Elemental mercury, the kind you can see, is only one part of the problem, it is the presence of methylmercury, the organic form of mercury that accumulates in organisms that will be of most concern.

Government scientists have detected elevated levels of mercury in bass and catfish in the Bear and South Yuba River watersheds in the Sierra Nevada of northern California as part of the first major study of mercury concentrations in the area.

Mercury is a well-known environmental pollutant that can have serious effects on human health.

"Liquid elemental mercury, or quicksilver, was used extensively in the Bear and Yuba River watersheds since the early gold mining days," said Charlie Alpers, US Geological Survey (USGS) research chemist and the study's chief scientist. "Our fish survey is part of the first comprehensive investigation in the Sierra Nevada region of mercury distribution in water, sediment, and biota, and the potential risks to human health and ecosystems."

Mercury, which is slightly more common than gold in the earth's crust, was heavily heavily mined in California, where the California Coast Ranges were among the most productive mercury districts in the world. They produced more than 220 million pounds of elemental mercury from the 1840s to the early 1960s.

Following the discovery of gold in California in 1848, mercury was mixed with gold to increase gold recovery. Millions of pounds of mercury were used as gold-bearing sediments were washed through sluice boxes over mercury which was loosely held in riffles and troughs.

An estimated 10 - 30% of mercury used in these operations was lost to the environment and transported into streams and reservoirs with other discharged sediments produced in hydraulic mining operations.

In many gold mining areas where mercury was used, it is still relatively easy to find quantities of liquid elemental mercury in sediments and stream channels.

The new USGS report contains mercury data on 141 samples of fish collected during September and October 1999 from reservoirs and stream environments in Nevada, Placer, and Yuba counties.

The five reservoirs sampled were Englebright Lake, Scotts Flat Reservoir, Rollins Lake, Lake Combie, and Camp Far West Reservoir. The main target species in these reservoirs was largemouth bass - other species that were sampled included smallmouth and spotted bass, channel catfish, crappie, green sunfish, and bluegill. Brown trout and rainbow trout were sampled predominantly from 17 stream sites, although a small number of trout were also taken from some of the reservoirs.

Mercury concentrations ranged from: bass - 0.20 - 1.5 parts per million (ppm),wet basis; sunfish - less than 0.10 -0.41 ppm; channel catfish - 0.16 - 0.75 ppm; rainbow trout - 0.06 - 0.38 ppm; and brown trout - 0.02 - 0.43 ppm.

For reference, the Food and Drug Administration action level for commercial fish is 1.0 ppm and the State of California considers mercury levels above 0.3 ppm indicative of the need for further study.

"Elemental mercury, the kind you can see, is only one part of the problem," said Jason May, USGS biologist. "It is the presence of methylmercury, the organic form of mercury that accumulates in organisms, that will be of most concern."

Methylmercury is a potent neurotoxin that is known to be especially detrimental to developing fetuses and young children. Methylmercury is known to biomagnify, or increase in concentration, as it moves up the food chain. Concentrations tend to be highest in predatory fish - those that eat other fish.

Some examples of predatory fish are bass and brown trout. The predominant form of mercury in edible fish tissue is methylmercury.

The USGS has submitted its data to the California Environmental Protection Agency and their Office of Environmental Health Hazard Assessment (OEHHA). Environmental health officials will be working closely with OEHHA toxicologists to determine whether formal notification to the public will be made concerning potential risks from fish consumption in these areas.

According to OEHHA, a total of 12 water bodies in California - including San Francisco Bay, Clear Lake, and Lake Berryessa - have fish consumption advisories related to elevated mercury levels. Some of the mercury levels reported by USGS are in a similar range compared with fish from other water bodies for which there are consumption advisories.

At this time, there are no fish consumption advisories for mercury in California's historic gold mining areas, including the Sierra Nevada and the Trinity-Klamath Mountains.

The USGS fish data report can be found at http://ca.water.usgs.gov/mercury/bear-yuba/. The full title is: 'Mercury Bioaccumulation in Fish in a Region Affected by Historic Gold Mining: The South Yuba River, Deer Creek, and Bear River Watersheds, California, 1999,' by Jason T. May, Roger L. Hothem, Charles N. Alpers, and Matthew A. Law, USGS Open-File Report 00-367.

Federal, state, and local agencies funding the USGS investigation include the Bureau of Land Management, the US Department of Agriculture - Forest Service, the California State Water Resources Control Board, and the Nevada County Resource Conservation District.

Other organizations cooperating in the effort by providing in-kind services and access to lands include the US Environmental Protection Agency, the California Department of Parks and Recreation, and the Nevada Irrigation District.

As the nation's largest water, earth, and biological science and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers.