Levels of Mercury Likely to Rise in Some Pacific Ocean Fish

Mercury levels in fish that feed deep in the North Pacific Ocean are likely to rise in coming decades, a recent study suggests. The mercury found in these fish appears to come from coal-fired power plants in industrializing countries in Asia, highlighting the international dimension of the issue, the researchers say.

“Our results are consistent with (but do not prove) that the ultimate source of mercury in the ocean is deposition of elemental mercury from the atmosphere,” says Brian N. Popp, PhD, a professor of geology and geophysics at the University of Hawaii at Manoa, one of the paper’s senior authors. “This finding supports a lot of previous research.”

Dr. Popp was a coauthor of a 2009 paper showing that the depth at which a species of fish feeds is important in determining how much mercury it contains. Fish that feed at deeper depths in the ocean, such as opah and swordfish, have higher mercury concentrations than those that feed nearer the surface—despite the fact that the mercury appears to come from the atmosphere above, carried on the wind from power plants thousands of miles away. The current study helps to explain why this is so.

The researchers showed that up to 80 percent of methylmercury, the toxic form of mercury, is produced below the shallowest ocean layer, and the methylation continues down to a depth of approximately 2,000 feet. This is likely caused by bacteria attached to sinking particles of decaying plant and animal matter that contain organic mercury. By contrast, in clear surface waters, sunlight helps to break down methylmercury.

If current trends continue, conditions will favor increased production of methylmercury and a resulting increase in mercury levels in fish from the North Pacific fisheries, a globally important source of seafood, the researchers say.

“One implication of our work is that if you are concerned with mercury exposure from eating fish and wish to maximize the healthy benefits of fish consumption (polyunsaturated fatty acids) and minimize exposure to mercury, eat only fish that live in the upper ocean, like mahi-mahi and yellowfin tuna,” says Dr. Popp via email.

Earlier this year, the United Nations Environment Programme announced agreement on an internation convention aimed at reducing mercury emissions. The Minimata Convention, four years in negotiation, will be open for signatures at a meeting in Japan next month.