Warming oceans may cause higher mercury levels in fish

Poisonous fish may be just another unforeseen consequence of climate change Photo: Killifish, NOAA

WASHINGTON, October 4, 2013—Higher ocean surface temperatures triggered by global warming may cause fish to accumulate larger amounts of mercury, posing an increased health risk for seafood eaters, according to a Dartmouth University study published in the Journal “PLOS One.”

In the first study of its kind, researchers examined the effect of rising ocean temperatures on mercury bioaccumulation in fish using both laboratory and field tests.

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Mercury pollution is mainly caused by coal-fired power plants, steel producers, incinerators, and cement makers that release it into the air. Other sources of mercury pollution are thermometers, batteries, consumer electronics and automotive parts that are manufactured, utilized or disposed of incorrectly. This pollution accumulates in oceans and waterways and is turned into methylmercury by bacteria found in the water.            

Moving its way up the food chain as larger fish eat smaller contaminated fish, mercury fails to dissolve and instead accumulates in a fish’s body at increasing levels. The mercury concentration in the bodies of large predatory fish who live a long time, including swordfish, tuna, mackerel and sharks, can be over 10,000 times higher than that of the surrounding environment, according to the Nature Conservancy.

Odorless and invisible, mercury in fish is difficult to detect and cannot be eliminated by trimming the skin or removing the bones. In the human body, mercury acts a neurotoxin that interferes with nervous system and brain functions.

Pregnant women and children are especially at risk for mercury poisoning, since a child’s brain is developing and absorbing a large amount of nutrients during the first years of life. High doses of mercury exposure prenatally and in infants can cause cerebral palsy, deafness, blindness and other developmental problems. Lower doses cause a host of development issues, including learning disabilities, shorter attention spans, and a delay in walking and speaking.

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Mercury poisoning in adults can cause tumors, memory and vision loss, difficulty regulating blood pressure, fertility issues and is increasingly being linked to heart disease.

For the study, the scientists analyzed the mercury levels in killifish, Fundulus heteroclitus, in a series of different water temperatures, both in laboratory tanks and six coastal salt flat marsh pools in Little River Estuary, Maine. The fish in the lab were fed mercury-enriched food, while the fish in the marsh pools fed on natural food sources including plankton, worms and insects living in the water.

Fish in the marsh pool were tested using the natural temperature variations found in the pools, while scientists manipulated water surface temperature in the lab tanks to be consistent with predicted climate change trends. Researchers then compared bioaccumulation in each group, finding that in both cases mercury concentration rose significantly at elevated water temperatures.

Researchers also found that in field experiments, temperature—rather than salinity or mercury concentration in sediment—determined mercury accumulation in the fish’s bodies.

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Finding that both groups of fish ate more but grew less at higher water temperatures, researchers hypothesize that an increase in the killifish’s metabolic rate caused by warmer water is responsible for the rise in mercury. In other words, the more the fish eat, the more mercury that is left over in their bodies. 

The study is the first to link changes in water temperature to higher concentrations of mercury in fish through field and scientific experiments. Even though more research is needed, the results of the study indicate that warmer oceans could result in higher mercury levels in fish, which could in turn increase human exposure.


READ MORE: A World in Our Backyard by Laura Sesana

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Laura Sesana

Laura Sesana is a writer and DC, Maryland attorney, joining the Communities in 2012.  She is the author of Colombia: Natural Parks, and has also written several articles on literary criticism.  She writes about food, health, nutrition, women’s legal issues, and the environment.  

In addition to writing for the Communities, Laura also works as an attorney and legal content writer.


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