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32
<br />COPEIA, 1990, NO. 1
<br />not all, of the aggressive behavior we observed
<br />in this species was due to reproductive events.
<br />Both N. lutrensis and P. ~iromelas vigorously pur-
<br />sued food items while Ptychocheilus lucius re-
<br />mained relatively "quiescent" following food
<br />addition. The indication that agonistic behavior
<br />and overall movements of P. lucius may have
<br />been inhibited in the Pimephales promelas tank
<br />suggests that young Ptychocheilus lucius may be
<br />disadvantaged in backwater habitats where
<br />quality resources (i.e., food or space) may be
<br />limiting.
<br />There was little indication of a widespread
<br />negative interaction between similar-sized R.
<br />balteatus and young P. lucius. Other studies have
<br />also noted a lack of interspecific overt aggres-
<br />sion (i.e., nipping, chasing) by R. balteatus Qo-
<br />hannes and Larkin, 1961; Beamesderfer and
<br />Congelton, unpubl. ms.; Reeves et al., 198'7).
<br />The distribution of R. balteatus in the Green
<br />River basin is now restricted to higher elevation
<br />tributaries (Banks, 1964; Vanicek and Kramer,
<br />1969; Tyus et al., 1982), and thus, opportunity
<br />for interaction with young P. lucius is probably
<br />low (Tyus et al., 1982; USFWS, unpubl.). How-
<br />ever, interaction between young P, lucius and
<br />R. balteatus may have been more frequent in the
<br />past due to the former abundance of this intro-
<br />duced species in the mainstream Green River.
<br />Behavior of young A. melas may have been
<br />most strongly influenced by confinement in
<br />aquaria without suitable cover, and thus, we
<br />hesitate to speculate on their potential inter-
<br />action with P. lucius. Although this introduced
<br />species is relatively uncommon in the Green
<br />River (USFWS, unpubl.), voracious feeding by
<br />larger individuals (Tyus and Minckley, 1988)
<br />together with their propensity to proliferate
<br />(Minckley, 19'73), suggests that this species could
<br />affect native fish populations.
<br />We noted a high incidence of mortality due
<br />to "white-spot disease" in P. lucius and C. lati-
<br />pinnis. In contrast, of the non-natives, only N.
<br />lutrensis became infected. This suggests that
<br />some native fishes may be more susceptible to
<br />outbreaks of this disease.
<br />Distribution patterns and behavioral inter-
<br />actions we noted may have been influenced by
<br />many factors including fish density, confine-
<br />ment in aquaria, water clarity, lack of cover,
<br />and study design. Although we do not presume
<br />that this study accurately depicted behavior of
<br />young P. lucius and the other fishes in the nat-
<br />ural environme-nt, fish densities used were with-
<br />in the range of those noted in backwater hab-
<br />itats in the Green River: P. lucius, range 0-110
<br />fish/10 m2; C. latipinnis, range 0-31 fish/10 m2;
<br />N. lutrensis, range 0-877 fish/10 m2; R. baltea-
<br />tus, range 0-100 fish/10 m2; Pimephales prome-
<br />las, range 0-756 fish/10 m2; L. cyanellus, range
<br />0-410 fish/10 m2; A. melas, range 0-13 fish/
<br />10 m2 (USFWS, unpubl.). These field densities
<br />are rough approximations of backwater con-
<br />ditions and are influenced by susceptibility to
<br />seine capture, relative geographic abundance,
<br />and daily, seasonal, and annual flow fluctua-
<br />tions.
<br />Ptychocheilus lucius evolved in the isolated and
<br />depauperate Colorado River basin (Miller,
<br />1959). Rapid habitat change caused by altera-
<br />tion of the natural hydrologic regime and wide-
<br />spread proliferation of numerous potential
<br />competitor and predator fish species, have no
<br />doubt stressed the competitive ability of young
<br />P. lucius. The relatively low number of aggres-
<br />sive acts and the high degree of intraspecific
<br />aggression (>90%) noted for young P. lucius,
<br />coupled with its apparent naivete towards other
<br />fishes and its slower pursuit of prey items (at-
<br />tacks were initiated 20-40 min following ad-
<br />dition of live fish larvae), suggests that young
<br />P. lucius may be competitively inferior in a re-
<br />source-limited environment.
<br />The rarity of juvenile P. lucius (70-400 mm
<br />TL) in the Green River system (Tyus et al.,
<br />1982; Tyus and Karp, 1989) suggests that early
<br />life history stages may be experiencing high
<br />mortality. Ptychocheilus lucius become piscivo-
<br />rous sometime during their first year (Vanicek
<br />and Kramer, 1969; Jacobi and Jacobi, unpubl.
<br />ms.; McAda and Tyus, 1984), and the abun-
<br />dance of small non-native prey fishes may ac-
<br />tually air growth of juvenile P. lucius. This
<br />study indicates that age-0 P. lucius may be neg-
<br />atively affected by presence of small aggressive
<br />non-native fishes, particularly during periods of
<br />overcrowding and resource ]imitation. We sus-
<br />pect that interspecific aggressions exhibited by
<br />N. lutrensis, Pime~hales~romelas, and L. cyanellus,
<br />their tolerance for warm, turbid conditions
<br />characteristic of backwater habitats, and their
<br />rapid proliferation throughout the Green River
<br />system, may have an adverse affect on growth
<br />and survival of young Ptychocheilus lucius.
<br />ACKNOWLEDGMENTS
<br />Funding for this project was provided by the
<br />U.S. Bureau of Reclamation and the U.S. Fish
<br />and Wildlife Service. G. L. Dean and C. Wil-
<br />
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