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DISPERSAL PATTERNS OF COLORADO SQUAWFISH
<br />the upper reach is probably low given the relative
<br />rarity of these endangered fish. Also, nursery areas
<br />might not have been exclusively located in the
<br />lower reach as is largely the case today: data from
<br />the mid-1970s indicate that small size-classes of
<br />Colorado squawfish were much more abundant in
<br />the upper reach (see Osmundson and Burnham
<br />1998). Predation from introduced largemouth bass
<br />Micropterus salmoides and green sunfish Lepomis
<br />cyanellus, now commonly found in upper-reach
<br />backwaters, may partly explain the current lack of
<br />young Colorado squawfish there. Also, prey of
<br />large Colorado squawfish (native suckers and
<br />chubs) may have once been more abundant in the
<br />lower reach than is the case today. The overall
<br />pattern observed in this study (i.e., large fish up-
<br />stream; younger fish downstream), makes sense as
<br />a life history strategy and may partially reflect an
<br />adaptation to modified riverine conditions. The
<br />lower reach provides warmer water and ample food
<br />for early life phases when rapid growth is most
<br />important for survival; the upper reach provides
<br />moderated temperatures and more appropriately
<br />sized prey for adults when maintaining body con-
<br />dition may be important in gamete production and
<br />in fueling spawning migrations.
<br />Long-distance dispersal is a manifestation of the
<br />individual's solution to spatial and temporal prob-
<br />lems (Baker 1982), and exploratory movements
<br />across considerable distances are probably search-
<br />es for suitable habitats (McKeown 1984). The dis-
<br />tribution and dispersal patterns of Colorado
<br />squawfish in the Colorado River indicate that dis-
<br />persal movements of this species occur at the land-
<br />scape scale, and large sections of river are required
<br />to meet the changing needs of different life stages.
<br />Acknowledgments
<br />We thank Bruce Bonar, Tom Fresquez, Mike
<br />Montagne, and Dale Ryden for assisting with data
<br />collection and Frank Pfeifer for administrative
<br />support. We also thank Mike Farmer of the Larval
<br />Fish Laboratory, who performed diet analysis, and
<br />Charles McAda, who provided data on backwater
<br />area and small fish abundance. Kevin Bestgen,
<br />John Hawkins, Patrick Martinez, Tom Nesler, Da-
<br />vid Propst, Harold Tyus, Richard Valdez, and an
<br />anonymous reviewer provided valuable comments
<br />on earlier versions of this manuscript. This re-
<br />search was funded by the U.S. Fish and Wildlife
<br />Service and the Recovery Implementation Pro-
<br />gram for Endangered Fish Species in the Upper
<br />Colorado River basin.
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