USGS Finds Mercury in Fish from Idaho, Nevada Reservoirs

Fish tissue samples taken from rainbow trout collected from four Idaho and Nevada reservoirs revealed elevated concentrations of methylmercury, according to data released recently by the U.S. Geological Survey (USGS) and the Shoshone-Paiute Tribes of the Duck Valley Indian Reservation.

The tribes and USGS collaborated on and jointly funded the study. The tissue samples were analyzed at the USGS Mercury Research Laboratory in Middleton, Wis.

"The concentrations found were generally elevated for rainbow trout," said Terry Maret, the USGS scientist who directed the sampling. Maret referred to a study conducted by the U.S. Environmental Protection Agency, which found that the average mercury concentration in rainbow trout across the United States was about 0.11 parts per million (ppm), wet weight. "About 70 percent of the fish filets analyzed were above this," he said.

"Also, many of the larger fish we sampled exceeded Idaho's fish tissue methylmercury criterion of 0.3 ppm, a level established for the protection of the adult consumer." Older, larger fish tend to show greater concentrations of methylmercury due to biomagnifications in the food chain.

Three of the four sample sites (Lake Billy Shaw, Mountain View Reservoir, and Sheep Creek Reservoir) are located on the Duck Valley Indian Reservation, which spans the border between southwestern Idaho and northeastern Nevada. At the tribes' request, public health toxicologists from the Idaho Department of Health and Welfare reviewed the USGS lab data for fish collected from those three reservoirs and recommended that the tribes establish the following fish consumption guidelines:

  • Up to 2 meals per month of fish taken from Mountain View Reservoir and Lake Billy Shaw, and up to 5 meals per month of fish taken from Sheep Creek Reservoir can be consumed safely by children and pregnant women.
  • Up to 8 meals per month of fish taken from Mountain View Reservoir and Lake Billy Shaw, and up to 20 meals per month of fish taken from Sheep Creek Reservoir can be consumed safely by the general public other than children and pregnant women.

The meal size is 4 ounces for children and 8 ounces for adults. The toxicologists emphasized that rainbow trout from the reservation reservoirs remain a healthy food choice within the recommended consumption guidelines.

Kyle Prior, Business Council chairman for the Shoshone-Paiute Tribes, responded to the study findings: "We are proactively addressing the problems that might possibly impact the health of our people and the environment, upon which we rely." He noted that the Tribes had already established a mercury workgroup to actively monitor and respond to the threat of contamination to Tribal resources.

Methylmercury is the most toxic form of mercury, and methylmercury contamination is an issue of concern in the intermountain West. Mining operations, coal-fired power plants, and other sources emit inorganic mercury to the atmosphere. Inorganic mercury is converted to methylmercury, an organic form, by anaerobic organisms living in water, sediments, and soils. Several other Idaho fisheries have been found to contain unsafe levels of methylmercury. "Reservoirs and wetlands offer an environment favorable to methylation of mercury," said Maret.

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