Mercury levels in yellowfin tuna caught in the Pacific Ocean have been rising at a 3.8% 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 coauthor Carl Lamborg, who conducted the research while at Woods Hole Oceanographic Institution in Massachusetts, and now is at UC 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.
The state Superior Court agreed in 2006 and also ruled that the Proposition 65 warning was preempted by federal rules, and that methyl mercury levels were too low to merit warnings. The ruling was upheld on appeal in 2009.
The U.S. Food and Drug Administration and the Environmental Protection Agency are updating their recommendations about fish consumption and, for the first time, are recommending that pregnant women's weekly diet include a minimum of 8 to 12 ounces of fish known to be low in mercury, to promote fetal development and growth.
The agencies' draft recommendations still caution pregnant or breastfeeding women to avoid tilefish from the Gulf of Mexico, shark, swordfish and king mackerel. They also recommend that such women limit consumption of white (albacore) tuna to 6 ounces a week.
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.
The researchers re-analyzed data on yellowfin caught near Hawaii in 1971, 1998 and 2008. After adjusting for the age and weight of the fish, which affect mercury accumulation, they confirmed there was no detectable rise from 1971 to 1998. But mercury contamination, nearly all of it in the more hazardous methylated form, started to accumulate rapidly after that date, according to the study. The researchers calculated the rate at 3.8% per year in that time frame.
Mercury in the atmosphere has risen by a factor of three during industrial times, according to several studies. At the same time, levels have crept up in several ocean basins. But tying the two together is not easy, because of complicated chemical and biological processes that change the form of mercury.
A study in 2013, however, used a kind of isotope "fingerprinting" to determine that a substantial portion of the methyl mercury in nine species of fish from the deep northern Pacific Ocean came from elemental mercury in the atmosphere, downwind from a rapidly industrializing eastern Asia.