Native fishes of the upper Colorado River UCR basin are declining in abundance. The most likely cause of their decline is anthropogenic alteration of the natural environment. Habitat of the native Colorado River fishes has been changed greatly during the last 100 years by human actions including physical alterations and the introduction of nonnative species. Changes in the physical and biological characteristics of riverine habitat have contributed to the endangerment of four native fish species Colorado (pikeminnow, humpback chub, bonytail, and razorback sucker). These and other fishes native to the main channels of the Colorado River system ("big river fish community") have disappeared from most of their original habitat. Concern about the decline and endangerment of four species from the big river fish community resulted in Federal and state endangerment listings.
