Mercurial tuna: Study explores sources of mercury to ocean fish

March 2, 2010

With concern over mercury contamination of tuna on the rise and growing information about the health effects of eating contaminated fish, scientists would like to know exactly where the pollutant is coming from and how it's getting into open-ocean fish species.

A new study published in the journal & Technology uses chemical signatures of nitrogen, carbon and mercury to get at the question. The work also paves the way to new means of tracking sources of mercury poisoning in people.

The study, by researchers at the University of Michigan, Harvard School of Public Health, the Louisiana Universities Marine Consortium and the National Institute of Nutrition and Seafood Research in Norway, appears in the journal's March 1, 2010 issue.

Mercury is a naturally occurring element, but some 2,000 tons of it enter the global environment each year from human-generated sources such as coal-burning power plants, incinerators and chlorine-producing plants. Deposited onto land or into water, mercury is picked up by microorganisms, which convert some of it to methylmercury, a highly toxic form that builds up in fish and the animals---and people---that eat them.

The primary way people in the United States are exposed to methylmercury is by eating fish and shellfish. Health effects include damage to the central nervous system, heart and immune system, and the developing brains of young and unborn children are especially vulnerable.

In the current study, the researchers wanted to know if tuna and other open-ocean fish pick up methylmercury by eating contaminated fish that live closer to shore or by some other means. They studied 11 species of fish, including red snapper, speckled trout, Spanish mackerel and two species of tuna. Seven of the species studied live in the shallow, coastal waters of the Gulf of Mexico; the two tuna species live far out in the and are highly migratory; the remaining two species spend parts of their lives in both habitats.

It's no mystery how the coastal fish acquire methylmercury, said Joel Blum, who is the John D. MacArthur Professor of Geological Sciences at U-M. "We know that there's a lot of mercury pollution in the coastal zone. A large amount of mercury comes down the Mississippi River, and there's also air pollution and deposition of mercury from the highly industrialized coastal Gulf region." In this environment, methylation occurs in the low-oxygen conditions of the lower water column and sediments, and the methylmercury wends its way up the food web, becoming more concentrated at each step along the way.

"It's much less clear how methylmercury gets into open-ocean fish species, some of which don't come anywhere close to shore but can still have very high levels," said the study's lead author, David Senn, formerly of the Harvard School of Public Health, and now a senior researcher at the Swiss Federal Institute of Aquatic Science and Technology. Scientists have proposed three possibilities.

One is that open-ocean fish visit coastal areas to feed, picking up methylmercury from the coastal food web. Another possibility is that small organisms that acquire methylmercury in coastal regions are washed out to sea, where they enter the open-ocean food web. In the third scenario, mercury is directly deposited into the open ocean, where it undergoes methylation.

By looking at three chemical signatures in the fish---nitrogen isotopes, carbon isotopes and mercury isotopes--- Senn, Blum and colleagues learned that coastal fish and open-ocean fish are feeding from two separate food webs.

"That rules out the first explanation, that these were getting their methylmercury by feeding off coastal fish," Senn said.

"We think it's unlikely that the mercury is being methylated in coastal sediments and then washed out to the open ocean, so the most likely alternative is that there is deposition and methylation of mercury in the open ocean," Blum said. The finding runs counter to the long-held view that the open ocean is too oxygen-rich to support methylation, but it is consistent with recent studies suggesting more methylation may be occurring in that environment than was previously thought.

"It turns out there are probably low-oxygen microenvironments on tiny particles of organic matter, where methylation may be able to occur," Blum said.

One of the biggest differences the researchers found between coastal and open-ocean fish was in their mercury "fingerprint." The fingerprint is the result of a natural phenomenon called isotopic fractionation, in which different isotopes of mercury react to form new compounds at slightly different rates. In one type of isotopic fractionation, mass-dependent fractionation (MDF), the differing rates depend on the masses of the isotopes. In mass-independent fractionation (MIF), the behavior of the isotopes depends not on their absolute masses but on whether their masses are odd or even.

The researchers found that open-ocean fish have a much stronger MIF fingerprint than do coastal fish, a discovery that opens the door to new ways of analyzing human exposure to mercury.

"We can do an isotopic analysis of the mercury in your hair, and by looking at this mass-independent signal, tell you how much of the mercury is coming from inorganic sources, such as exposure to mercury gas or amalgams in your dental fillings, versus how much is coming from the that you eat," Blum said. "We think this could become a widespread technique for identifying sources of contamination."

Explore further: 'Fingerprinting' method tracks mercury emissions from coal

More information: Environmental Science & Technology:

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3.5 / 5 (2) Mar 02, 2010
German u boats with tons and tons of mercury in their hulls sunk during the war?

Back during the war German u boats routinely replaced the ballast in their subs with Mercury bound to japan, This was because Japan needed it for their war effort. As U boats were sunk the tons and tons of mercury were never recovered and sit in decaying canisters on the ocean floor , to this day.
1.8 / 5 (5) Mar 02, 2010
Nice myth, with just a little bit of truth to it. Of course, most of the mercury has been recovered from the two wrecks that had mercury in them as a partial ballast compensation.

Try not using the word "routinely" with this myth in the future if you want to try to pass it off as some sort of truth again. There were only 3 uboats that had any mercury in them.

As well, you might want to realize that most of the tuna they are talking about are Pacific tuna, while the mercury you're talking about is confined mostly to the Atlantic. You also might want to note that most methylated mercury in our environments comes from burning coal and waste, a much greater amount of mercury than what the few uboats had.
4 / 5 (3) Mar 02, 2010
"According to decrypted intercepts of German naval communications with Japan, U-864's mission was to transport military equipment to Japan destined for the Japanese military industry, including approximately 67 tons of metallic mercury in 1,857 32 kg steel flasks stored in her keel."

Source Wikipedia

U-869, the submarine profiled in the NOVA program "Hitler's Lost Sub," was just one of the more than 1,100 Unterseeboote, or U-boats, sunk, scuttled, captured, or otherwise lost to German forces during World War II.


So we only intercepted one (which we have located and video of the canisters laying on the ocean bed) of 1100 u boats that were lost. Is it impossible there could be more?
1 / 5 (4) Mar 02, 2010
One sub, out of how many? As I pointed out, though, there were 3 subs that were ever verified to have been carrying any mercury. You claimed it was routine, which is part of a myth that went around for awhile.

Again, you also don't address the point of the amounts of methylmercury from coal burning power-plants and waste burning far exceed the amounts ever carried by these UBoats.
5 / 5 (1) Mar 02, 2010
No indeed I do not. I only set out to state the fact that there are other sources to look into, I am not trying to discount the fact that other things contribute. I thought it irresponsible to completely leave out the fact that these things happened.
1 / 5 (2) Mar 03, 2010
They have a negligible small effect, addidis. Maybe they should also look into the bird droppings from European Sparrows, the ones laden with coconuts, because it might add 0.000001% mercury to the oceans, and we wouldn't want to leave out that fact, now would we?
5 / 5 (1) Mar 03, 2010
I would look at the deep ocean vents as a source of mecury. There isn't a river in California that doesn't have mercury in it. Some can be attributed to naturally occuring and some as a result off gold mining activity.
not rated yet Mar 03, 2010
The majority of the mercury in the ocean is natural. The majority of the mercury in the shallow ocean is not.

The above article speaks mainly to the latter and not the former which is primarily introduced by man's industrial waste and not so much from coal burning. Methyl mercury is created biotically in the ocean, as so far I'm unaware of abiotic aqueous methyl mercury sources that would have a significant impact, but I haven't really looked at the issue in depth since the disproving of photodegradation.
not rated yet Mar 04, 2010
They have a negligible small effect, addidis. Maybe they should also look into the bird droppings from European Sparrows, the ones laden with coconuts, because it might add 0.000001% mercury to the oceans, and we wouldn't want to leave out that fact, now would we?

It'd be a higher mercury percentage than the percentage of CO2 that humans emit.

Count the small things or don't count the small things, but pick a consistent scale on which to determine relevance.

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