There’s some good news for sushi lovers. A new report finds that over an 8-year period, mercury levels in Gulf of Maine tuna declined 2 percent a year — a decline that parallels reductions in mercury pollution from Midwest coal-fired power plants.
Two years ago, Dr. Nicholas Fisher, a professor of marine sciences at Stony Brook University in New York, had a bit of luck — he found out that a colleague had established a collection of 1,300 western Atlantic bluefin taken from the Gulf of Maine between 2004 and 2012.
“They were frozen, wasn’t the entire fish, just about a pound from each fish or so. And then my colleagues and I in New York dissected out muscle tissue from each sample and analyzed it to determine the mercury content of each fish,” he says.
And as they created a timeline for mercury content for each year, taking into account the age and size of each fish sampled, a clear picture emerged.
“There was a fairly steady decline for all ages of fish, and the decline rate was approximately 2 percent per year, which doesn’t sound all that dramatic, but over 10 years it’s about 20 percent. Over two decades its about 40 percent,” Fisher says.
Most mercury pollution in this region originates from coal-fired plants in the Midwest, drifting east on the prevailing winds to drop on the coast and coastal waters. In response to regulatory and industry efforts, and to market forces, those emissions happen to have been declining by about 2 percent a year.
Same for mercury measured in eastern waterways and off the coast — down roughly 2 or even 3 percent a year.
“So it seemed as if the fish were responding in real-time to the declines of loadings of mercury into the north Atlantic,” Fisher says.
If the trend has continued since the last sample examined, from 2012, Fisher says it’s reasonable to speculate that the amount of mercury in West Atlantic bluefin has dropped by 30 percent since 2002. That could be important for human health, he says, because the highly toxic metal works up the oceanic food chain to accumulate in the flesh of top predators such as tuna and swordfish, and then into the American diet.
“We analyzed these fish for mercury because there is concern about mercury levels in tuna. Tuna are thought to account for up to about 40 percent of all the human exposure to mercury in the United States,” he says, “so they are the single greatest source of mercury for human beings.”
But Fisher is not a toxicologist, and he warns that even with the declines, tuna still carry a very heavy load of mercury. He says more work is necessary to determine whether his findings have any bearing on current government guidelines for fish consumption.
There’s also not much known about whether mercury affects fish health or behavior. He says one of his graduate students is embarking on a research project to try to find out.