Ui(r Living Resources — Fishes 



151 



special concern. On the other hand, a few small 

 tributaries, by virtue of their isolation, rare 

 intemiittent flows in lower reaches, and physi- 

 cal baiTiers, have been spared significant alter- 

 ations or invasions by non-native species and 

 retain an intact native fauna (e.g., Redfleld 

 Canyon, Arizona, Table 2). 



In the larger rivers of the Upper Basin, such 

 as the Green, lower Yampa. and most of the 

 upper Colorado, most native taxa are extant but 

 one or two (razorback sucker [Xyrauchen tex- 

 aniis], possibly bonytail [Gila elegans]). are re- 

 presented by very rare individuals that may not 

 be reproducing; all native fishes are greatly 

 exceeded in numbers and kind by non-native 

 taxa. In smaller tributaries of that region, varied 

 numbers of native taxa persist; in the worst 

 affected streams (e.g., most Green River tribu- 

 taries in Utah), most taxa have been replaced by 

 non-native taxa (author's observation). 



Case studies of two endangered Colorado 

 River species, which are hallmarks to conserva- 

 tionists, further elucidate patterns of decline 

 among these fishes. They are large, long-lived 

 (20-50 years) species that inhabit larger 

 streams. The Colorado squawfish (Prycho- 

 cheilus Indus) is a highly migratory (Tyus 

 1990) predatory minnow. Perhaps because of 

 fragmentation or impediment of migratory 

 routes, its original extensive range has been 

 reduced by roughly two-thirds, and it is uncom- 

 mon where it remains. The last confirmed report 

 in the Gila River was in 1950 and the last in the 

 Lower Basin in 1975 (Miller 1961; Minckley 

 1973; Maddux etal. 1993). 



The fourth species, the humpback chub 

 (Gila cypha), is strictly a denizen of turbulent 

 canyon reaches so difficult to sample that it was 

 not discovered until 1946; it ranged from 

 Boulder Canyon on the lower Colorado 

 throughout canyon reaches of the Upper Basin 

 well into Wyoming. Today, it occurs only in 

 Grand Canyon. Arizona (Maddux et al. 1993). 

 near the confluence of the Colorado and Little 

 Colorado rivers, and in five Upper Basin canyon 

 areas (rare in three), although the genetic "puri- 

 ty" of the Upper Basin populations is ques- 

 tioned. Recovery plans are in place for these 

 fish as well as the bonytail and the ra/orback 

 sucker. These fish are all easily propagated in 

 captivity. It is otherwise difficult to find any- 

 thing positive in the history of these or other 

 Colorado Basin native fishes over the past sev- 

 eral decades. 



Non-native Species 



Concomitant with the pervasive physical 

 alteration of the Colorado River ecosystem has 

 been both purposeful and accidental introduc- 

 tions of at least 72 non-native fish taxa (Maddux 



et al. 1993), including those indigenous to other 

 North American basins and more exotic species. 

 Alterations of the ecosystem's natural charac- 

 teristics have apparently tipped the ecologic 

 balance in favor of many of the non-native 

 species that now vastly outnumber natives in 

 numbers of species (Table 2), population densi- 

 ty, and often biomass at most localities. There is 

 evidence that some, such as the extremely per- 

 vasive red shiner (Cyprinella lutrensis), dis- 

 place native taxa (Douglas et al. 1994) while 

 others, such as channel and flathead catfish 

 (hraliinis piinctatus and Pylodictis olivahs), 

 are known predators on larval and juvenile 

 native species (several references in Maddux et 

 al. 1993). The introduced white sucker 

 {Catostomus commersoni) is hybridizing exten- 

 sively with native suckers throughout much of 

 the Upper Basin (author's observation), possi- 

 bly threatening the genetic integrity of those 

 taxa. These and other interactions between non- 

 native and native taxa may have significant neg- 

 ative effects on native fishes. The dominance 

 held by non-native fishes may be symptomatic 

 of the overall degree of alteration of the 

 Colorado River ecosystem and could potential- 

 ly confound future studies of biodiversity. 



Table 2. Overall and relative 

 abundance of native and non- 

 native fishes from various locali- 

 ties in the Colorado River Basin. 

 Numbers for 1 800's represent 

 original complements of native 

 taxa. For subsequent years, total 

 abundance is followed by ratio of 

 non-native to native taxa in paren- 

 theses. Sources: Miller 1961; 

 Taba et al. 1965; Vanicek et al. 

 1970; Stalnaker and Holden 1973; 

 Cross 1975; Holden and Stalnaker 

 197.5a.b;Sutlkusetal. 1976; 

 Carison et al. 1979; Miller et al. 

 1982; Valdez et al. 1982; Valdez 

 1984.1990; Wick etal. 1985; 

 Platania and Bestgen 1988; 

 Gnffith and Tiersch 1989. 



Altered Species Diversity and 

 Biodiversity Studies 



While native taxa have declined, there have 

 actually been two- to threefold increases in the 

 number of species at most localities in the 

 Colorado Basin because of the success of intro- 

 duced taxa (Table 2). If future biodiversity mon- 

 itoring is to truly gauge positive and negative 

 shifts in the health of the Colorado River 

 ecosystems, then an accurate baseline is neces- 

 sary. A baseline describing unaltered native 

 fauna might be an ideal but unattainable goal. 

 That line could be approached, however, by 

 divesting faunal lists of all non-native taxa and 

 determining, as much as possible, the true 

 extent of diversity of that which remains. In 

 fish, it is practical to do so to the level of dis- 

 tinctive populations through studies of genetic 

 variability. With luck, it is even possible to 



