On the relations between hydrography and the ranges of freshwater fish species and subspecies

Ranges of freshwater fish species depend on river basins. However, most species are confined to a limited area of a basin. Some limited distributions are determined by the ecology of the species, most have historical grounds. Many species are endemic to a restricted area of a river basin (e.g., 7 in the River Danube; more than 80 in Mississippi river basin). Most non-endemics confined to a sector of a river basin also live in one or more adjacent basins. Cases are known of conspecific subspecies inhabiting distinct areas within a river basin, some of them being also present in a neighbouring basin. Restricted distributions are determined by the fact that river captures usually involve tributaries. not the main rivers and the inhabitants of the upper and middle reaches of the river do nor disperse through the lower reaches.