9474 - Laserfiche WebLink

Home Browse Search

This should be of concern, in that it also demonstrates #hat hybrid individuals can be fertile, and that introgressed genes can be perpetuated in the population. Interestingly, only large (i.e., old) native suckers were found in the Little Yampa Canyon reach, whereas a greater size distribution (and thus age structure) was maintained in the introduced White Sucker (and to some extent hybrids). Lower reaches of the Yampa River, where natives are more abundant, were not evaluated for this trend. However, the hybrid zone appears localized and its effects dwindle as one moves downstream. Thus, the spread of individuals with alien genomes may not be of immediate concern. INTRODUCTI©N Numerous factors have contributed to the decline of indigenous fishes in western North America (Miller, 1961; reviewed by Hinckley and Douglas, 1991). Yet, the most serious stem from planned or impromptu introduction of alien species (both vertebrate and invertebrate). There are 36 indigenous fish species that comprise the Colorado River Basin ichthyofauna, and they have been subjected to over 70 nonnative fish introductions (Harsh and Douglas, 1997). Three indigenous species are now extinct, and 22 are either federally endangered or threatened. As a result, southwestern United States is now recognized as a major "hotspot" for reduction of endemic fish biodiversity (Dobson et al., 1997:figure 1 c). There are several mechanisms by which exotic fishes impact indigenous ones. From an ecological standpoint, overt predation of the former upon the latter is most visible and apparent (Heffe, 1985; Marsh and Langhors#, 198$; Marsh and Douglas, 1997). Here, impacts are immediate and severe, such that native fish densities become depressed and recruitment curtailed (Hinckley, 1973; Bestgen and Propst, 1989). However, exotic fishes also affect endemics in ways that occur more slowly and over longer periods of time, yet with results equally as devastating as overt predation. Exotics, for example, may out-compete and ultimately replace endemics, particularly when the realized niches of the two overlap considerably (reviewed by Ross, 1991; 4