That Chilean sea bass from the local grocery store could have twice the methylmercury that's expected - if it comes from a region other than indicated on the label, a new study says.

While fish certified by the Marine Stewardship Council (MSC) is generally considered safe, seafood from regions with high levels of contamination are not. And researchers studying samples from U.S. retail stores found that many fish are indeed the species they are claimed to be, but not from the region claimed.

"Chilean sea bass is already known to sometimes have high mercury levels," lead author Peter Marko, of the University of Hawai'i at Manoa, Honolulu, told Reuters Health.

"If women are pregnant or nursing, they probably shouldn't buy that fish, to be safe," he said.

Past research has found that fish sold in retail markets is not always the species it's advertised to be. And that even within a given species, mercury levels can vary widely.

Methylmercury, the type of mercury found in fish, is an organic compound that can be absorbed into living tissue.

Pregnant and nursing women and kids have been advised by the U.S. Food and Drug Administration to avoid shark, tilefish, swordfish and King Mackerel because these species have a mean mercury level of 0.73 to 1.45 parts per million. The FDA's limit for mercury in fish for human consumption is 1.0 ppm.

Normally, the mercury content of Chilean sea bass, also known as Patagonian toothfish, is 0.35 ppm, according to the FDA.

In the current study, published in the journal PLOS One, researchers used sea bass tissue samples from retailers in 10 U.S. states. They measured the total amount of mercury in 25 of the MSC-certified and 13 of the uncertified Chilean sea bass samples.

They found that fish labeled as certified had less than half the mercury (0.35 ppm) of uncertified fish (0.89 ppm).

But when the researchers excluded the fish that actually belonged to other species and were not genetically sea bass, they found no significant difference in the mercury levels of certified and uncertified fish.

"We then said, 'that can't be because certified is supposed to come from South Georgia, where the mercury level is low, why do we see such a difference in mercury?'" said Marko, referring to a fishery area close to the South Pole and known to have less mercury contamination than fish from waters off South American. "It's these fishery stock substitutions," he said.

The researchers tested the DNA of the fish and found those from outside the MSC-certified South Georgia/Shag Rocks fishery had twice as much mercury (0.63 ppm) as those genetically confirmed to be South Georgia stock (0.31 ppm).

"Regular mercury exposure is potentially dangerous to developing nervous systems, so this and other studies like it are of greatest concern to pregnant women, children, and women planning on having children," Marko said in an e-mail.

"Our study demonstrates that accurate labeling of seafood - not just with respect to what species but also what country or region the seafood came from - is essential to consumers, particularly in the aforementioned demographic, to make informed choices at the seafood counter," he said.

Marko pointed out that fish from South American waters can have two-to-three times as much mercury as fish from MSC-certified regions.

Roberta White, professor and chair of Environmental Health at Boston University School of Public Health, who was not involved in the study, told Reuters Health in a phone interview the findings were another reminder that consumers need to be careful when purchasing fish.

"What's really disturbing is how do people choose to eat fish that are safe?" said White, who has studied the effects of industrial pollutants on the brain.

"Everybody wants people to eat fish because it is good for the brain and heart, but we also don't want them to be poisoning their children because they're pregnant," she said.

White said future studies needed to focus on different species of fish and the genetics within species, as well as variations in neurotoxicants. Other contaminants in fish could also pose a health danger, including Polychlorinated Biphenyls (PCBs), which are synthetic organic chemicals, organic tin and different pesticides, she said.

"As this article points out, sometimes you think something is safe because of the way it's labeled and maybe it isn't, but that's true of all our food," White said.

"This is where you have to start, the simple stuff," White said. "I think what's important about the study is the public health message that we need to be careful about this and figure it out," said White.

SOURCE: bit.ly/1vBpKRH PLOS One, online August 5, 2014.