<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 />
|