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3.l <br />it was historically. The magnitude of mean annual flows and the frequency of <br />over bank flooding also has been significantly reduced (Brink and Schmidt <br />1996). The Service believes that these significant alterations in hydrology <br />and geomorphology of the Duchesne River has reduced its ability to maintain <br />aquatic endemic species like the Colorado squawfish and razorback sucker whose <br />life cycles depend on access to floodplain and particular in-channel habitats. <br />In addition, the impoundment of tributaries and mainstem waters also has <br />resulted in the stocking of a number of nonnative sport and bait fishes for <br />use by local residents and visitors to the basin. While the acceptance of <br />these fishes has been generally favorable to the public, their presence has <br />led to predation, competition and the general demise of native fishes (Tyus <br />and Saunders 1996). The stocking of nonnative warm water fishes such as <br />channel catfish, smallmouth bass, and walleye have resulted in the continuing <br />high probability of predation on soft-rayed native fishes. Red shiners, for <br />example, have been documented as preying on larval suckers, including <br />razorbacks (Rupert et. al. 1993. Modde and Irving 1997). Other exotics sucf~ <br />as sand shiner and fathead minnow compete for food and space in remaining <br />habitats. <br />As reported by the Interagency Aquatic Nuisance Species Task Force (ANSTF <br />1994), introductions of nonnative species into aquatic habitats has become <br />problematic worldwide. Not unique to the Colorado River system, the threat <br />that nonnative fisl~ies pose to native fishes, particularly when coupled with <br />significant changes to the physical environment (as in the case of the <br />Duchesne River), is hastening the decline of native species. <br />Tyus and Saunders (1996) reported species commonly identified as endangering <br />native fishes nationwide are centrarchids such as largemouth bass (Micronterus <br />salmoides), green sunfish (Len_. orris ~yanellus), bluegill (Ig omi ~n~~rochirus), <br />crappie (Pomoxis ~.). and smallmouth bass (Micropterus dolomieu). The <br />second most cited group are from the family Ictaluridae, which includes <br />channel catfish (~ lurus ~). and bullheads (Amirus ,~.).` Other <br />species such as red shiner (~,Yorinalla lutrensis) and fathead minnow also have <br />been identified as problematic to Colorado River endangered fishes. All of <br />these species currently inhabit the Colorado River Basin. <br />Some scientists believe (Tyus and Saunders 1996) that .changes in the <br />biological environment as a result of fish introductions may currently be the <br />most significant threat to the native fish fauna of the Colorado River Basin. <br />Since the late 1800's, over forty species of nonnative fishes have been <br />stocked (primarily to enhance recreational fishing opportunities) or <br />