Bluefin tuna, which migrate long distances and accumulate mercury as they age, can serve as global barometers of mercury pollution, according to a new study.

Health officials in the United States recommend eating no more than a few ounces of tuna per week, and women who are pregnant are advised to abstain.

That's because tuna, especially older, bigger specimens, accumulate methylmercury in their muscle tissue.

Exposure to signficiant levels of methylmercury can disrupt the nervous system. Contamination has been linked to cancer in adults and cognitive impairment in children.

In bluefin tuna, mercury contamination can reach concentrations unsafe for human consumption. However, mercury concentration varies across bluefin tuna populations.

Unfortunately, the global distribution of mercury pollution is poorly understood.

To shed light on the problem, scientists conducted a global survey of mercury contamination levels, revealing geographic variations, as well as the influence of age, size, sex and position in the food web on mercury concentrations.

Using data from dozens of tissue sample surveys conducted between 1998 to 2019, researchers determined Atlantic bluefin tuna caught in the Mediterranean feature the highest rates of mercury accumulation.

Scientists published the results of their survey in a new paper, published online on Monday in the journal PNAS.

Atlantic tuna are the most important tuna species for commercial fishing, and most are caught in the Mediterranean. Atlantic tuna from the North Atlantic feature lower levels of mercury, as do related species, Pacific tuna and Southern tuna, caught in the Pacific and Indian oceans.

The data also showed variation in mercury concentrations correspond with mercury pollution levels measured in seawater and zooplankton samples.

"Our study shows that mercury accumulation rates in bluefin tuna may be used as a global pollution index that can reveal patterns of mercury pollution and bioavailability in the oceans, natural and human caused emissions and regional environmental features," senior author John Reinfelder, a professor of environmental sciences at Rutgers University, said in a press release.

Mercury leaches naturally into the ocean from rocks. It's also released by metal mining, the burning of fossil fuels and other human activities.

Researchers confirmed that geographic variation of mercury pollution reflects the distribution of mercury-emitting human activities.

Previous studies have shown that as mercury pollution levels drop, so do concentrations in tuna and other fish populations.

Unfortunately, in some places, mercury levels continue to rise, increasing concentrations in local tuna populations.

"Overall, mercury accumulation rates provide a means to compare mercury bioavailability among geographically distinct populations of upper trophic level marine fish across ocean sub-basins, to investigate trophic dynamics of mercury in marine food webs and to improve public health risk assessments of mercury exposure from seafood," Reinfelder said.

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