Higher priced foods usually mean healthier options – but that may not be the case with tuna, new studies suggest. The large wild tuna fish that are favored by high end sushi brokers for their unique taste are probably the most likely to contain high levels of mercury, experts say.

In a survey conducted by New Jersey researchers for The New York Times that was published this week, laboratory tests found high concentrations of mercury in a sampling of tuna used in sushi in New York City restaurants, some so high that the U.S. Food and Drug Administration could legally remove them from the market because mercury concentrations exceeded 1 part per million.

To scientists, the findings make sense: Mercury accumulates in fish over the life of the animal, and is concentrated when predators eat other smaller fish. That means that long-lived and predatory species, like the blue fin tuna, are especially effective mercury banks. And the biggest tuna – those that make the best sushi – are the biggest storehouses.

“The first thing you can say is that these levels are very high,” said Grimur Valdimarsson, Director of the Fisheries Industries Division at the United Nation’s Food and Agriculture Organization in Rome. “But if sushi bars in New York are using big old blue fins, I guess you could expect it. The smaller tuna tends to end up in cans, the bigger tuna goes up-market.”

In contrast, smaller fish or even farmed tuna are probably less likely to have high levels of contaminants, experts said. Likewise, canned tuna – generally made from lower quality meat, and smaller types of tuna – is generally less likely to be a problem.

Valdimarsson said that tuna farms now often sell fish before they become too big in part because of rising mercury content as they grow, adding: “They are very much aware of this problem.”

A number of other predatory fish can contain high mercury levels, including shark, swordfish and mackerel. The higher the fish in the food chain, the more it collects mercury. Sharks often have levels of mercury over 2 parts per million, but their meat is rarely consumed in the developed world.

Scientists do not know for sure if the mercury levels seen in the New York sushi samples are typical of sushi in other U.S. cities or in other countries, though they believe they are because most high end tuna globally comes from the same sources. They do not know if tuna from some regions of the world tend to have higher mercury levels than others.

Since tuna spend most of their time far out at sea, scientists believe that most of the mercury they absorb occurs naturally and is dispersed in the oceans by historic volcanic activity. Close to land, industrial sources can emit mercury as a pollutant. And since tuna spawn in shallow waters, local industrial pollution could have some influence on contamination of tuna flesh.

A recent study of frigate tuna in Ghana, found low, safe levels in all the fish there, a maximum of 0.2 parts per million, well under the FAO safe limit and well under results from other parts of the world. “The results of the study suggest a relatively clean marine environment that has not been significantly impacted by mercury contamination, probably due to minimal industrial activity in the region,” the researchers wrote.

In Spain, researchers tested for mercury in a number of fish species and found that fresh tuna generally had the highest levels, up to .662 part per million, although there was wide variation among different tuna fish.

But its hard to know much about the piece of sushi on your plate or the tuna steak in the market. Mercury level “depends on the species, the size, the age and perhaps the fishery,” said John Kaneko at the University of Hawaii. “Tuna supply is global in nature.”

Full Story: http://www.iht.com/articles/2008/01/25/healthscience/tuna.php