Introduced Fish

Abstract

AbstractThere is concern that the non‐native bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix), now found in many large rivers of the Mississippi River Basin, may spread to other regions. However, evidence suggests that their eggs may not be able to survive in soft water. We used the distribution of water hardness values from rivers where bighead and silver carps appear to be reproducing to approximate the potential minimum water hardness needed for egg survival. We then used water hardness data from over 3,000 stream and river sites across the contiguous United States to classify ecoregions as soft water or hard water, relative to carp reproduction. In 24 ecoregions, comprising 20.7% of land area, the 75th percentile of water hardness values were less than 65 mg/L CaCO3. These soft‐water areas included New England, most of the Southeast, and western portions of the Pacific Northwest. Nearly all areas with bighead and silver carp reproduction were in hard‐water ecoregions. Exceptions in soft‐water regions were in rivers originating in hard‐water regions.