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I- ~ i 23 temperature habitat in the spring. This could be affecting squawfish'growth and spawning success. A]so, the quantity and frequency of availability of inundated floodplain depressions used by razorback suckers for spawning is dependent on the magnitude and frequency of spring flows necessary to inundate these areas. The decrease in the magnitude and frequency of spring flows necessary to inundate floodplain depressions is believed to be largely responsible for poor razorback sucker spawning success. Under adjusted baseline conditions for the 1975-1993 period of record, spring flows have fa]len below recommended levels in 55 months out of 76 months. Biolo4ical Environment Food supply, predation, and competition are important elements of the biologics] environment. Food supply is a function of nutrient supply and productivity, wliich could be limited by the presence of contaminants. The modification of flow regimes, water temperatures, sediment levels, and other 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, Osmundson 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 Osmundson 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 contro]led 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. Consequent]y, 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 OF THE ACTION r'°~.'~ ,~ Water depletions r a major factor contributing to the reductions in the .Xr ,- populations of the Colorado squawfish, humpback chub, bonytail, and razorback (~` sucker that have caused the Service to list these species as endangered and to ~~~~~'' implement programs to prevent them from becoming extinct.