Do You Know Which Sushi Contains the Least Mercury and Why?

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In a recent study published in the journal Nature Geoscience, researchers have made a startling new discovery that explains why deep-water fish from the oceans have greater amounts of mercury contamination than shallower water species. It turns out that organic mercury is converted into its poisonous methyl-mercury form by bacteria that thrive in oxygen-depleted ocean depths where particular species of fish used in sushi feed and accumulate mercury poisoning.

Mercury contamination in our oceans is a growing problem that researchers predict will reach highly toxic levels at particular depths in the next few decades. This growing contamination is continuing in spite of environmental laws in the West that limit the amount of organic mercury released by coal and oil burning plants. Researchers have suspected that a major source of organic mercury comes from industrialized countries like China and India via trade winds that deposit mercury in western waters where many species of fish are caught and marketed for western consumers.

Once the mercury is deposited in the ocean, bacteria digest the mercury and convert it into its poisonous methyl-mercury form that eventually makes its way through the ocean food chain into fish. Health officials often warn pregnant women to avoid eating too much seafood during their pregnancy to avoid an increased risk of mercury poisoning that could adversely affect fetal development.

The significance of this research is that it was previously believed that fish species captured closest to the ocean surface would be the ones most likely to have the highest levels of mercury contamination. However, past comparisons between fish species over a range of ocean depths have indicated that just the opposite is true.

Using isotope measurement techniques developed by researchers at the University of Michigan that can identify the type (organic or methylated forms) and sources (western or eastern origins) of mercury in contaminated fish, lead author Joel D. Blumm and colleagues analysed tissue samples from nine species of Hawaiian marine fish that were caught at varying depths where feeding takes place for each particular species.

According to a news release issued by the University of Michigan, what the researchers found was that that perhaps as much as 80% of methyl-mercury found in ocean fish is at depths below what is called the surface mixed layer―a region extending down to about 165 feet below the surface where oxygen levels then become increasingly depleted. The researchers found that methylation continues down to a depth of about 2,000 feet and is most likely the work of anaerobic bacteria attached to sinking particles of dead plant and animal matter containing inorganic mercury.

Why methyl-mercury levels are actually lower in shallow-water fish is believed to be due to a process called photochemical degradation where sunlight penetrates into the shallower water regions destroying up to 80% of methyl-mercury that is formed. At deeper levels, the methyl-mercury made by other types of bacteria remains intact and accumulates in deeper water feeding fish.

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"The crystal-clear waters surrounding Hawaii and the unique information that we had about the depths at which our local fish feed allowed us to clearly identify both the photochemical degradation of methyl-mercury at surface levels and the microbial production of methyl-mercury from inorganic mercury in deeper waters," said study co-author Brian N. Popp.

The species of fish tested in the study, listed from shallowest- to deepest-feeding, were flying fish, mahi-mahi, yellowfin tuna, skipjack tuna, moonfish (opah), big eye tuna, swordfish, and two species of lantern fish.

"We found that predatory fish that feed at deeper depths in the open ocean, like opah and swordfish, have higher mercury concentrations than those that feed in waters near the surface, like mahi-mahi and yellowfin tuna," said Popp.

Furthermore, the researchers determined that the isotope composition of the mercury found in the fish tissues was "a nearly perfect match" with the chemical signature of mercury known to travel from pollution sources such as coal-burning power plants from China and India.

For additional information by health authorities on the risk and benefits of eating fish, click on the following link titled “Which Fish Should I Eat?”

For a warning about one particular type of fish that can cause some embarrassing results, click on the following link titled, “Beware a Fish Laxative Counterfeit Food Warns Dr. Oz Show.”

Image Source: Courtesy of PhotoBucket

Reference: Methylmercury production below the mixed layer in the North Pacific Ocean Nature Geoscience (2013) doi:10.1038/ngeo1918; Joel D. Blum et al.

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