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<br />, <br /> <br />ECOLOGY OF RIVERINE FISHES IN REGULATED STREAM SYSTEMS <br /> <br />69 <br /> <br />rearing areas that are not found in undisturbed areas. The red <br />shiner (Notropis lutrensis), also an exotic, is very abundant in <br />the Colorado River, but is also very abundant in the Green River. <br />The channel morphology of the Colorado River in this area appears <br />to be somewhat entrenched compared to the lower Green River. This <br />observation is not based on any quantitative data and, therefore, <br />is the opinion of the author. Because of the great decrease in May <br />flows in this system, it appears probable that the channel morphology <br />may have changed over the last 10-15 years. <br /> <br />Therefore, it appears possible that a combination of factors, <br />such as low flows, abundant exotic competitors, predators, and <br />long-term habitat changes, may be working in combination to the <br />detriment of squawfish. Because our ability to determine successful <br />reproduction is limited to finding young-of-the-year, it is feasible <br />that sufficient habitat for fry is no longer available, therefore <br />leaving them very susceptible to exotic centrarchid predators. <br />Also, present information indicates that red shiners probably com- <br />pete primarily with the young of native fishes and that in areas of <br />marginal habitat for the native juveniles, red shiners predominate <br />(Holden and Irvine, 1975; Holden, 1977). Therefore, it is also <br />possible that, in the Colorado River, altered flows have reduced <br />the amount of preferred habitat for native species to the point <br />where red shiners are favored. <br /> <br />The major reason for making these speculations is to suggest <br />that the regulation of rivers by dams may well cause more detri- <br />mental problems to riverine fishes than is presently known. It is <br />hoped that future investigations will include more in-depth studies <br />in sections of regulated rivers far below the dams and at time <br />periods fairly well removed from the injtiation of the regulations. <br /> <br />One other factor that often affects fishes when dams are built <br />on rivers is the rather widespread practice of preimpoundment fish- <br />eradication programs. This has little to do with the ecology of <br />riverine fishes in regulated streams, except that these programs <br />may well cause the extirpation of a riverine species before impound- <br />ment. Eradication programs in the Colorado River Basin were <br />conducted at the Flaming Gorge Reservoir site and the Navajo Dam <br />site. Both were conducted to eradicate "rough fish," which were <br />primarily native species. No preimpoundment eradication program <br />was conducted in the Glen Canyon (Lake Powell) area. After closure <br />of Flaming Gorge and continuing until the present, large numbers of <br />exotic Utah chub (Gila atraria) have filled the reservoir and have <br />been a major management problem. In Navajo Reservoir, large numbers <br />of native round tail chubs (Gila robusta) were abundant for. several <br />years after formation, but have since declined in numbers drasti- <br />cally. At Glen Canyon, no increase in rough fish was noted. This <br />points out a fairly understandable ecological axiom: Obligatory <br />