Fish Depth May Affect Mercury Content

A fish auction in Hawaii sells  local a local shallow swimmer called moonfish, or Opah. (Photo: C. Anela Choy).
A fish auction in Hawaii sells local a local shallow swimmer called moonfish, or Opah. (Photo: C. Anela Choy).

By Poncie Rutsch
BU News Service

For years, scientists have noticed that fish swimming the deep seas contain more mercury than their shallow swimming friends. Now, a recent study from the University of Michigan and the University of Hawaii shows why that discrepancy exists.

The researchers collected nine different fish species and measured the mercury accumulated in the fish tissue. The fish they collected ranged from a lanternfish, which swims as deep as five thousand feet below the surface, to flying fish, which leap out of the water and glide through the air.

Researchers used a large net to catch the nine species of fish tested for mercury in the study.  The net weighs 2000 pounds when it's dry, and consists of different bundles that can be selectively opened to catch fish at specific depths. (Photo: Jeff Drazen).
Researchers used a large net to catch the nine species of fish tested for mercury in the study. The net weighs 2000 pounds when it’s dry, and consists of different bundles that can be selectively opened to catch fish at specific depths. (Photo: Jeff Drazen).

The researchers used isotope analysis to determine what kinds of chemical reactions the mercury had undergone. They found that sunlight may help mercury to degrade and published their findings in Nature Geoscience at the end of August.

Fish eat mercury every day when they consume sea plants containing mercury-consuming bacteria. In a pristine environment, their mercury levels remain constant. But because of human activity, fish mercury levels have increased in the past hundred years.

Scientists worried about mercury in fish for decades and have been studying its origins. Mercury in the atmosphere exists in a vaporous, inorganic state that does not cause significant damage to humans. It settles on the sea’s surface, and collects on sea plants. Here tiny microbes digest the mercury and convert it to methylmercury, or organic mercury, which impairs human brain development. The microbes can convert mercury as deep as two thousand feet below the surface. Fish eat the sea plants with methylmercury accruing microbes, and the mercury accumulates in their tissue.

Because the mercury from each source undergoes different chemical reactions, each has a different chemical fingerprint. This makes it fairly easy for scientists to trace mercury in the atmosphere back to its source. The researchers started with the mercury in fish tissue and determined each reaction that had happened to it on its way to the fish.

Although mercury can cause significant damage to developing human brains, lead author Joel Blum said that the increasing mercury doesn’t seem to harm the fish themselves. “Yet,” he said, “if mercury levels double, will there even be fish?”

Previous research linked the mercury in Pacific fish to coal-fired power plants in Asia. According to co-author Brian Popp, mercury levels in Pacific fish are on the rise as Asian manufacturing continues to increase.

“But in the Atlantic,” Popp said, “we see the opposite, probably because of new regulations in North America and Europe.”

The research connects the mercury cycle to the fish we eat. Popp and Blum agree that because even tiny amounts of mercury can cause so much damage, understanding the cycle is vital for choosing which fish to eat and which to avoid.

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