Summary

Description

Common loons Gavia immer that stage during migration at Walker Lake, Nevada, were found to have elevated concentrations of mercury in their blood. The source of the mercury was unknown, although fish from Walker Lake were known to contain moderate concentrations. Therefore, an investigation was initiated to determine the sources of mercury in the Walker River basin and the concentrations of mercury in Walker Lake tui chub Gila bicolor of the size consumed by common loons to determine risk to this fisheating species. A total of 12 fish and 29 aquatic invertebrate samples were collected in 19992001 at 19 sites in the Walker River basin. Mercury concentrations in aquatic invertebrate samples collected at sites downstream of historic mine sites, where mercury was used in precious metal recovery in the late 1800s, were often higher than at sites where no historic mining influences were present. Mercury concentrations in tui chub from Walker Lake increased with size of the fish, with a major increase for fish greater than 25 cm in length. Mercury concentrations in fish were below those associated with adverse effects to fish. Mercury concentrations in larger tui chubs may be a risk to fisheating birds, including common loons. Data on mercury concentrations in stream sediment from the Walker River basin that were provided by the U.S. Geological Survey and the U.S. Environmental Protection Agency were examined in relation to the concentrations found in this study in aquatic invertebrates. These data sets suggested that mercury source areas included: an area of the Virginia Creek watershed, south of Bridgeport, California; several sites on the southeast side of the Sweetwater Range near the CaliforniaNevada state line, which included areas along Sweetwater and Fryingpan Creeks; Bodie, California; and the nearby Aurora district in Nevada. Additional studies are needed to further define mercury source areas for possible stabilization or cleanup. Additional information is also needed on common loons which include the following: fall migration patterns and wintering areas to determine if the birds frequent known areas of mercury contamination; mercury exposure on the breeding grounds in Saskatchewan; and mercury in blood of loons using other Nevada lakes and reservoirs during migration to determine if Walker Lake is a significant source of mercury.