Chilean sea bass is associated with higher levels of methylmercury, a highly toxic substance that fish absorb at a greater rate — but hardly in the way we once thought, according to new study published in the journal PLOS ONE.

A large reason for this has to do with "substitutions," or when fish of the same species are generically labeled depite their different geographic origins. Sea bass caught in the higher latitudes of the Southern ocean are often contaminated with more mercury than sea bass caught in lower latitudes. Not acknowledging this difference conceals the likelihood of mercury poisoning to the consumer.

"Accurate labeling of seafood is essential to allow consumers to choose sustainable fisheries," Peter B. Marko, lead study author and biologist at the University of Hawaii-Mānoa, said in a press release. "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."

Marko and his team looked at two kinds of fish: Chilean sea bass labeled by the Marine Stewardship Council (MSC), known for their sustainable, low-mercury seafood, and Chilean sea bass not labeled by the MSC. The fish labeled by the MSC come from the South Georgia waters, near Antartica, where there is considerably less pollution.

While reasearchers did not find a huge difference in mercury levels between the two fish, they found the MSC-certified fish to have high levels of mercury — much more than is allowed to be imported into most countries, including the United States. "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," Marko said. "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."

Chilean sea bass is larger in size than most fish. According to Marko, the larger (and older) the fish, sea bass or not, the more mercury they absorb. Pregnant women should be especially cautious, since seafood can complicate pregnancy.

Since fish is such a great source of the heart-healthy fats that lessen a person's risk for heart disease, the solution isn't to eliminate it altogether. A better idea is to get in the habit of referencing sustainable food resources, like the Monterey Bay Aquarium, in order to stay on top of which fish are the least and most contaminated.