Salmonid response to superfund remediation in Silver Bow Creek, Montana
Naughton, Joseph Patrick.
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Uncontrolled disposal of hard rock mining wastes in the Butte hill mining district of Montana, led to the extirpation of fish from Silver Bow Creek throughout the 20th century. Superfund remediation has been ongoing since 1998 and metal concentrations are reduced. However, water quality remains poor due to inadequate treatment of municipal sewage. To assess the effectiveness of Superfund remediation in reestablishing salmonid populations, we evaluated seasonal salmonid abundance and movement in the Silver Bow Creek watershed over a 3-year period. Spatially-continuous abundance surveys were conducted in 34 main stem stream km and each sampled westslope cutthroat trout Oncorhynchus clarkii lewisi (n = 787) and brook trout Salvelinus fontinalis (n = 1,846) was PIT-tagged. Movements of PIT-tagged individuals were monitored at seven stationary antenna sites and during six seasonal portable antenna surveys. Monthly synoptic water quality samples were collected. In the main stem, water quality was poor below the wastewater effluent and was characterized by acutely toxic copper concentrations, elevated ammonia levels (e.g., NH 3-N = 2.8 mg/L), and hypoxia during summer nights (e.g., DO = 1.4 mg/L). Longitudinal abundance of salmonids closely resembled the longitudinal trend in DO. Regression analysis revealed strong associations between salmonid occurrence and abundance with DO (positive) and copper (negative) concentrations during the summer. However, westslope cutthroat trout relative abundance increased between summer and winter in remediated segments that had been hypoxic during the summer. Few brook trout recolonized the remediated main stem during the study period and the wastewater effluent may have deterred brook trout movement. Westslope cutthroat trout moved into remediated segments during the late summer and early fall as hypoxia subsided. The majority of westslope cutthroat trout sampled in the main stem were large-bodied adults (> or = 200 mm TL) contrasting with the predominantly small-bodied counterparts in the tributaries. Despite hypoxia and copper toxicity, recolonization of indigenous westslope cutthroat trout apparently was driven by the reexpression of a fluvial-adfluvial migratory behavior, a pattern that was not possible during the 100-150 years of main stem contamination.