Cutthroat trout avoidance of metals and conditions characteristic of a mining waste site: Coeur d'Alene River, Idaho

The South Fork basin of the Coeur d'Alene River, Idaho has been an area of heavy mining activity since the 1880s. The mining operations have resulted in elevated concentrations of metals in surface water, most notably cadmium, lead, zinc, and, to a lesser extent, copper. The metals affected surface water quality downstream in the Coeur d'Alene basin and are suspected to be one of the primary reasons for the reduction in populations of native westslope cutthroat trout Oncorhynchus clarki lewisi. The avoidance response of a surrogate species, Snake River cutthroat trout O. clarki (unnamed subspecies), was evaluated against conditions simulating those in the Coeur d'Alene River basin. Cutthroat trout avoided a metals mixture of these concentrations: Cd (0.30 ??g/L), Cu (6.0 ??g/L), Pb (0.6 ??g/L), and Zn (28 ??g/L). The avoidance response to either Cu or Zn alone was similar to the avoidance response to the mixture, suggesting that avoidance to the mixture was due to these metals. After acclimation to Zn at 55 ??g/L for 90 d, cutthroat trout detected and preferred a lower Zn concentration of 28 ??g/L. The lowest Zn concentrations avoided (28 ??g/L) were 1/6 to 1/78 the Zn concentrations measured in the South Fork and lower Coeur d'Alene River basins. Avoidance of metals-contaminated habitats by cutthroat trout may be, in part, responsible for reduced fish populations.