SUNDAY, June 27 (HealthDay News) -- Seawater itself is the reason why mercury in saltwater fish poses more of a health threat to humans than freshwater fish, even though concentrations of the chemical are much higher in freshwater species, according to new research.

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

Methylmercury is a potent neurotoxin that can cause kidney and brain disorders, and even death, the study authors explained in a university news release.

"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 the 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.

The findings, released online in advance of publication in an upcoming print issue of the journal Nature Geoscience, suggest that scientists and policy makers should focus their attention on the effects of mercury in the ocean, rather than in freshwater, she added.

More information

The U.S. Environmental Protection Agency has more about mercury.