Pollution can seem like a vague, general problem, but sometimes it is specific and personal. People with asthma living in some major cities know to keep tabs on the ozone report in the weather forecast, for example. And frequent anglers should be keenly aware of how much of their catch they put on the dinner table because of mercury contamination in fish. Mercury is a problem for marine fish, as well—particularly the ever-popular tuna.

Mercury emitted by burning coal finds its way from the atmosphere into seawater, but an additional step is necessary to weaponize the heavy metal. Bacteria convert mercury into methylmercury by attaching a carbon atom and three hydrogens, making a molecule that freely wanders into biological tissue and hangs around.

Since predators are made up of all the many critters they eat, this mercury accumulates to greater and greater levels at each step in the food chain. The meaty tuna sits at the top of its food chain, and that means it can contain a lot of accumulated mercury. Because of how much tuna is consumed in the US, the fish actually accounts for about 40 percent of the mercury ingested from fish.

A recent study led by Stony Brook University’s Cheng-Shiuan Lee took advantage of an archive of Atlantic bluefin tuna tissue samples from fish caught between 2004 and 2012 to find out whether mercury levels are improving as regulations and declining coal use reduce North American emissions that drift over the Atlantic.

The researchers analyzed almost 1,300 samples from 9- to 14-year-old fish, each of which were measured, weighed, and examined when they were caught. The fish were caught on commercial boats working the waters of the Gulf of Maine and Gulf of St. Lawrence, mainly, but these tuna travel pretty widely. Older and bigger fish tend to contain more mercury, so the researchers compared fish of the same age over time—all the 9-year-old fish caught each year, for example.

The results showed just how effectively mercury gets concentrated in these apex predators. Tissue samples contained as much as 100 million times the concentration of mercury in seawater. But each age group showed a consistent pattern of declining mercury concentrations—a drop of about 19 percent between 2004 and 2012. Because of the age of the fish, that actually represents changes in the Atlantic Ocean going back to about 1990.

The researchers stress that they can’t say if the same mercury reductions are occurring in other species of tuna or other Atlantic fishes, but nothing we know indicates that the bluefin tuna is unusual in this regard.

The trend measured is remarkably similar to the improvement in North American mercury emissions and Atlantic seawater mercury content. (Mercury pollution in Asia, on the other hand, is still increasing.) The fish seem to be responding pretty quickly to changing emissions, which tells you that reducing mercury pollution further could pay pretty immediate dividends. Federal guidelines have long warned pregnant women to watch how much tuna they eat, although the balance of risks and benefits has recently been reevaluated. But this is not some unavoidable, natural risk—humans put that mercury there.

Environmental Science & Technology, 2016. DOI: 10.1021/acs.est.6b04328 (About DOIs).