According to a new study, mislabeling on seafood is exposing consumers to surprising levels of mercury.

The study by University of Hawaii at Mānoa published in PLos ONE is titled "Seafood Substitutions Obscure Patterns of Mercury Contamination in Patagonian Toothfish (Dissostichus eleginoides) or 'Chilean Sea Bass.'" It found that the use of fish "substitutions," or fish that is presented as the same species but comes from a different place of origin, are misleading and distort "the true abundance of fish in the sea, defrauds consumers and can also cause unwanted exposure to harmful pollutants."

"Accurate labeling of seafood is essential to allow consumers to choose sustainable fisheries," Peter Marko, lead author of the study and UH Mānoa biologist, said in a press release. "But consumers also rely on labels to protect themselves from unhealthy mercury exposure."

In an earlier study, scientists discovered that 20 percent of fish labeled as Chilean sea bass did not match the species' genetic data.

In the most recent study, researchers examined Marine Stewardship Council (MSC) certified Chilean sea bass, which consumers are led to believe come from South Georgia's Southern Ocean waters. The waters are near Antarctica and thus reportedly far away from pollution. They also studied non-certified sea bass, but surprisingly found minimal discrepancies between mercury levels.

"What's happening is that the species are being substituted," Marko said. "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."

In the new study, some samples of verified, MSC-certified Chilean sea bass were found to have two to three times more mercury than expected, sometimes surpassing certain countries' import limits.

"It turns out that the fish with unexpectedly high mercury originated from some fishery other than the certified fishery in South Georgia," Marko said. "Most of these fish had mitochondrial DNA that indicated they were from Chile. Thus, fishery stock substitutions are also contributing to the pattern by making MSC-certified fish appear to have more mercury than they really should have."

Researchers concluded that the source of the fish is very important in understanding its mercury and potential risk levels.

"Because mercury accumulation varies within a species' geographic range, according to a variety of environmental factors, the location where the fish is harvested matters a great deal," Marko said.

"Although on average MSC-certified fish is a healthier option than uncertified fish, with respect to mercury contamination, our study shows that fishery-stock substitutions can result in a larger proportional increase in mercury. We recommend that consumer advocates take a closer look at the variation in mercury contamination depending on the geographic source of the fishery stock when they consider future seafood consumption guidelines."

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Follow Scharon Harding on Twitter: @ScharHar.

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