Fish species, particularly endemics, are good indicators of the importance of freshwater biodiversity. Many lakes, such as Lake Victoria and Lake Tanganyika in Africa, have unique species assemblages with many species adapted to very specific habitats and ecological niches. This map presents the pattern of fish endemism by river basin for selected major watersheds of the world. The pattern of endemism shows strong similarities to the pattern of species richness shown in the previous map, particularly in Central Africa, South America, and Southeast Asia. In temperate regions, the Colorado, Rio Grande, and Alabama basins in North America stand out as having large numbers of endemic fish.

Because there is a correlation between number of species and basin area, large watersheds tend to have more fish species than smaller ones. To help eliminate this bias and classify the top basins in terms of fish species endemism, we grouped basins according to size: large (more than 1,500,000 square kilometers), medium (between 400,000 and 1,499,999 square kilometers) and small (less than 400,000 square kilometers). Following these categories, the pattern of fish endemism shows that the top three large basins in terms of number of fish endemics, is the Amazon with 1,800 endemic fish species, the Congo with 500, and the Mississippi with 107. Within the medium-sized basins the top three include the Xun Jiang or Pearl River with 120, the Orinoco with 88, and the Paraguay subbasin with 85. Finally, among the small basins, Lake Victoria (309), Lake Tanganyika (238), and the Salween (46) are the top three basins in terms of endemic fish (Revenga et al.1998).