Pollution from Asia Linked to High Mercury Levels in Pacific Fish

Getting India and China to reduce their mercury emissions will likely lead to healthier fish in the Pacific. A new study has found that mercury levels in fish that are caught off the coast of Hawaii are high due to toxins released from Asian countries. Researchers have even predicted that levels of mercury in North Pacific fish will rise with increase in global pollution.

The study was conducted by researchers at University of Michigan and their colleagues at University of Hawaii who found that about 80 percent of all methylmercury found in deep-water fish is produced by bacteria that reside deep in the ocean. Methylmercury is a toxic form of mercury. Their research also found that nearly all the mercury found in Pacific fish likely comes via air before being deposited on ocean surface.

"This study reinforces the links between mercury emitted from Asian countries and the fish that we catch off Hawaii and consume in this country," Joel Blum U-M environmental scientist and lead author of the study said. According to Blum, North Pacific fisheries are downwind from developing countries such as China that use coal as a major source of energy.

"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 in a news release. "Cleaning up our own shorelines is not going to be enough. This is a global atmospheric problem."

Mercury exposure is associated with brain and kidney damage and poor health of heart and immune system. Pregnant women can pass mercury to their babies and so are advised to lower their consumption of foods that are known to contain high levels of mercury such as sea-food.

Previously, it was believed that mercury is accumulated in fish via a process known as bio-accumulation where fish get mercury in their system by eating smaller, mercury-laden fish.

The latest study shows that a major source of methylmercury are the anaerobic bacteria that live in a region called surface mixed layer, which extends to about 165 feet below the ocean surface. Researchers found that methylation also occurred at 2,000 feet, mostly carried out by bacteria that feed on dead matter such as plants or animal bits that have inorganic mercury.

Other researchers estimate that the levels of mercury, in the future, will rise at the intermediate depths (660 to 3,300 feet) in the North Pacific region. Also, many regions of the world will see a rise of oxygen-deprived zones, meaning that anaerobic bacteria will be producing plenty of  methylmercury in the future.

In the study, Researchers used isotopic measurement techniques developed at U -M to measure the levels of mercury in the tissue samples obtained from nine species of marine fish that live at different depths in Hawaii.

The research team found that methylation does occur at open, well-lit ocean surface, but this mercury usually breaks-down in the presence of sunlight, a process called as photochemical degradation.

The mercury in the samples, however, matched with those found in pollutants from Asian countries.

"These results strongly support the hypothesis that long-range transport of mercury deposited to the ocean surface is ultimately what's ending up in these fish," Blum said.

The study is published in the journal Nature Geoscience.

The Dietary Guidelines for Americans recommends that women, pregnant or breastfeeding, to eat at least 8 ounces but no more than 12 ounces of seafood each week but avoid certain types of fish that are high in mercury. Fish to be avoided include shark, swordfish, king mackerel, and tilefish along with any uncooked fish.