Mercury levels rising in Pacific tuna, study says

Mercury levels in yellowfin tuna caught in the Pacific Ocean have been rising at a 3.8 percent annual rate since 1998, according to a new study.

The findings, published online Monday in the journal Environmental Toxicology and Chemistry, add to evidence that air pollution, particularly from burning coal, is pumping mercury into the ocean food chain, potentially posing a hazard to human health.

“Evidence is piling up that the methyl mercury has an anthropogenic source,” said University of Michigan eco-toxicologist Paul Drevnick, lead author of the study. “It’s coming from mercury emissions that are falling into the ocean.”

The levels found in yellowfin, a species that is not at the top of the food chain and could be considered a bellwether, are “concerning,” said co-author Carl Lamborg, who conducted the research while at Woods Hole Oceanographic Institution in Massachusetts and now is at the University of California, Santa Cruz.

“What this number is saying is that the amount of mercury in fish is getting higher and higher all the time, and if it keeps going like that, at some point, most every kind of fish is going to be potentially hazardous,” Lamborg said. “Where that point is, I don’t know.”

None of the measured levels of methyl mercury, the kind that is absorbed by the body, are likely to be a current hazard to health, and they probably don’t outweigh the health benefits of a fish-enriched diet, according to the researchers.

But the data appear to undermine an element of the legal argument that kept tuna canning companies from having to post warnings on products sold in California, under Proposition 65. At the time, canners offered scientific studies suggesting that methyl mercury in tuna was coming from natural sources, such as deep ocean vents.

Yellowfin is more often sold to consumers in steak form, but is used in small amounts in canned “light” tuna, which is mostly skipjack, according to SeafoodHealthFacts.org, an academic coalition based at Oregon State University. The FDA has found slightly higher levels in “white” or albacore tuna, but still below one part per million.