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<br />r G{u;rl?-<-J <br />" \ q ~o <br /> <br />'" <br /> <br />. <br /> <br />~ <br /> <br />e <br /> <br />Great Basin Naturalist 50(1), 1990, pp. 33-39 <br /> <br />e <br /> <br />02Z?L_ <br /> <br />---. <br /> <br />ESOX LUCIUS (ESOCIDAE) AND STlZOSTEDION VITREUM (PERCIDAE) <br />IN THE GREEN RIVER BASIN, COLORADO AND UTAH <br /> <br />Harold M. Tyus1 and James M. Beard1,2 <br /> <br />ABSTRACT.-Northern pike, Esox lucius, stocked in the Yampa River in 1977, invaded the mainstream Green River <br />by 1981 and subsequently increased in range and abundance. The speed of this invasion is indicated by two recaptured <br />pike that moved 78 and llO km, respectively, downstream in about one year. Pike stomachs (n = 123) were usually <br />empty (54,5%), but some contained fish (43%) and nonfish items (2.4%). Red shiner, Notropis lutrensis, and fathead <br />minnow, Pimephales promelas, predominated among the 12 fish species eaten. Walleye, Stizostedion vitreum, <br />presumably introduced to the Green River drainage in the 1960s, was widely distributed but low in abundance. Most of <br />61 adult walleye stomachs contained food (60.7%); of 6 fish species eaten, channel catfish, Ictaluruspunctatus, and <br />fathead minnow were most frequently consumed. Northern pike and walleye were captured in habitats occupied by <br />endangered Colorado River fishes, particularly Colorado squawfish, Ptychocheilus lucius. Predation on endangered <br />fishes was not detected, but northern pike and walleye consumed at least three other native fishes. The northern pike <br />may pose a threat to endangered fishes due to its population expansion, piscivory, and resource sharing. Diets of <br />northern pike and walleye species should be further evaluated if their abundance increases. <br /> <br />Northern pike were introduced into Elk- <br />head Reservoir, an impoundment on the <br />Yampa River drainage, in 1977 (P, J. Mar- <br />tinez, personal communication) and collected <br />in the mainstream Yampa River as early as <br />1979 (E. J. Wick, personal communication), <br />Their numbers increased in the upper Yampa <br />River in the early 1980s (Wick et al. 1985), and <br />a downstream movement into the Green <br />River was subsequently documented in 1981 <br />(Tyus et al. 1982, Green River fishery investi- <br />gations). Northern pike reproduction has <br />been reported in the upper Yampa River <br />drainage, where it has access to the main- <br />stream river (T. P. Nesler, personal commu- <br />nication). <br />Walleye presumably accessed the main- <br />stream Green River by moving downstream <br />from various tributaries. The fish was first <br />reported in Utah in 1951 (Sigler and Miller <br />1963), and reproducing populations of wall- <br />eye were established by fish stockings in <br />Duchesne River reservoirs (Fig, 1) in the <br />1960s and 1970s (G. M. Davis, personal com- <br />muniCation). <br />The Green River basin of Colorado and <br />Utah is an important recovery area for four <br />rare and endangered Colorado River fishes <br />(reviewed by Joseph et al. 1977, Carlson and <br /> <br />lU.S. Fish and Wildlife Service, 1680 W. Highway 40, Vernal, Utah 84078. <br />2Present address: 1361 Vernon St., Eureka, California 95501. <br /> <br />Carlson 1982, U.S, Fish and Wildlife Service <br />1987). However, over 20 nonnative fishes <br />have been introduced into the basin for sport, <br />forage, food, or by accident (Tyus et al. 1982, <br />Fishes of upper Colorado), Impacts of these <br />introduced fishes on the native fauna are not <br />well understood, but the presence of two <br />large piscivores, northern pike, Esox lucius, <br />and walleye, Stizostedion vitreum, in areas <br />presently occupied by endangered fishes, is <br />cause for concern. Control of nonnative fishes <br />has been identified as a recovery measure <br />under provisions of an interagency recovery <br />program for endangered fish species in the <br />upper Colorado River basin (U. s, Fish and <br />Wildlife Service 1987). Fish introductions in <br />other locations have eliminated or partially <br />extirpated native fish faunas, and the instabil- <br />ity of resultant communities has caused man- <br />agement problems (Moyle et al. 1986). <br />The purpose of this study was to determine <br />diets of northern pike and walleye in the Green <br />River, and to evaluate the degree of predation <br />on native and endangered fishes. We also doc- <br />ument the recent invasion of northern pike <br />into the Green River basin, and the abun- <br />dance and distribution of northern pike and <br />walleye in the mainstream Green River, The <br />results of this study are interpreted relative <br /> <br />33 <br />