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149 



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For further iniormation: 



David P Philipp 



Illinois Natural History Survey 



Center for Aquatic Ecology 



607 E. Peabody Dr. 



Champaign, IL 61820 



Tlie Colorado River and its tributaries have 

 undergone drastic alterations I'rom their nat- 

 ural states over the past 125 years. These alter- 

 ations include both physical change or elimina- 

 tion of aquatic habitats and the introductions of 

 numerous non-native species, particularly tlsh. 

 Ironically, several more species occur at most 

 localities today than were historically present 

 before these alterations. This situation compli- 

 cates the use of biodiversity as a litmus test for 

 monitoring trends of either the deterioration or 

 the health of an aquatic ecosystem. 



An Altered Ecosystem 



Over its entire basin (Figure), the Colorado 

 River has been changed from its natural state 

 perhaps as much as any river system in the 

 world. The demands for water and power in the 

 arid West have drastically altered the system by 

 impoundments, irrigation diversions, diking, 

 channelization, pollutants, and destruction of 

 bank habitats by cattle grazing and other prac- 

 tices. Some reaches, ranging from deseil spring 

 runs to main rivers, have been completely dewa- 

 tered or, seasonally, their flows consist almost 

 entirely of irrigation return laden with silt and 

 chemical pollutants. The Gila River of Arizona, 

 one of the Colorado's largest tributaries, has not 



Captive bonytail (Gila elegans), rarest of the larger river 

 species in the Colorado River Basin. 



flowed over its lower 400 km (248 mi) since the 

 early 1900's. These alterations and their effects 

 on the fish fauna have been discussed by sever- 

 al authors (Miller 1961; Minckley and Deacon 

 1968: Stalnaker and Holden 1973: Carlson and 

 Muth 1989; Minckley and Deacon 1991 ). Only 

 a few small tributaries, mostly at higher eleva- 

 tions, retain most of their natural characteris- 

 tics. 



Native Fish Fauna 



Despite the expansive drainage basin 

 (631.960 km- [243.937 mi^]) of the "Colorado 

 River, the system supported only a relatively 

 small number of native fish species compared 

 with basins of inuch smaller size east of the 

 Continental Divide. The Colorado Basin's 

 native fauna, however, was nearly unique. If 

 two fomier marine invaders are removed from 

 the 51 native taxa known from the system 

 (Table 1 ). 42 of the 49 that remain (86%) are 

 considered endemic to the system. The greatest 

 diversity of taxa (44) was distributed in the 

 Lower Basin downstream of the Arizona-Utah 

 border, in a variety of habitats that include 

 mainstem rivers, smaller tributaries, and isolat- 

 ed springs. The Upper Basin was much less 

 diverse, containing 14 species, including a sub- 

 set of the Lower Basin fauna plus 4 headwater 

 species that occur in cooler water and a warm 

 spring endemic. Basinwide. about 5 species 

 occurred mostly in mainstem river or larger trib- 

 utary habitats, 37 were restricted to smaller, in 

 some cases isolated, habitats, and 7 were more 

 generally distributed among different habitat 

 types. 



Trends 



As a consequence of habitat alterations, the 

 prevailing trend among native fish populations 

 in the Colorado River Basin has been drastic 



Colorado 

 River Basin 

 Fishes 



by 



Wayne C. Starnes 



Smithsonian Institution 



