Mercury Found in All Sampled Fish in U.S. Streams

September 2, 2009

USGS researchers have found that it may be impossible for people eating fish caught in US waters to avoid eating mercury-contaminated fish. After reviewing data from 1998-2005 for 291 streams, they found that every single fish in every stream contained some mercury, and that 27% of the fish contained so much mercury that consuming a typical amount would exceed government guidelines for mercury intake.

The streams are located all over the country, in urban, agricultural, forest, shrubland, grassland, and wetland settings. However, the researchers note that the tested streams aren't technically representative of all streams in the country, and they point out that their testing was limited to predator fish species.

The highest mercury concentrations were in streams draining mined areas and blackwater coastal zones. Viewed another way, concentrations were highest in streams in watersheds that were among the least disturbed, with forests and wetlands, and which might typically be perceived as the cleanest by people who fish. The mercury pollution in these remote areas is likely due to the mechanics of methylmercury formation following deposition of airborne mercury. Some of the mercury also came from mining sources, but only about 20% of the sampled streams fall into that category.

The pervasive mercury contamination of fish highlights the relative futility of trying to use the fish consumption advisory systems that are in effect in almost all states, but which are routinely ignored by many people who fish. Long-term, it may be more effective to substantially reduce mercury contamination in air and water, rendering the advisories unnecessary.

For much more information on fish contaminants, see the TipSheet of Aug. 15, 2007.