Changes in stream fish community composition along a gradient of specific conductance

We assessed the impacts of a gradient of specific conductance on the fish communities of a southwestern Pennsylvania stream.  A total of 16 stations were established at 1.6 Km increments over a 26 Km reach of the South Fork of Tenmile Creek.  Two were located above a treated mine drainage outfall and the remainder extended downstream to its confluence with Tenmile Creek.  In addition, a reference station was established on Tenmile Creek above its confluence with the South Fork.  At each site, specific conductance (µS/cm) and pH were determined in the field; total alkalinity (mg/l) was measured in the laboratory by titration.  Total dissolved solids concentrations were determined from specific conductance values.  A total of 10,940 fish representing seven families and 42 species/hybrids were collected from 17 stations over the entire survey.  Species richness, abundance, and the coefficient of community loss (I) showed marked impairment at the two stations directly below the outfall.  Species richness declined from 27 above the outfall to 7 below. This study suggests that the threshold for in-stream conductivity impairment to fish communities in this region is in the range of 3,000–3,500 µS/cm and 2,000–2,300 mg/l of total dissolved solids, respectively.