Colorado River Basin Fishes
by
Wayne C. Starnes
Smithsonian Institution
| 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 practices. Some reaches, ranging from desert spring runs to main rivers, have been completely dewatered 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 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 several 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 elevations, retain most of their natural characteristics. |
Figure. Colorado River Basin. |
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Native Fish Fauna |
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| Despite the expansive drainage basin (631,960 km2 [243,937 mi2]) of the Colorado River, the system supported only a relatively small number of native fish species compared with basins of much smaller size east of the Continental Divide. The Colorado Basin's native fauna, however, was nearly unique. If two former 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 isolated springs. The Upper Basin was much less diverse, containing 14 species, including a subset 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 tributary habitats, 37 were restricted to smaller, in some cases isolated, habitats, and 7 were more generally distributed among different habitat types. |
Table 1. Native fish taxa of the Colorado River Basin including currently recognized subspecies. Taxa denoted by * may eventually prove genetically distinct from populations outside the Colorado River Basin. Those denoted "(m)" are marine invaders. Status of jeopardized and extinct species appears in parentheses: E = endangered; T = threatened; SC = special concern; X = extinct (based, in part, on Carlson and Muth 1989; Williams et al. 1989; and the National Biological Service's Category 2 list). Common names bracketed with quotation marks indicate that those species are undescribed and not officially named. | |
Trends |
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| In the Lower Basin, only 3 of the 10 native species that inhabited the mainstem of the lower Colorado River remained by the 1940's but by the 1960's, none remained. In the lower Salt River portion of the Gila River system, the original complement of 14 taxa was also reduced to 3 by the 1940's and to 2 by the 1960's; today, they are probably extirpated. In the early 1900's, the isolated springs of the Pluvial White River system in southern Nevada harbored 17 endemic taxa; today, 1 of those taxa is extinct, 9 endangered, 3 threatened, and the remainder of special concern. On the other hand, a few small tributaries, by virtue of their isolation, rare intermittent flows in lower reaches, and physical barriers, have been spared significant alterations or invasions by non-native species and retain an intact native fauna (e.g., Redfield 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 texanus], 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 tributaries 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 conservationists, further elucidate patterns of decline among these fishes. They are large, long-lived (20-50 years) species that inhabit larger streams. The Colorado squawfish (Ptycho-cheilus lucius) 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 uncommon 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 et al. 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 "purity" of the Upper Basin populations is questioned. Recovery plans are in place for these fish as well as the bonytail and the razorback sucker. These fish are all easily propagated in captivity. It is otherwise difficult to find anything positive in the history of these or other Colorado Basin native fishes over the past several decades. | ||
Non-native Species |
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| Concomitant with the pervasive physical alteration of the Colorado River ecosystem has been both purposeful and accidental introductions 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 characteristics 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 density, and often biomass at most localities. There is evidence that some, such as the extremely pervasive red shiner (Cyprinella lutrensis), displace native taxa (Douglas et al. 1994) while others, such as channel and flathead catfish (Ictalurus punctatus and Pylodictis olivaris), are known predators on larval and juvenile native species (several references in Maddux et al. 1993). The introduced white sucker (Catostomus commersoni) is hybridizing extensively with native suckers throughout much of the Upper Basin (author's observation), possibly threatening the genetic integrity of those taxa. These and other interactions between non-native and native taxa may have significant negative 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 potentially confound future studies of biodiversity. |
Table 2. Overall and relative abundance of native and non-native fishes from various localities in the Colorado River Basin. Numbers for 1800's represent original complements of native taxa. For subsequent years, total abundance is followed by ratio of non-native to native taxa in parentheses. Sources: Miller 1961; Taba et al. 1965; Vanicek et al. 1970; Stalnaker and Holden 1973; Cross 1975; Holden and Stalnaker 1975a,b; Suttkus et al. 1976; Carlson et al. 1979; Miller et al. 1982; Valdez et al. 1982; Valdez 1984,1990; Wick et al. 1985; Platania and Bestgen 1988; Griffith and Tiersch 1989. | |
| References | |
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Carlson, C.A., and R. Muth. 1989. The Colorado River: lifeline of the West. Pages 220-239 in D.P. Dodge, ed. Proceedings of the International Large River Symposium. Canadian Special Publ. in Fishery and Aquatic Sciences 106. Carlson, C.A., C.G. Prewitt, D.E. Snyder, and E.J. Wick. 1979. Fishes and macroinvertebrates of the White and Yampa rivers, Colorado. Bureau of Land Management, Biological Sciences Series 1. Denver, CO. 276 pp. Cross, J.N. 1975. Ecological distribution of the fishes of the Virgin River. M.S. thesis, University of Nevada, Las Vegas. 187 pp. Douglas, M.E., P.C. Marsh, and W.L. Minckley. 1994. Indigenous fishes of western North America and the hypothesis of competitive displacement: Meda fulgida (Cyprinidae) as a case study. Copeia 1994:9-19. Griffith, J.S., and T.R. Tiersch. 1989. Ecology of fishes in Redfield Canyon, Arizona, with emphasis on Gila robusta intermedia. Southwestern Naturalist 34:131-164. Holden, P.B., and C.B. Stalnaker. 1975a. Distribution of fishes in the Dolores and Yampa river systems of the Upper Colorado Basin. Southwestern Naturalist 19:403-412. Holden, P.B., and C.B. Stalnaker. 1975b. Distribution and abundance of mainstream fishes of the Middle and Upper Colorado River basins, 1967-1973. Transactions of the American Fisheries Society 104:217-231. Maddux, H.R., L.A. Fitzpatrick, and W.R. Noonan. 1993. Colorado River endangered fishes critical habitat draft biological support document. U.S. Fish and Wildlife Service, Salt Lake City, UT. 225 pp. Miller, R.R. 1961. Man and the changing fish fauna of the American Southwest. Papers of the Michigan Academy of Science, Arts, and Letters 46:365-404. Miller, W.H., D. Archer, H.M. Tyus, and R.M. McNatt. 1982. Yampa River fishes study, final report. Colorado River Fishery Project. U.S. Fish and Wildlife Service, Salt Lake City, UT. 79 pp. Minckley, W.L. 1973. Fishes of Arizona. Arizona Game and Fish Department, Phoenix. 293 pp. Minckley, W.L., and J.E. Deacon. 1968. Southwestern fishes and the enigma of endangered species. Science 159:1424-1432. |
Minckley, W.L., and J.E. Deacon, eds. 1991. Battle against extinction: native fish management in the American West. University of Arizona Press, Tucson. 517 pp. Platania, S.P., and K.R. Bestgen. 1988. An interim report on the fishes of the lower San Juan River, New Mexico, 1987. New Mexico Department of Game and Fish, Santa Fe. 61 pp. Stalnaker, C.B., and P.B. Holden. 1973. Changes in native fish distribution in the Green River system, Utah-Colorado. Utah Academy of Science, Arts, Letters Proceedings 50:25-32. Suttkus, R.D., G.H. Clemmer, C. Jones, and C.R. Shoop. 1976. Survey of fishes, mammals and herpetofauna of the Colorado River and adjacent riparian areas of the Grand Canyon National Park. Grand Canyon National Park Colorado River Research Serial Contribution 34. 48 pp. Taba, S.S., J.R. Murphy, and H.R. Frost. 1965. Notes on the fishes of the Colorado River near Moab, Utah. Utah Academy of Science, Arts, Letters Proceedings 42:280-283. Tyus, H.M. 1990. Potamodromy and reproduction of Colorado River squawfish (Ptychocheilus lucius). Transactions of the American Fisheries Society 119:1035-1047. Valdez, R. 1984. A survey of fish collections in the White River, Utah. Report for White River Shale Corporation, Ecosystem Research Institute, Logan, UT. 39 pp. Valdez, R. 1990. The endangered fish of Cataract Canyon. Bio/West Report 134-3 for Bureau of Reclamation, Salt Lake City, UT. 94 pp. Valdez, R., P. Mangan, M. McInerny, and R.P. Smith. 1982. Tributary report: fishery investigations of the Gunnison and Dolores rivers. Pages 322-365 in Colorado River Fisheries Project Final Report of Field Investigations, Part 2. U.S. Fish and Wildlife Service, Salt Lake City, UT. Vanicek, C.D., R.H. Kramer, and D.R. Franklin. 1970. Distribution of Green River fishes in Utah and Colorado following closure of Flaming Gorge Dam. Southwestern Naturalist 14:297-315. Wick, E.J., J.A. Hawkins, and C.A. Carlson. 1985. Colorado squawfish and humpback chub habitat monitoring, 1981-1982. Colorado State University Larval Fish Lab and Colorado Division of Wildlife Rep. SE 3-6. 41 pp. Williams, J.E., J.E. Johnson, D.A. Hendrickson, S. Contreras-Balderas, J.D. Williams, M. Navarro-Mendoza, D.E. McAllister, and J.E. Deacon. 1989. Fishes of North America endangered, threatened, or of special concern: 1989. Fisheries 14:2-80. |
