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30 habitat conditions caused by water depletions has contributed to the establishment of nonnative fishes. Predation and competition from nonnative fishes have been clearly implicated in the population reductions or elimination of native fishes in the Colorado River Basin (Dill 1944, Qsmundson and Kaeding 1989. Behnke 1980, Joseph et al. 1977. Lanigan and Berry 1979, Minckley and Deacon 1968. Meffe 1985. Propst and Bestgen 1991. Rinne 1991. and others). Data collected by Qsmundson and Kaeding (1991) indicated that during low water years nonnative minnows capable of preying on or competing with larval endangered fishes greatly increased in numbers. Nonnative fishes compete with native fishes in several ways. The capacity of a particular area to support aquatic life is limited by physical habitat conditions. Increasing the number of species in an area usually results in a smaller population of most species. The size of each species population is controlled by the ability of each life stage to compete for space and food resources and to avoid predation. Some nonnative fishes' life stages appear to have a greater ability to compete for space and food and to avoid predation in the existing altered habitat than do some native fishes' life stages. Nonnative fishes are often stocked in and enter rivers from off-channel impoundments. The periodic introduction of these nonnative fishes into a river allows them to bypass limitations to reproduction. growth. or survival that they might encounter in the river. Consequently. populations of nonnative fishes in the river are enhanced. Endangered and other native species in the river experience greater competition and predation as a result. EFFECTS QF THE ACTIQN Water depletions are a major factor contributing to the reductions in the populations of the Colorado squawfish. humpback chub, bony tail , and razorback sucker that have caused the Service to list these species as endangered and to implement programs to prevent them from becoming extinct. Water depletions reduce the ability of the river to create and maintain these important habitats and reduce the frequency and duration of availability of these habitats. Food supply. predation. and competition are important elements of the biological environment. Food supply is a function of nutrient supply and productivity; because high spring flows flood bottomland habitats increasing the nutrient supply and productivity of the river environment. reduction of high spring flows brought about by water depletions can reduce food supply. Predation and competition from nonnative fish species have been identified as factors in the decline of the endangered fishes. Water depletions contribute to alterations in flow regimes that favor nonnative