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<br />0Q2481
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<br />noted in Colorado by Ellis (1914) who attributed declines, in part, to competition by
<br />introduced fishes. Dill (1944) was one of the first to suggest that nonnatives were
<br />responsible for declines observed in native fish populations in the lower Colorado River
<br />basin. He recognized that the decline began about 1930, and that it was coincident
<br />with a large increase in the abundance of nonnative fishes, especially channel catfish
<br />and largemouth bass. By 1960, populations of the big river fishes had been reduced
<br />greatly. Miller (1961) noted "drastic changes" in the fish fauna and observed that the
<br />"most impressive documentation for changing fish fauna" occurred in the lower
<br />Colorado River where it was associated with a replacement by introduced fishes.
<br />Schoenherr (1981) considered the evidence "overwhelming" for replacement of native
<br />fishes by aggressive introduced fishes, and he provided examples in which predation
<br />resulted in extirpation. More recent studies document a decline in the abundance of
<br />native fish species as nonnative species increased in abundance (Joseph et al. 1977,
<br />Behnke 1980, Osmundson and Kaeding 1989, Quaterone 1993).
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<br />An increasing body of evidence characterizes the negative interactions of
<br />nonnative fishes with the endangered big river fishes (Hawkins and Nesler 1991,
<br />Minckley et al. 1991, Maddux et al. 1983, Lentsch et al. 1995). Many of the reports
<br />present evidence that is indirect because they lack direct observations or absolute
<br />proof of predation on natives. Such indirect evidence may include inferences from field
<br />dat or results of laboratory studies. Direct evidence of predation includes native fishes
<br />obtained from stomach contents of the nonnative fishes and by visual observation of
<br />predation.
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<br />Indirect evidence connecting the decline of native fishes to the proliferation of
<br />nonnative fishes has been given by many workers (Dill 1944, Wallis 1951, Jonez and
<br />Sumner 1954, Miller 1961, Vanicek 1967, Rinne 1971, Vanicek and Kramer 1979,
<br />Baxter and Simon 1970, Moyle 1976, Holden 1977, Joseph et al. 1977, Allan and
<br />Roden 1978, Deacon 1978, Behnke 1980, Miller et al. 1982 and references therein,
<br />Kaeding and Zimmerman 1983, Minckley 1983, Wick et al. 1985, Bestgen and Propst
<br />1989, Marsh and Minckley 1989, Tyus and Karp 1989, Tyus and Beard 1990, Tyus and
<br />Nikirk 1990, Valdez et al. 1990, Minckley and Deacon 1991 and references therein,
<br />Propst and Bestgen 1991, Rinne 1991, Rinne and Minckley 1991, Scoppertone 1993,
<br />Trammel et al. 1993, and Valdez and RyeI1995). Other workers have studied dietary
<br />overlap and postulated that competition for food and/or space was occurring (Jacobi
<br />and Jacobi 1981, MeAda and Tyus 1984, Grabowski and Hebert 1989, Muth and
<br />Snyder 1995, and Valdez and RyeI1995). Laboratory studies have documented
<br />agonistic behavior, resource sharing, and vulnerability to predation (Papoulias and
<br />Minckley 1990, Karp and Tyus 1990, Ruppert et al. 1993, and Johnson et al. 1993),
<br />
<br />A substantial body of indirect evidence for nonnative predation has been
<br />assembled for the razorback sucker. An almost total lack of recruitment to an adult
<br />size has been cited as the major cause of the decline and endangerment of the
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