Scientists at the US Geological Survey sampled fish in 291 streams across the US and found mercury in every one. About a quarter of the fish were found to contain enough mercury to push consumption of the metal above the Environmental Protection Agency (EPA) recommended limit of 0.33ppm in people who eat about two fish meals a week.
Mercury in fish has been a public health issue for some time. The FDA says the metal can build up in the blood stream provoking reproductive problems in women and damaging the development of the nervous system in children. The regulator therefore recommends that people limit their fish consumption to about two meals a week.
As well as investigating the extent of the mercury problem in fish, the latest study from the US Geological Survey identified causes and highlighted geographical variations.
It said that some of the highest levels of contamination were found in the south and south east of the US in states such as North and South Carolina, Georgia, Florida and Louisiana.
The reason for this is not high levels of pollution but rather the abundance of wetlands and forests, which USGS scientist Lia Chasar said aids the conversion of mercury to the toxic form, methylmercury.
People are not, however, exempt from blame because the original source of the mercury is emissions from industry that are then dumped in waterways by the rain. Coal-fired power plants and mining plants are the biggest sources of mercury emissions.
Reacting to the study, Secretary of the Interior Ken Salazar said: “This science sends a clear message that our country must continue to confront pollution, restore our nation’s waterways, and protect the public from potential health dangers.”
As well as uncovering the amount of mercury in fish, USGS scientist Barbara Scudder said the study would help decision makers better predict levels of mercury and methylmercury in different streams with comparable environmental settings.
But Scudder refused to comment as to whether the research should lead to changes in recommended fish consumption levels.