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direct than simply limiting reproduction, as few adults of this rela- tively long-lived minnow are found after impoundment. Adults of ell other native species (notably razorback suckers) seem to survive w in the reservoirs, but may have trouble reproducing. ' In addition, squawfish also have declined in the riverine portion above some of the reserwirs (e.g., Salt River above Roosevelt Lake, Arizona; Green River above Flaming Gorge, Wyoming; San Juan River above Navajo ' Reservoir, New Mexico). Reasons for these disappearances-are riot fully understood, but they may be related to the proliferation of populations of introduced fishes. Species such as the red shiner, Notropis lutrensis, 1 redside shiner, Richardsonius balteatus, and green sunfish, Lepomis cyanellus have become established in habitats formerly occupied only by endemic fishes. These and other non-native fishes may be subjecting young Colorado squawfish to biological interactions for which they are poorly adapted due to their "long isolation. In this respect, the squaw- fish may be similar to geographically isolated island fauna which can be quickly decimated by predation or competition from foreign species. Below reservoirs, reasons for native fish declines are more clearly understood (Vanicek and Kramer 19.69; Seethaler N.D.). In these areas, streams always undergo similar types of alterations: water temperature fluctuations and mean temperatures are reduced, seasonal and daily flow patterns are altered, turbidity declines, salinity increases and exotic fish are stocked. These influences are reported to have stopped squawfish reprrx7uction in the Green River below Flaming Gorge Dam for 65 miles (105 ~) to the confluence of the Yampa River; the Yampa River presently provides habitat for squawfish and modifies the Green River below that point (Vanicek et al, 1970: Holden and Stalnaker 1975). The rotenone fish eradication program in 1962 carried out on the Green River in conjunction with the closure of Flaming Gorge Dam and a similar program on the San Juan River in conjunction with the closure of Navajo Dam resulted in the direct destruction of Colorado squawfish. The precise magnitude of the squawfish loss was not 1 doc~mpnted in either case. But, a reduction in squawfish numbers followed immediately by extreme habitat modification (which would limit re-population) no doubt contributed to the decline of this endemic species. Perhaps Hinckley and Deacon (1968) best summarize the problems that confront Colorado squawfish, and native Southwestern fishes in general: "Declines in the populations of native fishes in the American Southwest are largely due to habitat changes associated with man's modification of various aquatic environments. Early deci- mation of the fauna was mainly a result of large-scale physical change, such as the diversion and impoundment of river and down- cutting of streams in their formerly stable floodplains...P4ore subtle physical or chemical changes...eutrophication and other pollutional effects, and biological phenomena associated with the ever-increasing introduction of exotic species--all are ac- celerating the extirpation of remnant- populations. 7 L