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below the Grand Valley, particularly below
<br />Westwater Canyon, food is the limiting
<br />resource for adult Colorado pikeminnow,
<br />whereas at some point upstream of the
<br />Grand Valley, temperature becomes limiting.
<br />The Grand Valley appears to represent the
<br />best available balance between these two
<br />constraining resources. Also, floodplain
<br />habitats, believed critical in meeting the life
<br />history needs of razorback sucker, are more
<br />plentiful in the floodplain of the Grand Valley
<br />than in more confined reaches downstream.
<br />Based on this understanding of the life
<br />history and ecological requirements of Colo-
<br />rado pikeminnow and razorback sucker, it is
<br />the position of the U. S. Fish and Wildlife
<br />Service that the prime adult habitat found in
<br />the Grand Valley, including the 15-mile
<br />reach, must be preserved and enhanced. The
<br />population of subadult and adult Colorado
<br />pikeminnow in the Colorado River currently
<br />consists of only about 600-800 individuals
<br />(Osmundson and Burnham 1998, Osmund-
<br />son 1999b). An increase in the frequency of
<br />strong year classes is needed to boost the
<br />size of this population. For the nearly extir-
<br />pated razorback sucker, stocking will first be
<br />required to establish an effective breeding
<br />population; naturalized reproduction and
<br />recruitment will then be necessary for the
<br />population to become self-sustaining. How-
<br />ever, in addition to recruitment limitations,
<br />quality adult habitat for both species is cur-
<br />rently in short supply. To address this
<br />shortfall, not only will access to historically
<br />occupied reaches need to be restored, but
<br />management efforts must also focus on
<br />preserving and enhancing conditions in the
<br />Grand Valley (including the 15-mile reach),
<br />where the best available habitat for adult
<br />Colorado pikeminnow and razorback sucker
<br />currently exists.
<br />ACKNOWLEDGMENTS
<br />This report was made possible by the collective
<br />efforts of numerous researchers and field techni-
<br />cians working on a variety of projects. Helpful
<br />reviews of earlier versions of this report were
<br />provided by Daniel Byers, John Hawkins, Henry
<br />Maddux, Thomas Nesler, Frank Pfeifer, David
<br />Propst and Rich Valdez. Former Recovery Program
<br />Director John Hamill originally perceived the need
<br />for a synthesis of information on the 15-mile reach.
<br />LITERATURE CITED
<br />Anderson, R M. 1997. An evaluation of fish
<br />community structure and habitat potential for
<br />Colorado squawfish and razorback sucker in
<br />the unoccupied reach (Palisade to Rifle) of the
<br />Colorado River, 1993-1995. Final Report.
<br />Colorado Division of Wildlife, Fort Collins,
<br />Colorado.
<br />Anderson, R M. 1999. Aspinall Studies: annual
<br />assessment of Colorado pikeminnow larval
<br />production in the Gunnison and Colorado
<br />rivers, Colorado 1992-1996. Final Report.
<br />Colorado Division of Wildlife, Grand Junc-
<br />tion.
<br />Bestgen, K. R, R. T. Muth, and M. A. Trammel.
<br />1998. Downstream transport of Colorado
<br />squawfish larvae in the Green River drainage:
<br />temporal and spatial variation in abundance
<br />and relationships with juvenile recruitment.
<br />Final Report. Colorado State University, Fort
<br />Collins.
<br />Black, T., and R. V. Bulkley. 1985a. Growth rate
<br />of yearling Colorado squawfish at different
<br />water temperatures. Southwestern Naturalist
<br />30:253-257.
<br />Black, T., and R V. Bulkley. 1985b. Preferred
<br />temperature of yearling Colorado squawfish.
<br />Southwestern Naturalist 30:95-100.
<br />Brandt, S. B., and J. Kirsch. 1993. Spatially
<br />explicit models of striped bass growth poten-
<br />tial in Chesapeake Bay. Transactions of the
<br />American Fisheries Society 122:845-869.
<br />Burdick, B. D. 1992. A plan to evaluate stocking
<br />to augment or restore razorback sucker in the
<br />upper Colorado River. Final Report. U. S.
<br />Fish and Wildlife Service, Grand Junction,
<br />Colorado.
<br />Chart, T. E., D. P. Svendson, and L. Lentsch.
<br />1999. Investigation of potential razorback
<br />sucker (brauchen texanus) and Colorado
<br />pikeminnow (Ptychocheiluslucius) spawning
<br />in the lower Green River, 1994 and 1995.
<br />Final Report. Utah Division of Wildlife Re-
<br />sources, Moab, Utah.
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