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e 38 e H. M. TYUSAND]. M. BEARD [Volume 50 only one female walleye with developed ovaries, and that was in May at a water tem- perature of 13 C. Walleye in other locations usually spawn at cooler water temperatures (3.3-7.2 C, Sigler and Miller 1963; 5.6-11.1 C, Scott and Crossman 1973). No small wall- eye (< 395 mm TL) were captured in this study. Young of the endangered humpback chub, Gila cypha; bony tail chub, G. elegans; razor- back sucker, Xyrauchen texanus; and Colo- rado squawfish, Ptychocheilus lucius, may be potential prey for northern pike and walleye. None of these fishes were identified in stom- achs of northern pike or walleye, but our abil- ity to detect such predation was constrained by a small sample size of stomachs that con- tained food, rarity of endangered fishes, and inability to identify all of the fishes eaten. Sympatry of adults of northern pike, wall- eye, and endangered fishes is a cause for con- cern, particularly if resource sharing occurs during periods oflimited availability. We col- lected northern pike, walleye, and Colorado squawfish in similar shoreline habitats in the mainstream Green River; in addition, radio- tagged northern pike and Colorado squawfish were syntopic in the Green and Yampa rivers (Valdez and Masslich 1989, Wick and Hawkins 1989). Northern pike were captured in shal- low, flooded habitats also utilized by razor- back sucker. Stocking programs for northern pike and walleye have been discontinued by state agencies in Colorado and Utah (G. M. Davis and P. J. Martinez, personal communication), and the relative absence of small fish of both species suggests that reproduction in the mainstream Green River is low or nonexistent during most years. However, the continuing invasion of northern pike and walleye into the Green River from established, reproducing stocks should be monitored, and their interac- tions with endangered fishes further evalu- ated until it can be more clearly demonstrated that competition or predation on endangered fishes does not occur or pose a serious threat. The increasing abundance and spread of northern pike, the diversity of fishes con- sumed, and its syntopy with endangered fishes make this voracious piscivore a poten- tial threat to endangered Colorado River fishes. ACKNOWLEDGMENTS This work was funded by the Bureau of Reclamation and the Fish and Wildlife Ser- vice. Numerous Fish and Wildlife Service employees assisted with data collection. We thank S. R. Cranny and M. Moretti of the Utah Division of Wildlife Resources and R. A. Valdez of BIO/WEST Incorporated for pro- viding northern pike and walleye stomachs. G. B. Haines and D. Moses aided in data summarization and word processing. C. A. Karp, P. Pister, and an anonymous reviewer improved an earlier draft manuscript. LITERATURE CITED BEYERLE, G. B., AND J. K WILLIAMS. 1968. Some observa- tions of food selectivity by northern pike in aquaria. Transactions of the American Fisheries Society 97: 28-31. CAR LANDER, K D. 1969. Handbook of freshwater fishery biology. Vol. I. Iowa State University Press, Ames. 752 pp. CARLSON, G A, AND K M. CARLSON. 1982. Review of selected literature on the upper Colorado River system and its fishes. Pages 1-8 in W. H. Miller, H. M. Tyus, and GA. Carlson, eds., Fishes of the upper Colorado River system: present and future. American Fisheries Society, Bethesda, Maryland. 131 pp. COOK, M. F., AND K P. BERGERSEN. 1988. Movements, habitat selection, and activity periods of northern pike in Eleven Mile Reservoir, Colorado. Transac- tions of the American Fisheries Society 117: 495-502. FROST, W. K 1954. The food of pike, Esox lucius L., in Windermere. Journal of Animal Ecology 23: 339-360. HOLDEN, P. B., AND G B. STALNAKER. 1975. Distribution and abundance of mainstream fishes of the middle and upper Colorado River basins, 1967-1973. Transactions of the American Fisheries Society 104: 217-231. JOSEPH, T W., J. A SINNING, R J. BEHNKE. AND P. B. HOLDEN. 1977. An evaluation of the status, life history, and habitat requirements of endangered and threatened fishes of the upper Colorado River system. FWS/OBS-77-62. U.S. Fish and Wildlife Service, Office of Biological Services, Fort Collins, Colorado. 194 pp. LAWLER, G. H. 1965. The food of the pike, Esox lucius, in Hemming Lake, Manitoba. Journal of the Fish- eries Resource Board of Canada 22: 1357-1377. MILLER, R B. 1948. A note on the movement of the pike, Esox lucius. Copeia 1948: 62. MOYLE, P. B., H. W. LI, AND B. A BARTON. 1986. The Frankenstein effect: impact of introduced fishes on native fishes in North America. Pages 415-426 in R. H. Stroud, ed" Fish culture in fisheries management. American Fisheries Society, Be- thesda, Maryland. 481 pp. ~