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ti)
<br />88 CALIFORNIA FISH AND GAME
<br />Missouri River (Berner 1951 ), 541/6 of materials ingested by fishes was of terres-
<br />trial origin. The sparse terrestrial vegetation of most of the Colorado River basin,
<br />and relatively large size of the stream in proportion to its narrow, water-limited
<br />riparian zone, diminishes the importance of allochthonous input. As emphasized
<br />by Minshall (1978), autochthonous conditions are far more prevalent in open,
<br />western streams than has been generally recognized.
<br />Organic and inorganic transport in the Colorado River are curtailed by reser-
<br />voirs, e;:cepting for downstream passage of plankton and associated suspended
<br />debris through penstocks. These materials and nutrients not trapped by im-
<br />poundments IPaulson and Baker 1980), nonetheless form a basis for down-
<br />stream production. The few backwaters that remain along the river appear highly
<br />productive, supporting large standing crops of plankton, rooted aquatic vegeta-
<br />tion, and fishes (Minckley 1979, Nicola 1979), all of which are flushed into the
<br />channel by almost-tidal fluctuations in the stream resulting from hydroelectric
<br />generation and pulses of irrigation deliveries. Less modified reaches of the river
<br />and those which are relatively stabilized have proportionately more backwater
<br />habitat. In-stream productivity is also locally high, enhanced by current under
<br />abundant insolation without interference of shading by turbidity or riparian
<br />vegetation, and is at present further augmented by addition of nutrients through
<br />return flow of irrigation systems. A trophic economy based upon autochthonous
<br />detritus establishes quickly under such conditions.
<br />Introduced forage species that have survived and flourished in the lower
<br />Colorado River mainstream all depend heavily upon detritus or primary produc-
<br />ers, or upon secondary consumers such as zooplankton, chironomids, or other
<br />invertebrates. Large fishes with less piscivorous tendencies feed directly upon
<br />detritus- or plant-dependent clams or crayfish. The introduced fish fauna thus
<br />appears to have relatively simple food interrelations.
<br />ACKNOWLEDGMENTS
<br />Research was supported in part by Contract 14-06-300-2529 from the U.S.
<br />Department of Interior, Bureau of Reclamation, Lower Colorado River Division,
<br />Boulder City, Nevada, which also gave permission for this publication. J. J.
<br />Landye, J. Warnecke, and W. T. Kepner. plus numerous fisheries students at
<br />Arizona State University helped collect and analyze the data; their assistance is
<br />gratefully acknowledged.
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