High levels of methylmercury are known to accumulate in the fatty tissues of large ocean-going fish such as tuna, swordfish and marlins. Though most people do not eat enough of these types of fish to result in mercury poisoning, pregnant women are advised to restrict their intake because of the risk to the fetus.

Now it looks like climate change may make the consumption risk even greater.

Biologists observed higher accumulation of methylmercury in killifish, a small fish, as temperatures increased both in the wilderness of Maine and in laboratories.

The contamination levels in the fish may increase with temperature because the metabolism of the tiny fish speeds up in warmer water, according to the recent research published in PLOS ONE. The cold-blooded fish eat more and thereby pick up more methylmercury from the environment.

Coal-burning power plants produce most of the mercury pollution in the atmosphere. When the mercury falls back to Earth’s surface it lands either in the ocean or on land where it can be washed into lakes, streams, and eventually the ocean. Certain bacteria transform the mercury into methylmercury, a more toxic form. After this bacterial conversion, the methylmercury enters the food chain, accumulating in fatty tissues. When people worry about mercury in their fish and other seafood, it is actually this bacterially-altered form of methylmercury.

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In the lab, the biologists conducting the study replicated the increasing temperatures likely to occur as climate change continues. The highest levels of mercury contamination occurred in the fish in the warmest water (27 degrees Celsius or 80°F). They also fed the fish mercury-enriched food. “The fish in warmer waters ate more but grew less and had higher methylmercury levels in their tissues,” the authors reported in a press release.

In nature, killifish from different salt marsh pools on the coast near Wells, Maine, served to also replicate the effects of varying degrees of temperatures. The water in some pools reached higher temperatures than others, ranging from 18 to 22 degrees Celsius (65°F to 72°F). Once again, the warmer pools held fish with greater levels of mercury, even though they were only feeding on insects, worms and other natural food sources.

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Many larger fish feed on the killifish, varieties of which live around the globe. Mercury in the smaller fish transfers to the larger fish. Mercury continues to build up in the food web, until it reaches the top predator on the planet, us.

IMAGE: Reddish egret catching killifish at north beach, Fort Desoto Park, Florida. (jimgrayimages.com,Flickr/Getty Images)