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<br />~~ <br /> <br />Dam) where nonnative fishes have replaced native fish species (Miller <br />(1961). <br /> <br />Many of the nonnative fishes introduced into the Colorado River basin <br />are suspected of adversely affecting the native mainstem fishes in some <br />fashion. Warmwater gamefish are thought to have the greatest adverse <br />effect on endangered native fishes. Centrarchids (e.g.. largemouth <br />bass [Micropterus sa7moides], green sunfish [Lepomis cyane77us]. <br />bluegill [L. macrochirus], black crappie [Pomoxis nigromacu7atus], and <br />smallmouth bass [M. do7omieui]), ictalurids (e.g.. channel catfish <br />[Icta7urus punctatus] and black bullhead [Ameiurus me7as]), and esocids <br />(northern pike [Esox 7ucius]) are listed as frequent contributors to <br />the decline of native fishes. An increasing body of evidence <br />characterizes the negative interactions of nonnative fishes with the <br />endangered big river fishes (Hawkins and Nesler 1991; Minckley et al. <br />1991: Maddux et al. 1993; Lentsch et al. 1996). Some of this evidence <br />is indirect that includes inferences from field data or results from <br />laboratory studies of predation by nonnatives on natives. Laboratory <br />studies have documented agonistic behavior. resource sharing, and <br />vulnerability to predation (Papoulias and Minckley 1990; Karp and 1yus <br />1990: Ruppert et al. 1993; Johnson et al. 1993). Direct evidence or <br />predation includes native fishes obtained from stomach contents of <br />nonnative fishes and by visual observation of predation. Other means <br />by which nonnative may adversely affect native fishes is by competition <br />for food which limit the success of razorback sucker (Papoulias and <br />Minckley 1990) and interactions that do not involve predation. 1hese <br />include hybridization of white suckers with other native Colorado River <br />suckers (Burdick 1995) that could compromise the genetic integrity of <br />native suckers, and adult Colorado squawfish that prey on channel <br />catfish may choke on the catfish's pectoral spines (McAda 1983; <br />Pimental et al. 1985; Quartarone 1993). <br /> <br />A substantial body of indirect evidence for nonnative predation has <br />been assembled for the razorback sucker. The loss of early-life <br />history stages of the razorback sucker has been linked to predation .by <br />nonnative fishes, and the loss of those stages has led to a virtual <br />absence of recruitment to adult size. Recruitment failure has been <br />cited as the major cause of the decline and endangerment of the <br />razorback sucker. <br /> <br />Tyus and Saunders (1996) recently produced a comprehensive review of <br />evaluating the impact of nonnative fishes on aquatic ecosystems, <br />identifying and discussing the effects of introductions of nonnative <br />fishes on big river fishes of the mainstem Colorado River, and <br />proposing strategies for mitigating the negative impacts of nonnative <br /> <br />s- <br />