New research on the mislabeling of fish sold at retailers in 10 different states demonstrates that this false advertising can expose consumers to high levels of the harmful pollutant mercury.

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As well as allowing consumers the option to select fish harvested from sustainable fisheries, accurate labeling is important for protecting consumers against unhealthy mercury exposure.

Although there are many health benefits associated with eating fish, most commercially harvested fish are contaminated with mercury – a neurotoxin that is especially damaging to the developing nervous system.

Within the general environment, mercury is present only in small quantities, but it accumulates in living organisms, such as fish. The amount of mercury prevalent in fish varies among species but tends to be found in higher concentrations in the larger, longer-lived predatory fish.

The Food and Drug Administration (FDA) advises pregnant and nursing women, women who may become pregnant and young children not to eat shark, swordfish, king mackerel and tilefish – which are known to have high concentrations of mercury – and to eat no more than 12 oz per week of species of fish that have lower levels of mercury.

However, mercury concentration can also vary within species, depending on a variety of factors, though this is less widely known. The region from which fish were harvested can strongly influence mercury concentrations. For example, recent studies have shown that Chilean sea bass harvested from higher latitudes in the Southern Ocean have lower mercury concentrations, compared with fish from lower latitudes.

As well as allowing consumers the option to select fish harvested from sustainable fisheries, the researchers behind the new study argue that accurate labeling is important for protecting consumers against unhealthy mercury exposure.

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It transpired that the fish with the very high mercury content originated from fisheries other than the MSC-certified fishery in South Georgia.

Mislabeling may occur when less desirable species of fish are substituted for more expensive ones, but also when fish of the same species are labeled as being from incorrect fisheries.

The researchers, from the University of Hawai’i at Mānoa (UH Mānoa), argue that these fishery stock substitutions are the most dangerous mislabeling offense. They publish their findings in PLOS One.

The study examined brands of Chilean sea bass that had been labeled as certified by the Marine Stewardship Council (MSC) and brands that had not been certified, purchased from retail counters across 10 different states. MSC-certified Chilean sea bass are advertised as being sourced from the Southern Ocean waters of South Georgia – higher latitude waters far away from man-made pollution – and should be consistently low in mercury.

However, when the researchers compared the MSC-certified and non-certified Chilean sea bass, they found no significant differences in the levels of mercury.

UH Mānoa biologist Peter B. Marko explains:

“What’s happening is that the species are being substituted. 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.”

What is more, even within the MSC-certified samples, Marko’s team found that some fish had mercury levels up to two or three times higher than expected, even exceeding the import limits to some countries.

It transpired that the fish with the very high mercury content originated from fisheries other than the MSC-certified fishery in South Georgia.

“Most of these fish had mitochondrial DNA that indicated they were from Chile,” Marko says. “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.”

The researchers emphasize that because accumulation of mercury varies within a species’ geographic range according to environmental factors, the location where fish are harvested is very important. By misrepresenting the origins of their fish, the researchers state that retailers are not only defrauding customers but also putting their health at risk.

Marko concludes:

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.”