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Diet & Nutrition

Seafood Substitutions High In Mercury

New measurements from fish purchased at retail seafood counters in 10 different states show the extent to which mislabeling can expose consumers to unexpectedly high levels of mercury, a harmful pollutant.
Fishery stock “substitutions”—which falsely present a fish of the same species, but from a different geographic origin—are the most dangerous mislabeling offense, according to research by University of Hawai’i at Mānoa scientists and published in August 2014 in PLOS One.

A release from the university quotes lead author Peter B. Marko, a biologist at UH Mānoa, as saying, “Accurate labeling of seafood is essential to allow consumers to choose sustainable fisheries. But consumers also rely on labels to protect themselves from unhealthy mercury exposure. Seafood mislabeling distorts the true abundance of fish in the sea, defrauds consumers, and can cause unwanted exposure to harmful pollutants such as mercury.”

The study included two kinds of fish: those labeled as Marine Stewardship Council- (MSC-) certified Chilean sea bass, and those labeled simply as Chilean sea bass (uncertified). The MSC-certified version is supposed to be sourced from the Southern Ocean waters of South Georgia, near Antarctica, far away from man-made sources of pollution. MSC-certified fish is often favored by consumers seeking sustainably harvested seafood but is also potentially attractive given its consistently low levels of mercury.

In a previous study, the scientists had determined that fully 20 percent of fish purchased as Chilean sea bass were not genetically identifiable as such. Further, of those Chilean sea bass positively identified using DNA techniques, 15 percent had genetic markers that indicated that they were not sourced from the South Georgia fishery.

In the new study, the scientists used the same fish samples to collect detailed mercury measurements. When they compared the mercury in verified, MSC-certified sea bass with the mercury levels of verified, non-certified sea bass, they found no significant difference in the levels. That’s not the story you would have expected based on what is known about geographic patterns of mercury accumulation in Chilean sea bass.

“What’s happening is that the species are being substituted,” Marko explained. “The ones that are substituted for MSC-certified Chilean sea bass tend to have very low mercury, whereas those substituted for uncertified fish tend to have very high mercury. These substitutions skew the pool of fish used for MSC comparison purposes, making certified and uncertified fish appear to be much more different than they actually are.”

But there’s another confounding factor. Even within the verified, MSC-certified Chilean sea bass samples, certain fish had very high mercury levels—up to 2 or 3 times higher than expected, and sometimes even greater than import limits to some countries.

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