Study links warmer water temperatures to greater levels of mercury in fish

Correction: An earlier version of this article incorrectly attributed a report to Scientific American. The report, about a U.S. delegation that was unable to attend an international convention on mercury because of the government shutdown, appeared in that publication, but Environmental Health News was its original source. This version has been corrected.

October 13, 2013

Under the watchful eyes of scientists, a little forage fish that lives off the southern coast of Maine developed a strangely large appetite.

Killifish are not usually big eaters. But in warmer waters, at temperatures projected for the future by climate scientists, their metabolism — and their appetites — go up, which is not a good thing if there are toxins in their food.

In a lab experiment, researchers adjusted temperatures in tanks, tainted the killifish’s food with traces of methylmercury and watched as the fish stored high concentrations of the metal in their tissue.

In a field experiment in nearby salt pools, they observed as killifish in warmer pools ate their natural food and stored metal in even higher concentrations, like some toxic condiment for larger fish that would later prey on them.

The observation was part of a study showing how killifish at the bottom of the food chain will probably absorb higher levels of methylmercury in an era of global warming and pass it on to larger predator fish, such as the tuna stacked in shiny little cans in the cupboards of Americans and other people the world over.

“The implication is this could play out in larger fish . . . because their metabolic rate is also increasing,” said Celia Chen, a professor at Dartmouth College in New Hampshire and one of six authors of the study. “Methylmercury isn’t easily excreted, so it stays. It suggests that there will be higher methylmercury concentrations in the fish humans eat as well.”

Methylmercury is linked to high blood pressure, kidney disease and heart attacks in adults and slow neuro-behavioral development in children. A thousand tons of the contaminant drops onto oceans every year from power plant emissions, and more than 250 tons pour from the land into various waters as a result of deforestation.

Top predators on land and sea have higher levels of mercury because of their prey. It is hard for any organism to release the metal, causing it to accumulate, or biomagnify, as scientists say.

The study, “Experimental and Natural Warming Elevates Mercury Concentrations in Estuarine Fish,” was published in the journal PLOS One in April, and officials at Dartmouth called attention to it ahead of last week’s Minamata Convention on Mercury in Japan.

Delegates from 130 nations at the three-day convention that ended Friday met to sign a treaty that seeks to greatly limit emissions from coal-fired power plants from industrial nations, mining operations in Africa and other sources that pollute oceans.

Every U.S. state has issued fish consumption advisories for mercury, and there is a particular concern among states bordering the Gulf of Mexico over health risks related to eating seafood containing mercury.

In spite of these concerns, there was no U.S. delegation in Japan. A small U.S. contingent rushed there before the convention but was recalled to the United States when the federal government shut down, according to a report by Environmental Health News that appeared in Scientific American.

“No one from the U.S. is here and no one from the U.S. government will walk to the front of the room and sign the treaty in front of the global community,” Joseph DiGangi, an adviser at the International POPs Elimination Network, a group devoted to reducing toxic chemicals, or “persistent organic pollutants,” was quoted as saying.

A 2007 World Health Organization report warned that “eating contaminated fish and shellfish is the main source of methylmercury exposure” and that the metal cannot be cooked out. The WHO recommended that mercury should be “eliminated wherever possible” and that exposure should be reduced.

But the killifish study suggested a future of fish with higher levels of mercury in a warming world, not less.

The list of sources that place mercury in the air and water is long. In addition to power plants and deforestation, there are industrial boilers, tooth fillings, car batteries, cosmetics, medical tools, vaccines and even some soaps.

“The study is the first of its kind to demonstrate, in both field and laboratory conditions, that methylmercury concentrations in killifish increase with temperature,” said the study’s lead author, Jennifer A. Dijkstra, a University of New Hampshire professor who was a researcher for the Wells National Estuarine Research Reserve in Maine when the killifish were observed between July 2009 and September 2010.

“This increase can be propagated up through the food web to fish that are consumed by humans, resulting in greater human exposure to methylmercury,” she said.

The other authors of the study were Kate L. Buckman of Dartmouth; Michele Dionne of the Wells research reserve; David W. Evans, a researcher for the National Oceanic and Atmospheric Administration’s Center for Coastal Fisheries and Habitat Research in Beaufort, N.C.; and Darren Ward, a researcher for the Department of Fisheries Biology at Humboldt State University in Arcata, Calif.

The scientists decided to measure outdoor temperatures in the pristine salt pools where killifish dwell in Maine and set temperatures in lab tanks at the research reserve that matched air and marine warming projections by the world’s top climate scientists.

They found six salt pools of about the same size in wetlands of the Little River estuary along the Gulf of Maine that had higher temperatures at different elevations.

Killifish in the field ate what they normally eat. In the lab they ate feed tainted with methylmercury. In both cases, they fed greedily in warmer water. Because of their higher metabolism, killifish did not gain weight, but they gained more metal than usual.

To determine that, the scientists collected the fish from the wild and labs using nets, then severed their little spines to euthanize them for tests.

Methylmercury accumulation in killifish in a salt pool where the water temperature reached 71 degrees was 400 percent higher than killifish in a pool with cooler water, 64 degrees, over four months of study ending in October 2010.

In the lab, methylmercury accumulation in killifish in tanks with the water temperature set at 80 degrees was 30 percent higher than those in water set at 59 degrees. But that study was shorter, 30 days each in March and May 2011.

“What it suggests is with increased temperature the uptake of methylmercury is going to be higher. . . . You can have higher contamination of fish tissue,” Chen said. “One of the most important effects will be the temperature effect.”

Darryl Fears has worked at The Washington Post for more than a decade, mostly as a reporter on the National staff. He currently covers the environment, focusing on the Chesapeake Bay and issues affecting wildlife.
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