Reduction is based on the number of native species determined to be present as of 2015, compared with historical numbers documented prior to 1970. Data are displayed by 6-digit hydrologic unit code (HUC-6) watershed. A species is considered "present" if there is at least one record of its presence in any 8-digit HUC within the 6-digit HUC. This indicator presents a summary of data available from the NatureServe Explorer database (https://www.natureserve.org/conservation-tools/data-maps-tools/natureserve-explorer). The identity and status (current vs. historical) of all native fish species recorded in each 8-digit HUC are available from this database, along with species-by-species distribution maps at the 8-digit HUC level. Analyses based on these data have previously been reported in Master et al. (1998, 2003) and Stein et al. (2000). Data were provided by NatureServe.