Nearly half of U.S. lakes and reservoirs contain fish with potentially harmful levels of the toxic metal mercury, according to a federal study released in November.

The EPA found mercury-a pollutant primarily released from coal-fired power plants-and polychlorinated biphenyls in all fish samples it collected from 500 lakes and reservoirs from 2000-2003. At 49 percent of those lakes and reservoirs, mercury concentrations exceeded levels that the EPA says are safe for people eating average amounts of fish. Mercury consumed by eating fish can damage the nervous system and cause learning disabilities in developing fetuses and young children.

Fewer lakes and reservoirs -17 percent- had fish containing polychlorinated biphenyls, or PCBs, which were banned in the late 1970s because of their potential impacts on human health, although they can linger in the environment.

The study is the latest to highlight how widespread mercury pollution has become.

In August, the U.S. Geological Survey released a study of fish contamination based on a survey of 300 streams nationwide. That research found mercury in all fish sampled, but only about a quarter of the fish had mercury levels exceeding EPA levels.

The EPA said its findings underscore the need to further reduce mercury pollution and make sure that fish consumption advisories are followed.

Earlier this year, the Obama administration said it would begin crafting new regulations to control mercury emissions from power plants after a federal appeals court threw out plans drafted by the Bush administration and favored by industry.

Bay region scientists over the years have considered mercury to be one of the contaminants of greatest concern in the watershed, and numerous fish consumption advisories exist in the Bay and its rivers. The Chesapeake Bay Foundation in October threatened to file suit over mercury emissions from a proposed new coal-fired power plant in Virginia.

Figures in a 2007 fact sheet produced by the Bay Program Scientific and Technical Advisory Committee showed that a bit more than half of the mercury deposition on the watershed originates from the six states that drain into the Bay. Since that time, Maryland, Delaware and New York have required further mercury emission reductions from power plants, according to an October report from the congressional Government Accountability Office.