Mercury in saltwater fish poses more of a health threat to humans than freshwater fish, a new study suggests.

The potentially harmful form of mercury called methylmercury attaches onto dissolved organic matter in freshwater, but latches onto the salt (chloride) in seawater, explained researchers at Duke University.

Methylmercury is a potent neurotoxin that can cause kidney and brain disorders, and even death, the researchers said in the study appearing online Sunday in the upcoming print issue of the journal Nature Geoscience.

"The most common ways nature turns methylmercury into a less toxic form is through sunlight," study author Heileen Hsu-Kim, an assistant professor of civil and environmental engineering, said in a news release.

"When it is attached to dissolved organic matter, like decayed plants or animal matter, sunlight more readily breaks down the methylmercury. However, in seawater, the methylmercury remains tightly bonded to the chloride, where sunlight does not degrade it as easily. In this form, methylmercury can then be ingested by marine animals," Hsu-Kim explained.

She suggested that scientists and policy makers should focus their attention on the effects of mercury in the ocean, rather than in freshwater.