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A total of 43 Colorado squawfish were radio-tracked to the Yampa Canyon spawning reach by the U.S. Fish and Wildlife Service (N = 38) and Colorado Division of Wildlife (N = 5) from 1981 to 1988. These included 28 fish from the upper Yampa River, 13 fsh from the Green River, and 2 fish from the White River. Average one-way movement of migrants radio-tracked by Service biologists was about 124.8 km. One of the White River fish migrated about 372.8 km to reach the Yampa River spawning grounds. This fish may have been en route for more than a year, because it was tagged at RK 164.8 of the White River in 1983, tracked to RK 49.6 of the Yampa River in spring 1984, and recaptured at RK 156.8 of the White River in 1985. These radio-tracking data show that migrating Colorado squawfish arrive in the Yampa Canyon in early summer from many areas throughout the upper Green River basin, including the White, Green, and upper Yampa rivers. The initiation of the spawning migration is an important component of the reproductive cycle of the Colorado squawfish. To better evaluate some factors influencing the onset of migration, adult Colorado squawfish were radio-tracked in early spring in the upper Yampa River by U.S. Fish and Wildlife Service biologists in 1981 and 1983 (Tyus and McAda 1984; Tyus 1985), by Colorado Division of Wildlife and National Park Service personnel in 1982 (Wick et x1.1983), and by U.S. Fish and Wildlife Service and Colorado Division of Wildlife biologists in 198$. These fish initiated spawning migrations from 27 May to 13 June, depending on the type of water year (Table 1). Flows and water temperatures were highly variable within each migration period and among years. However, spawning migrations were initiated earlier. in low-water years (e.g., 1981) and later in higher-water years (e.g, 1983; Table 1). Although radio-tracking studies were not conducted all years, data from 1981 (low-water year), 1982 and 1988 (average-water years), and 1983 (high-water year) were used to investigate relations between discharge, water temperature, and date of initiation of spawning migration (Figs. 7 and 8). As shown in Fig. 8, spawning migrations of radio-tagged Colorado squawfish (N= 24) were associated with highest spring flows and river temperatures generally exceeding 14°C. However, the actual period of initiation of spawning migration of Colorado squawfish maybe longer because radio-tagged fish may not have included early and lake migrants. Timing of the reproductive cycle is influenced first by intrinsic biological mechanisms and secondly by en- vironmental stimuli (Brown et al. 1970). Identification of these stimuli is made difficult by synergistic and other confounding interrelations (Bye 1984). We believe that some complex combination of endogenous (e.g., stage of maturity, physiological condition, genetic lineage) and exogenous factors (e.g., substrate, temperature, dis- charge, photoperiod) are necessary and that neither discharge nor temperature alone is sufficient to induce spawning migrations or spawning. For example, radio- trackingdata suggest that all adult Colorado squawfish do not spawn each year. Of four fish radio-tracked to spawning grounds at least once in consecutive years (1 + years) during the spawning season, two migrated only 1 year and presumably did not spawn each year. Effects of exogenous factors on reproductive cycles of cyprinid fishes are well known and many physicochemical variables have been implicated (reviewed by Brown et al. 1970; Bye 1984; McKeown 1984). We hypothesize that inputs of certain chemical substances from runoff and inundated shorelines during spring snowmelt, in concert with increasing river level and temperatures, may act to influence genetic, physiological, and behavioral mechanisms in Colorado squawfish that are associated with spawning migrations. We assume these mechanisms can only be activated at Table 1. Initiation of Colorado squatvfish spawning migrations, Yampa River spawning grounds, 1981-83, 1988. Movei~ier:t recorded for 24 radio-tagged fish (1981 = 7 fish, 1982 = 5 fish, 1983 = 7 fcslt, 1988 = 5 fish). Data for 1982 after Wrck et al. (1983). Dischargec(m3/s) Water temperature (°C)` Type water Initiation of Year yeas migrationb Mean Range Mean Range 1981 Low 27 May-20 June 121.1 33.6-171.8 14.9 12.5-18.5 1982 Average 10 Jun~3 July 200.7 175.5-235.2 13.7 11.6-15.7 1983 High 13 June-12 July 240.7 127.4-322.6 13.8 9.2-17.3 1988 Average 9-23 June 129.8 93.4-193.1 15.8 13.8-19.4 a Designation of low, avers e, and high water years based on average annual discharge for 1922-87: low, <43.15 m3/s; average, 43.15- 77.83 m3/s; high, > 77.83 m~/s (M. Butler, personal communication}. b Initiation of migration is the period between first and last departure of radio-tagged fish in upper Yampa River to Yampa Canyon spawning grounds. c Data based on daily averages during indicated period (USGS flow records, Yampa River near Maybell, Colorado).