Mercury, an industrial toxin often found in the ocean, may have a harsher or milder affect on fish depending on their depth, according to a press release.
Researchers from the University of Michigan and the University of Hawaii at Manoa School of Ocean and Earth Science and Technology (SOEST), conducted the study, published online Saturday in Nature Geoscience.
The scientists found the fish people eat contain more mercury when they are from the ocean's deeper depths. Photochemical reactions toward the well-sunned surface waters cause a breakdown of organic mercury, contributing in part to the distribution of the toxin.
"A few years ago we published work that showed that predatory fish that feed at deeper depths in the open ocean, like opah and swordfish, have higher mercury concentrations than those that feed in waters near the surface, like mahi-mahi and yellowfin tuna," said co-author Brian Popp, professor of geology and geophysics at UH Manoa. "We knew this was true, but we didn't know why."
As the mercury sinks, ocean bacteria causes it to turn into the monomethylmercury that can accumulate in a fish's tissue. Predatory fish are often found with the most of this kind of mercury because they eat the smaller fish that accumulate it.
"The implication is that predictions for increased mercury in deeper water will result in higher levels in fish," said lead author Joel Blum of the University of Michigan, professor of earth and environmental sciences. "If we're going to effectively reduce the mercury concentrations in open-ocean fish, we're going to have to reduce global emissions of mercury, including emissions from places like China and India."
The research will benefit any fishing areas, like Hawaii, because it will help better understand the distribution of mercury in the ocean. Larger fish like swordfish and tuna are the main ways monomethylmercury make it to humans.
"The results of our research allow us to determine which marine fish are likely to have lower mercury concentrations, and why mercury concentrations are higher in some fish compared to others," said Jeffrey Drazen, associate professor of oceanography at UH Manoa. "This research is exciting because it allows us to gather new insight about both the biogeochemistry of mercury and the ecology of animals living in Earth's largest habitat-the open ocean."
The research will also affect mercury deposition policies and other international treaties on industrial emissions.
"In the next few decades there will be changes in mercury concentrations in the Pacific Ocean, and those changes are likely to be different for surface waters than for deep waters," Popp said. "Understanding the competing processes that produce and destroy monomethlylmercury at different depths in the ocean is critical to tracing its bioaccumulation in fishes and the potential consequences for human food supply."
(The author of this article edited the photo title and caption to remove the reference to a swordfish.)
