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h ~" ~. ~ ~~ „" ..~ 4 -. American Fisheries Society Symposium 5:68-79, 1988 '~""' Evidence for Baseline Flow Spikes as Spawning Cues for Colorado squawfish in the Yampa River, Colorado THOMAS P. NESLER Fish Research Section, Colorado Division of Wildlife, Fort Collins, Co-orado 80526, USA ROBERT T. MUTH Larval Fish Laboratory, Department of Fishery and Wildlife Biology Colorado State University, Fort Collins, Colorado 80523, USA ANTHONY F. WASOWICZ U.S. Fish and Wildlife Service, Utah Cooperative Fisheries Research Unit Utah State University, Logan, Utah 84321, USA Abstract.-The Colorado squawfish Ptychocheilus Lucius is an endangered species in the Colorado River basin. Understanding the spawning ecology of this cyprinid has been an important focus of research duetted at recovery of the species. Comparison of yearly river flow regimes to estimated Colorado squawfish spawning periods in the Yampa River, Colorado, suggested that the first major flow spike in early summer, as river flows decline to near baseline, may act as an environmental cue that stimulates substantial spawning. In five cases during 1983-1986, estimated spawning peaks were associated with flow spikes of 27.5-71.4 m3/s that occurred over a 2-3-d period sometime during late June to late July. These flow spikes represented 25-139% increases in river flow. Rainstorms representing 23-89% of total monthly precipitation appeared to cause the observed flow spikes. Spawning activity appeared to become negligible when Rows decreased below 57-66 mils. Flow spikes similar to those in 1983-1986 represented 34%n of the 68 spikes documented for the Yampa River during the June-August period fmm 1934 to 1986. These flow spikes occurred in 19 of 53 years with a mean interval of 1.7 years. Seventy-four percent of the flow spikes of a magnitude similar to those in 1983-1986 peaked in 4d or less. Because of their recurrent nature, these flow spikes qualify as potential environmental cues serving to stimulate spawning by Colorado squawfish. These results may bear on recommended flow windows and water project operations criteria for the protection and management of this species in regulated river environments. The Colorado squawfish Ptychocheilus Lucius is a large piscivorous minnow endemic to the Colo- rado River basin. Presently, the Colorado squaw- fish is listed as endangered, and wild populations are found only in the upper basin in Colorado and Utah. Research on extant Colorado squawfish populations has been underway since the mid- 1960s. Recent work has emphasized reproductive and early life habits, particularly as they relate to present-day physical and biological conditions in the system. One of the primary objectives of recovery plans for this endangered species is to identify and safeguard its habitat and life history requirements. In this paper, we examine the rela- tionship between timing and extent of Colorado squawfish spawning (estimated from larval fish collections), and flow regimes and water tempera- tures in the Yampa River, Colorado, during 1980- 1986, and discuss the role these physical variables may play as environmental cues that stimulate spawning activity by the species in this river. The Green-Yampa river system, which drains to the Colorado River, is considered to be one of the few remaining strongholds where spawning popula- tions of wild Colorado squawfish still occur. Successful reproduction by Colorado squawfish in the upper Colorado River basin was first veri- fied by collection of young of the year from the Green River, Colorado-Utah, in 1964 (Vanicek and Kramer 1969). Evidence of successful repro- duction by Colorado squawfish in the Yampa River was first verified by capture of larval fish in 1980 (Haynes et al. 1984). Subsequent radiotrack- ing and sampling of adult Colorado squawfish have demonstrated that these fish migrate to and spawn in the lower 50 km of the Yampa River in Colorado (Figure 1) and in a 14.5-km reach of the Green River in Utah at Three Fords (Tyus et al. 1981; Wick et al. 1983; Tyus and McAda 1984; Tyus et al. 1987). The largest concentration of Colorado squawfish spawners in the Yampa River has been observed at river kilometers 24-30, 68