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t ~~ BASIS FOR BIOLOGICAL OPINION This biological opinion addresses an average annual depletion of approximately 56.4 acre-feet of water from the Upper Colorado River Basin (Upper Basin). Water depletions in the Upper Basin have been recognized as a major source of impact to endangered fish species. Continued water withdrawal has restricted the ability of the Colorado River system to produce flow conditions required by various life stages of the fishes. Impoundments and diversions have reduced peak discharges by 50 percent since 1942, while increasing base flows by 21 percent in some reaches. These depletions, along with a number of other factors, have resulted in such drastic reductions in populations of the Colorado squawfish, humpback chub, bonytail chub, and razorback sucker that the Service has listed these species as endangered and has implemented programs to prevent them from becoming extinct. BASIS FOR CONFERENCE OPINION In considering the biological basis for proposing critical habitat, the Service focused on the primary physical and biological elements that are essential to the conservation of the species without consideration of land or water ownership or management. The Service has identified water, physical habitat, and biological environment as the primary constituent elements. This includes a quantity of water of sufficient quality that is delivered to a specific location in accordance with a hydrologic regime that is required for the particular life stage for each species. Water depletions reduce the ability of the river system to provide the required water quantity and hydrologic regime necessary for recovery of the fishes. The physical habitat includes areas of the Colorado River system that are inhabited or potentially habitable for use in spawning and feeding, as a nursery, or serve as corridors between these areas. In addition to the river channels, oxbows, backwaters, and other areas in the 100-year floodplain serve as spawning, feeding, and nursery habitats or provide access to these habitats. Water depletions reduce the ability of the river to create and maintain these important habitats. Food supply, predation, and competition are important elements of the biological environment. Food supply is a function of nutrient supply and productivity, which could be limited by reduction of high spring flows brought about by water depletions. Predation and competition from nonnative fish species has been identified as a factor in the decline of the endangered fishes. Water depletions contribute to alterations in flow regimes that favor nonnative fishes. The Service concludes that water depletions impact the primary constituent elements; however, reasonable and prudent alternatives have been developed by the Service that would avoid destruction or adverse modification to the proposed critical habitat. BIOLOGICAL BACKGROUND Colorado Squawfish The Colorado squawfish evolved as the main predator in the Colorado River system. The diet of Colorado squawfish longer than 3 or 4 inches consists almost entirely of other fishes (Vanicek and Kramer 1969). The Colorado squawfish is the largest cyprinid fish (minnow family) native to North America