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<br />lJ02510 <br /> <br />larvae, because the larger size of the juveniles (130 to 150 mm at the end of the first <br />year for razorback suckers)would greatly reduce their vulnerability to predation. Green <br />sunfish, channel catfish, red shiner, and fathead minnow were considered the most <br />serious threats to larvae of both of the native species. Concern for interactions with <br />centrarchids was generally high. <br /> <br />As mentioned previously, negative interactions of nonnative fishes on the <br />juveniles of Colorado squawfish and razorback sucker were less than that for the <br />larvae. Channel catfish, small mouth bass, and pike were a greater concern for the <br />juveniles of the two natives. From a control perspective, channel catfish, small <br />cyprinids, and centrarchids merit attention. <br /> <br />The rarity of the humpback chub and lack of knowledge about its interactions with <br />nonnative fishes in the UCRB made it difficult to identify chief threats them. Most <br />participants felt they did not have sufficient information to evaluate those threats with <br />any certainty. Nevertheless, the general opinion held that threats from channel catfish <br />were the most serious. <br /> <br />Channel catfish was regarded as a highly significant threat to the listed species. <br />It was accorded a high level of concern for interactions with larvae and juveniles of all <br />listed fishes, and no other species received as many high rankings. The fish species <br />representing the next most important threats were the green sunfish, fathead minnow, <br />and red shiner. <br /> <br />Control Measures <br /> <br />Most of the control measures suitable for use with nonnative species in the UCRB <br />were reviewed recently in Lentsch et al. (1995). These fall into four general categories: <br />mechanical teChniques (traps, nets, seines, and electrofishing), chemical removal (i.e., <br />poisons), biological techniques (introduce other predators, infectious agents like <br />channel catfish viral disease), and physicochemical manipulations (altering flow regime <br />or water chemistry through reservoir releases), During the workshop, experts <br />suggested four measures not treated in Lentsch et al. (1995). Larry Hesse presented <br />insight into commercial harvesting, which relies on mechanical techniques, on a large <br />scale that could greatly reduce channel catfish populations. Todd Crowl discussed the <br />use of exclusion structures for protecting nursery habitat and enhancing the recruitment <br />of native fishes. Escapement controls were suggested for reservoir outlets to prevent <br />the nonnative fishes from reaching surface waters connected with the river. Larry <br />Hesse also brought up the possibility of using a Llewellyn weir, which incorporates <br />electroshock. <br /> <br />Not all nonnative species are equally susceptible to specific control measures <br />(Table 8). Furthermore, the listed species also will be removed by many of the control <br /> <br />29 <br />