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-r /~b > ,....- ,..~ ]ransactions of the American Fisheries Society 119: 103 5-1047, 1990 CJ'J31t(J Potamodromy and Reproduction of Colorado Squawfish in the Green River Basin, Colorado and Utah , " HAROLD M. Tyus u.s. Fish and Wildlife Service 1680 West Highway 40, Suite 1210, Vernal, Utah 84078, USA ...... Abstract. - Movements of Colorado squawfish Ptychocheilus lucius were studied by radio-track- ing and recapturing tagged fish in the Green River basin of Colorado and Utah, 1980-1988. Of 153 fish tracked, 63% were highly mobile: 41 % migrated to known spawning sites, 11 % migrated to suspected spawning sites, and 11% moved to other locations. Nonmigratory behavior of tracked fish (18%) was linked with non-annual spawning or sexual immaturity. Contact was lost with 14% of the fish, but five that Wllre lost one year migrated in subsequent years. Some fish (5%) were radio-tagged after the spawning season. Spawning migrations began about 28 d after highest spring flows when water temperatures were 9"C or greater. Movement offish to spawning areas (N = 63) averaged 140.7 kIn (range, 32-372.8 kIn) in downstream (73%) and upstream (23.8%)directions, and some fish (3.2%) moved downstream in tributaries, then upstream in the mainstream. An autumn-to-spring home range was indicated by radiotelemetry and recaptured fish. Colorado squawfish spawned as summer flows decreased and water temperatures increased, and capture of 13 ripe females indicated that spawning occurred in mean water temperatures of 23"C (range, 22- 25"C). Most ripe fish were males, and their mean total length was smaller than that of females (males, 555 mm, N = 194; females, 654 mm, N = 14). The possibility of stock differentiation was suggested by radio-tracked and recaptured individuals that spawned at the same site for more than 1 year, and by migrations to a spawning reach from both upstream and downstream areas. Innate and environmental factors influencing reproduction and recruitment should be fully considered in management of this endangered fish. . Colorado squawfish Ptychocheilus lucius Gi- . rard, formerly abundant and the largest minnow in North America (Jordan and Evermann 1896), has been listed by the U.S. Fish and Wildlife Ser- vice (USFWS) as endangered for over 20 years (USFWS 1967). Despite efforts to mount recovery programs (Rose and Hamill 1988), the identifi- cation and alleviation of causes for its extirpation from about 80% of its historic range has been dif- ficult. Extant reproducing populations are restrict- ed to the upper Colorado River basin, where the fish is most abundant in the mainstream Green River and its tributaries, the Yampa and White rivers (Holden and Wick 1982; Tyus, in press). During studies in the Green River basin, Wick et al. (1983) and Tyus (1985) identified freshwater, potamodromous migrations as important factors in Colorado squawfish reproductive ecology and discovered spawning sites in Yampa and Deso- lation-Gray canyons. However, the influence of environmental factors on reproduction is not fully understood. Recent changes in riverine habitats have been advanced as factors contributing to the decline of Colorado squawfish (Miller 1961; Holden and Wick 1982; Ono et al. 1983). However, the fish was extirpated from the lower Colorado River ba- sin before its habitat requirements were known (Minckley 1973), and present knowledge about its reproductive ecology is limited to only a few geo- graphic areas. Nesler et al. (1988) hypothesized that spawning activity was associated with flow peaks in the Yampa River, but the role of envi- ronmental cues in initiating spawning migrations remains little understood, and the influence of such variables on egg maturation and spawning needs further study. I have investigated potamodromous migrations and spawning of Colorado squawfish and evalu- ated the hypothesis that seasonal changes in major environmental variables are associated with the initiation of spawning migrations and reproduc- tion. I also explored the potential role of exoge- nous and endogenous factors in reproduction and related the findings to the management and po- tential recovery of the fish. Methods Radiotelemetry. - I combined data on Colorado squawfish movement behavior from 1980-1981 (Tyus and McAda 1984) with information from 1982-1988. In this and the former study, Colo- rado squawfish were usually captured in nearshore habitats by downstream electro shocking with a 1035