mercury contamination starts at the bottom of the food chain and the bottom of the ocean, where small aquatic organisms take up the chemical from contaminated silt as they feed. When they themselves become prey, they pass the mercury on to the fish that eat them. The pollutant is never metabolized and builds up in the fatty tissues of large animals. The result is that top predators--the fatty fish most prized by gourmets and health nuts alike--have ...