Distribution of fish in the Virgin River,a tributary of the lower Colorado River

Synopsis The Virgin River, a major tributary of the lower Colorado River, contains a unique, though impoverished, native ichthyofauna (six species) that is largely intact despite the introduction of exotic fishes (13 species) and alterations of the river. The main conclusions of the study are: (1) The longitudinal distribution pattern of the native species does not follow theoretical expectations of a downstream increase in species richness; (2) for the size of the basin (>15,000 km²), the native ichthyofauna is extremely depauperate; and (3) most of the exotic fish are not established in the river, but persist through repeated invasions from downstream reservoirs.