Note: The download link includes Fish Regions, Freshwater Ecoregions, and Freshwater Resilience.
Within each freshwater ecoregion, we defined one to four discrete fish regions using a cluster analysis of the USGS 8-digit Hydrologic Units (HUC) based on similarities in their native fish composition. The analysis was based on a previously developed list of native species present within each HUC (NatureServe, 2008). The cluster analysis defined up to four clusters within each freshwater ecoregion using similarity of composition (Linkage method: Flexible beta, Distance measure: Sorensen (Bray-Curtis), Flexible beta value of -0.250). To determine the faunal distinctiveness between clusters, we performed an indicator species analysis and calculated a Sorensen’s similarity index using relative frequency (i.e. the percent of HUC’s in a cluster where a given species was present). Clusters within a freshwater ecoregion were recognized as distinct if they were less than 80 percent similar in their respective fish compositions (Sorensen similarity index was <= 0.8). This resulted in four fish regions within the North Atlantic Ecoregion, three fish regions within the Chesapeake Bay Ecoregion, and three fish regions within the Ohio Basin Ecoregion. No distinct clusters were found within the St. Lawrence, Great Lakes, South Atlantic, and Tennessee freshwater ecoregions, probably because only a portion of each ecoregion was contained in our study area. For these ecoregions, the fish regions were identical to the ecoregion.
