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<br />Copeia, 1990(1), pp. 25-34
<br />Behavioral Interactions between Young ~-
<br />Colorado Squawfish and Six Fish Speci
<br />CATHERINE A. KARP AND HAROLD M. TYUS
<br />We evaluated patterns of space use, activity, and agonistic interactions between
<br />young Ptychocheilus lucius, an endangered cyprinid, and similar-size individuals
<br />of native and non-native fishes (i.e., Catostomus latipinnis, Notropis lutrensis,
<br />Richardsonius balteatus, Pimephales promelas, Lepomis cyanellus, and Ameiurus
<br />melas), which co-occur in shoreline riverine habitats. Vertical distribution of P.
<br />lucius was most similar to that of L. cyanellus and R. balteatus. We detected no
<br />overt shifts in vertical space use by P. lucius due to the presence of non-native
<br />fishes. Lepomis cyanellus, N. lutrensis, and Pimephales promelas initiated more
<br />interspecific aggression than Ptychocheilus lucius. Agonistic behavior in P. lucius
<br />and C. latipinnis was mostly inteaspecific. Richardsonius balteatus and P. lucius
<br />were the most active and widely ranging species, but only N. lutrensis, Pimephales
<br />promelas, and C. latipinnis significantly increased their activity during feedings.
<br />Predation by Ptychocheilus lucius on live fish was slower than that of L. cyanellus,
<br />N. lutrensis, and R. balteatus. Overt physical damage was infrequent, however,
<br />the aggressive nature of the non-natives, in concert with similar activity sched-
<br />ules, space use patterns, and shared habitat use, suggests that L. cyanellus, N.
<br />lutrensis, and Pimephales promelas may adversely affect growth and survival of
<br />age-0 Ptychocheilus lucius.
<br />THE native fish fauna of the Colorado River
<br />basin is characterized by low species di-
<br />versity and high endemism (Miller, 1959). His-
<br />toriccomposition ofthis unique fauna no longer
<br />exists due, in part, to the combined effects of
<br />human-induced habitat alteration, including
<br />regulation of flow, temperature, and sediment
<br />regimes, river impoundment, water diversion,
<br />and introduction of about 50 species of fish (Mil-
<br />ler, 1961; Minckley, 1973; Tyus et al., 1982).
<br />Mechanisms whereby introduced fishes affect
<br />the native Colorado .River fishes are not well
<br />understood, but widespread patterns of decline
<br />with the concomitant proliferation of intro-
<br />duced forms, suggests that interspecific inter-
<br />actions (e.g., aggressive behavior, competition,
<br />predation), among other factors, may be in-
<br />volved (Miller, 1961; Minckley and Deacon,
<br />1968; Minckley, 1973).
<br />Historically, Ptychocheilus lucius (Colorado
<br />r' squawfish) was common to abundant in main-
<br />-, stream rivers of the Colorado River basin (Ev-
<br />ermann and Rutter, 1895; Jordan and Ever-
<br />mann, 1896), but today occupies less than 25%
<br />of its former range. This species is listed as en-
<br />dangered by the U.S. Fish and Wildlife Service
<br />(USFWS) and protected under provisions of the
<br />Endangered Species Act of 1973. Factors im-
<br />plicated in the decline of P. lucius include block-
<br />~a.~©
<br />age of migration routes, loss of usable habitat,
<br />low numbers of reproducing adults, high ju-
<br />venile mortality, suboptimal temperature re-
<br />gimes, and the introduction of about 40 poten-
<br />tial predator and competitor fish species
<br />(Vanicek et al., 1970; Holden and Stalnaker,
<br />1975; Joseph et al., 1977; Seethaler, 1978; Tyus
<br />et al., 1982; Kaeding and Osmundson, 1988;
<br />Tyus and Karp, 1989).
<br />The Green River basin in Colorado and Utah
<br />supports the largest reproducing population of
<br />P. lucius in the United States (Seethaler, 1978;
<br />H. M. Tyus, unpubl.). Ephemeral shoreline em-
<br />bayments, or backwaters, in the mainstream
<br />Green River are preferred nursery habitats of
<br />young P. lucius (Holden and Stalnaker, 1975;
<br />Holden, 1977; McAda and Tyus, 1984), and
<br />may be occupied by as many as 16 other species
<br />(four native and 12 non-native, Tyus et al.,
<br />1982). These habitats may be areas of poten-
<br />tially high interspecific fish interaction because
<br />high numbers of fish species are present, and
<br />many of the non-native fishes are abundant.
<br />Patterns of shared habitat use from distri-
<br />butional surveys and food habit studies indicate
<br />that survival of young P. lucius in backwater
<br />habitat may be adversely affected by local and
<br />widespread establishment of introduced fishes
<br />(Holden, 1977; Joseph et al., 1977; McAda and
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