Assessing Mercury Levels in Fish Tissue, Duck Valley Reservation, Idaho and Nevada

Methods

At least 15 catchable (>12 inch) fish were taken from each reservoir. Three age classes (5 fish each) were taken to provide insight on variability in mercury concentration resulting from age differences. Tribal staff assisted with obtaining a fish collection permit for Wildhorse Reservoir because it is not part of the reservation. Each fish was measured for total length (mm) and weight (g). Individual samples, consisting of a one-inch skinless fillet were taken from each fish just below the pectoral fin. Samples were labeled and placed in self-sealing plastic bags and placed on dry ice for later shipment to the lab. The remainder of each fish carcass were kept frozen to provide backup samples in case additional laboratory analysis is desired.

Samples were analyzed for mercury to determine the magnitude and variability of concentrations within selected age classes of trout. Only total mercury was analyzed in fish tissue because it is assumed that the majority (95-99%) of the mercury present in fish tissue had already been converted to the methyl form.

Fish-tissue samples were analyzed for total mercury by the USGS Mercury Laboratory in Middleton, Wisconsin using standard laboratory and quality-control procedures. Direct combustion (USEPA method 7473) was used for determination of total mercury in tissue samples. Approximately 65 fish-tissue samples were analyzed, 15 from each reservoir. An additional 5 hatchery trout were provided by the tribe prior to stocking, and each fish was measured and analyzed for mercury.