Fish Can Recover Surprisingly Quickly From Mercury Pollution

If the chemical stops leaking into freshwater ecosystems, its concentration in some fish species can drop by more than 75 percent

A close-up image of a pike swimming. It has green-tinted scales, a protruding mouth and beady eyes.
Though pikes (picture above) had the highest concentration of methylmercury in their bodies, they recovered faster than other species.  Dirk Godlinski

In a hopeful new study, scientists found that fish populations can quickly bounce back from mercury pollution once it stops seeping into their ecosystem, Adam Vaughn reports for New Scientist.

Over the course of 15 years, scientists in Canada studied the effects of mercury on a lake and its inhabitants. For the first seven years, they leaked mercury into the experimental lake—set aside just for research purposes—by adding it the water or to upland areas that washed into the lake. For the next eight years, they monitored how the ecosystem recovered, Kate Baggaley reports for Popular Science.

Once mercury is in the water, microorganisms convert it into a more toxic form called methylmercury. Three years after researchers stopped adding mercury, the concentration of methylmercury in the water dropped by 81 percent. By the end of 15 years, the concentration in the fishes' bodies fell by 38 percent in lake whitefish, 76 percent in pike and 85 percent in small fish, Popular Science reports. The researchers published their findings last week in the journal Nature.

"I can’t imagine a much faster recovery," lead author Paul Blanchfield, a research scientist at Fisheries and Oceans Canada, tells New Scientist.

Mercury is a naturally occurring element in Earth's crust, but dangerous human activities—such as burning fossil fuels and gold mining—have driven widespread mercury pollution. When it ends up in water and converted into methylmercury, it can build up in critters like shellfish, fish and the animals that eat those organisms—including us. Nearly everyone in the world has some traces of methylmercury in their body, according to the Environmental Protection Agency.

"[Methylmercury] tends to increase with every step in the food chain," Blanchfield says. "The fish at the top of the food chain are the ones we humans eat, and they have the highest concentrations of methylmercury—often up to a million times higher than in [the surrounding] water."

Upon analyzing the tissue samples, the team discovered that an individual fish's mercury levels don't really change much over time. "However, when we looked at the entire population, we saw that it was declining quite rapidly," Blanchfield tells Popular Science.

This evidence suggests that young fish drove the recovery. By the time the researchers stopped adding mercury, the predatory pikes had the highest concentration of it in their bodies. At a population level, however, their concentrations decreased two times faster than the other large fish in the lake, the whitefish. This difference likely occurs because pike reproduce faster—and don’t live as long—as whitefish, so the methylmercury didn't build up as much in the young pike, New Scientist reports.

Though mercury pollution is an ongoing problem, the evidence that fish populations can bounce back so quickly offers a bit of good news for communities that rely on fisheries. Plus, the paper's findings can be used to advocate for stronger efforts to slash mercury population, New Scientist reports.

"These scientific advances will allow policymakers, resource managers and communities dependent on subsistence fishing to make better lake-specific predictions about the magnitude and timing of mercury reductions," co-author Brian Branfireun, an environmental scientist at Western University in Canada, says in a press release.

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