U-M researchers and their University of Hawaii colleagues say they’ve solved the longstanding mystery of how mercury gets into open-ocean fish, and their findings suggest that levels of the toxin in Pacific Ocean fish will likely rise in coming decades.
Using isotopic measurement techniques developed at U-M, the researchers determined that up to 80 percent of the toxic form of mercury, called methylmercury, found in the tissues of deep-feeding North Pacific Ocean fish is produced deep in the ocean, most likely by bacteria clinging to sinking bits of organic matter.
The study also confirmed that the mercury found in Pacific fish near Hawaii likely traveled through the air for thousands of miles before being deposited on the ocean surface in rainfall, said U-M environmental scientist Joel Blum. The North Pacific fisheries are downwind from rapidly industrializing nations such as China and India that are increasingly reliant on coal-burning power plants, a major source of mercury pollution.
“This study reinforces the links between mercury emitted from Asian countries and the fish that we catch off Hawaii and consume in this country,” said Blum, the lead author of a paper published online Aug. 25 in Nature Geoscience.
“The implications are that if we’re going to effectively reduce the mercury concentrations in open-ocean fish, we’re going to have to reduce global emissions of mercury, including emissions from places like China and India,” Blum said. “Cleaning up our own shorelines is not going to be enough. This is a global atmospheric problem.”
The main pathway for human exposure to methylmercury is the consumption of large predatory marine fish such as swordfish and tuna. Effects of methylmercury on humans can include damage to the central nervous system, the heart and the immune system. The developing brains of fetuses and young children are especially vulnerable.
In December 2011, the Environmental Protection Agency released new standards sharply limiting future emissions of mercury and other toxic pollutants from coal- and oil-burning power plants in the United States. Earlier this year, the United Nations Environment Programme negotiated the Minamata Convention on Mercury, an international treaty aimed at curbing future mercury emissions.
It has been known for some time that large predatory marine fish contain high levels of methylmercury in part because they eat lots of smaller, mercury-containing fish. The toxin builds up in the tissues of the top-of-the-food-chain predators through a process called bioaccumulation.
In 2009, researchers at the University of Hawaii determined that the depth at which a species of fish feeds is nearly as important as its position in the food chain in determining how much methylmercury it contains.
“We found that predatory fish that feed at deeper depths in the open ocean, like opah and swordfish, have higher mercury concentrations than those that feed in waters near the surface, like mahi-mahi and yellowfin tuna,” said Brian Popp, a professor of geology and geophysics at the University of Hawaii at Manoa and co-author of both the 2009 paper and the new Nature Geoscience paper. “We knew this was true, but we didn’t know why.”
That observation was difficult to explain because researchers had presumed that if methylmercury production occurs in the open ocean, it most likely takes place in the biologically active surface layer, carried out by microbes that convert inorganic mercury into the toxic organic form through a process called methylation.
