Ecoregion boundaries are taken from Abell et al. (2000) and are based on subregions defined by Maxwell et al. (1995). The boundaries were then modified based on the faunal similarity of 166 major watersheds based on a cluster analysis of freshwater fish occurrences in these watersheds. The Laurentian Great Lakes ecoregion includes the watersheds that drain into the Great Lakes and the Great Lakes themselves. This ecoregion has the greatest fish species richness of any ecoregion in Canada. This is a result of proximity and multiple connections to the Mississippian and Atlantic Coastal refugia, relatively moderate climate, and diversity of habitats.