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03/13/96 15:08 0303 245 6933 FISH & WILDLIFE -)44 R 0 4 Denver -.31004/011 INTRMXTION The Colorado River basin was originally occupied by only thirteen native fishes (Behnke and Benson 1980). However, native fishes of the Colorado River basin are threatened by major environmental changes caused by human-induced alterations to the ecosystem. As a result some populations of native fish fauna have declined; others are on the verge of extinction. Four of these fishes, the Colorado squawfish1, razorback sucker, bonytail, and humpback chub are all now 11sted as endangered. These four fishes are endemic to the Colorado River system and all receive full protection under the Endangered Species Act (ESA; U. S. F'isn and Wildlife Service [FWS]). The Colorado squawfish has been extirpated completely from the lower basin. and it now occupies approximately BOX of its historic range (Tyus 1990). The razorback sucker is extinct in riverine reaches of the lower Colorado River Basin, only occurring in Lake Mohave. In the Uppe^ Colorado River Basin, the largest populations of both Colorado squawfish ano adult razorback sucker are found in the Green and Yampa rivers (Holden and Wick 1982; Lanigan and Tyus 1989), A small remnant population of Colorado squawfisn occurs in the San Juan River; no wild razorback sucker have been found here in recent times (Ryden and Pfeifer 1993). In the Upper Colorado River, only 21 adult razorback sucker have been captured since 1980 from riverine habitats; only seven adult fish have been captured since 1990 (Valdez et al 1982; Osmundson and Kaeding 1991; Burdick 1992; unpublished FWS data). Colorado squawfish continue to persist in the Upper Colorado River, recruitment is occurring, but it is uncertain whether this is adequate to maintain a stable population size. Humpback chub now exist only in four widely-separated canyon habitats. Bonytail. the rarest native fish in the Colorado River, is considered "virtually" extinct