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Field data of fish occurrences and specific conductivity were used to estimate the tolerance of freshwater fish to elevated ion concentrations and to compare the differences between species- and genus-level analyses for individual effects. We derived extirpation concentrations at the 95th percentile (XC95) of a weighted cumulative frequency distribution for fish species inhabiting streams of the central and southern Appalachians by customizing methods used previously with macroinvertebrate genera. Weighting factors were calculated based on the number of sites in basins where each species occurred, reducing overweighting observations of species restricted to fewer basins. Comparing the species- and genus-level fish XC95 values, XC95s for fish genera were near the XC95s for the most salt-tolerant species in the genus. Therefore, a genus-level effect threshold is not reliably predictive of species-level extirpation, unless the genus is monospecific in the assessed assemblage. Of the 101 fish species XC95 values, 5% were <509 and 10% were <565 mu S/cm. The lowest XC95 for a species was 322 mu S/cm, which is >300 mu S/cm, the exposure estimated to extirpate 5% of macroinvertebrate genera in the central Appalachians. Above 509 mu S/cm, 41 of the 101 species are expected to decline in occurrence. Environ Toxicol Chem 2018;37:871-883. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.