The first American sushi bar opened in Los Angeles in 1964 and catered to business travelers and corporate expatriates before catching on with American businessmen and Hollywood celebrities in the 1970s. But while sushi becomes more popular, tuna is increasingly fingered as a cause for concern over its potentially toxic mercury levels, a worldwide problem for fish stocks.

Fish consumption is the main source of mercury in Americans' bodies. The mineral gets into waterways, mostly through the burning of fossil fuels like coal, where bacteria convert it to toxic methyl mercury. The smallest fish eat or absorb methyl mercury, which becomes more concentrated as it moves up the food chain into large predatory fish. Mercury binds to the meat and cannot be removed from fish by cooking or removing the fat.

Mercury accumulation in the body can lead to mercury poisoning, which damages the central nervous system. Fetuses and young children are most at risk because of their smaller size and developing nervous systems. Mercury exposure in pregnant women is linked to neurological and behavioral side effects in their children.

Studies haven't yet established minimums for the frequency, duration or amount of mercury exposure that is safe for a developing fetus. Once exposure stops, it takes about six months to a year for mercury to leave the bloodstream.

Long-living, large predatory fish like tuna are the most likely to have mercury contamination. "A lot of the sushi tuna that we have tested had really high levels of mercury," said Peter Fugazzotto of the Turtle Island Restoration Network, which publishes the Web site "Got Mercury?". "There's a lot of variation in mercury levels in tuna that people are eating, so it's a little bit of a Russian roulette game."

The Food and Drug Administration (FDA) has previously considered mercury levels above 1 part per million actionable, and the Environmental Protection Agency's reference dose -- the amount of mercury a person can safely be exposed to daily over a lifetime -- is 0.1 µg per kilogram of body weight daily, which corresponds to 5.8 parts per billion. According to "Got Mercury?" and using FDA data for mercury levels, eating five ounces of albacore tuna would put a 150-pound person at 110 percent of the EPA's limit for weekly mercury exposure.

But it's hard to know if the fish at your favorite sushi bar has unsafe mercury levels. Fish is not often tested for mercury, either by the FDA or by retailers, before it is distributed, and as fish passes from ocean to table its origins become clouded.

"Fish are often moving through six or eight or 10 different hands, from where they come out of the water to where they're being eaten, and that's across continents and days and weeks and sometimes longer," Issenberg said. "There's no piece of documentation that travels with the fish. The fish comes with no provenance."

Regardless of how much a sushi chef may want to provide customers with information on where their meal came from, it's nearly impossible to do. Because they cannot know for sure where their tuna came from, some European sushi restaurants are moving away from tuna altogether, Issenberg said. Others are working with different tuna species instead of the preferred bluefin, or experimenting with other red meats like whale or horse.

Variety may also help avoid accumulation of mercury in the body. Americans don't consume enough fish, studies from the National Academy of Sciences and the Journal of the American Medical Association have concluded, but they should vary their choices and avoid certain species.

Several smaller species like tilapia and trout have low mercury levels, as do shrimp, crab and scallops. Both farmed and wild salmon are low in mercury and high in essential omega-3 fats. As well, fish oil supplements are a safe way to get omega-3 fatty acids because they are generally made from smaller fish like sardines and anchovies.

In 2001, the FDA warned high-risk groups like pregnant women, women of childbearing age and small children not to eat certain fish -- shark, swordfish, king mackerel and tilefish -- and to limit overall seafood consumption to 12 ounces a week, with no more than six ounces of that being tuna.

Some states have tightened their warnings further. Washington advises women of childbearing age and children younger than seven years old not to eat fresh or frozen tuna and to limit canned tuna based on weight.

Wisconsin and Minnesota say at-risk groups should limit halibut, tuna steak and canned albacore to two meals per month.

Grocery chains in California are required to post mercury warnings at their fish counters, and some food retailers, including Safeway and Whole Foods provide the information voluntarily nationwide.

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