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.”