Why deep sea fish contain more toxic mercury than shallow water fish

Ocean fish plucked from great depths are packed with more mercury than are their counterparts fished from shallower waters, according to new research from the University of Michigan and the University of Hawaii at Manoa.

A new paper, published in Nature Geoscience, pinpoints the difference in mercury levels in shallow and deep-water fish as in the amount of sunlight to which the animals are exposed and proposes that most of the mercury that humans consume is produced in the deep ocean. The researchers propose that the findings will be critical in assessing which fish are safe to eat as the mercury content of the Pacific Ocean changes over the next few decades.

“How atmospheric mercury deposited in the surface ocean will impact fish mercury levels (and how it will change over time) requires understanding the mechanisms controlling the depths at which elemental mercury is transformed to organic mercury,” says Brian Popp, professor of geology and geophysics at UH Manoa and a co-author on the paper.

The process through which mercury ends up in fish bedding on supermarket ice shavings begins with oceanic bacteria in the deep, dark ocean. These microbes convert mercury from the atmosphere into monomethylmercury, a form of the compound especially toxic to humans that can accumulate in animal tissue. Little fish snack on those bacteria, taking in that organic compound. Big fish then feast on those little fish, building up mercury in their own bodies – and build up, and build up, since these large fish live long lives packed with mercury-laden meals. Some studies have indicated that high levels of mercury in pregnant or breastfeeding women have been linked to cognitive problems in their children.

Scientists had for a while now known that fish that feed in deep waters, where those bacteria are found, are more toxic than shallow water fish: in 2009, Popp and colleagues from University of Hawaii, Manoa, reported that big fish culled from large depths have higher mercury concentrations than fish harvested in shallower waters. Depth was important – but, at the time, the scientists could not explain why.

To find out, the team partnered with the University of Michigan to measure the stable isotopic compositions of mercury in nine species of marine fish – six predators and three non-predators – that all feed at different depths.

Their analysis found that sunlight initiates chemical reactions that eliminate up to about 80 percent of the toxic compound in the upper, sunlight-bathed portion of the central North Pacific Ocean, near Hawaii.

But the mercury found in fish has to come from somewhere, if not from the shallow water. That means that about 60 and 80 percent of the mercury that ends up in humans is formed in the oxygen-poor, deep water, home to the bacteria that underpin the long-food chain.

“Organic mercury in the well lit upper ocean appears to be destroyed by photochemical degradation faster than it is produced by microbes,” says Popp. “Our work shows that a substantial amount of organic mercury is formed below the surface mixed layer of the ocean.”

The finding comes at a time when booming Asian coal factories – the newest culprits in churning out the compound – are sending more and more mercury into the atmosphere. That mercury ends up in bacteria, and then gets carried up the marine food chain to humans. Research suggests that the Pacific’s intermediate depths are becoming increasingly polluted with emissions from those enterprises.

Still, some progress has been made in curbing mercury emissions. In December of 2011, the Environmental Protection Agency (EPA) announced the first set of national standards to reduce mercury pollution. Those standards are expected to push mercury levels down 80 percent by 2016, compared to 1990 levels. And, at the beginning of this year, some 140 nations adopted the first legally binding international treaty tackling the world’s mercury problem, outright banning or phasing out certain high-mercury content products and setting a timetable for emission reductions.

Fish with the highest levels of mercury include: the King Mackerel, 
Marlin, Orange Roughy, 
Shark, Swordfish, Tilefish, Bigeye, and Ahi Tuna. In the US, tuna is the biggest threat to human health, as a popular fish with a high mercury content. Safer choices with less than 0.09 parts per million of mercury include: Catfish; Tilapia; Whitefish; Haddock; Herring; Flounder; and Salmon.