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<br />OJ248Cl <br /> <br />perches from impoundments and other water bodies results in the input of a substantial <br />number of nonnatives to ma.instem habitat. <br /> <br />Over time, escapement from various reservoirs has contributed a steady supply <br />of predators and competitors to riverine habitats occupied by native Colorado River fish <br />species. However, the magnitude of this contribution has not been quantified. For a <br />perspective on the potential, escapement from reservoirs in the Missouri River system <br />included 16 fish species (Walburg 1991), many of which also occur in the Colorado <br />River basin (e.g., common carp, channel catfish, various centrarchids, and walleye). <br />The number of escapees was remarkable; peak numbers ranged from 170,000 channel <br />catfish to 10 million freshwater. drum in a single 24-hr period (Walburg 1971). <br />Although such releases have not been quantified in the Colorado River basin, there are <br />many examples of fish that have been stocked in one location that are now found in <br />other more distant locations (e.g., Courtenay and Robins 1989, Tyus and Beard 1990). <br /> <br />Escapement may eventually result in large numbers of individuals in the riverine <br />environment. In the Yampa River system, for example, northern pike and smallmouth <br />bass populations have increased dramatically since the 1970s when Holden and <br />Stalnaker (1975) did not collect either species. By the early 1980s, northern pike and <br />small mouth bass were present in extremely low numbers (Tyus et al. 1982). By the end <br />of the 1980s, northern pike were abundant in the Yampa system (Nesler 1995), and <br />had dispersed into the Green River system (Tyus and Beard 1990), Smallmouth bass <br />remained uncommon in the Yampa system as of 1991 (Nesler 1995). However, in <br />September, 1995, the authors seined shoreline eddies and found that small mouth bass <br />were more common than the native fishes in these habitats. It is probable that the <br />abundance of small mouth bass in the Yampa River has now increased several orders <br />of magnitude due to continued recruitment from stocked populations in the Yampa <br />River basin. Modde and Smith (1995) present evidence that the relative abundance of <br />northern pike and smallmouth bass increased greatly after 1992 due to escapement <br />from Elkhead reservoir in Colorado. <br /> <br />Not all nonnative fish introductions have been sanctioned by those agencies <br />officially responsible for managing fisheries. Some have been introduced unwittingly <br />through release of bait fish or unwanted pets, others have been introduced <br />accidentally, and some intentionally (Miller 1952, Minckley 1982, Taylor et al. 1984). <br />Illegal transfers, typically of gamefish, are now thought to be a major mechanism by <br />which some nonnative fish become established in new locations (L. Lentsch, Utah <br />Division of Wildlife Resources, personal communication, 1996). <br /> <br />Effects of Nonnatives on the Endemic Colorado River Fauna <br /> <br />For at least 50 years, scientists have been concerned about the role nonnatives <br />have played in the decline of native fishes. Early changes in the Western fish fauna <br /> <br />8 <br />