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March 1984 Haynes el aJ.-Larval Colorado Squawfish 29 TABLE ".-Earliest dates of spawning-related water temperatures, Colorado-Yampa Rivers, 1979-81 (U.S. Geol. Survey Data). Records were unavailable lor period 4/17-7/21,1980, Yampa River. Colorado River Yampa River 1979 1980 1981 1980 1981 Maximum ~18OC 6/29 6/25 4/10 6/19 180C + 30 days 7/28 7/24 5/10 7/18 Maximum ~ooC 7/12 6/26 5/28 6/20 Maximum ~2OC 7/22 61S0 6/20 6/24 Earliest Estimated Spawning Dale 6/29 6/28 6/18 6/16 6/25 The possible interrelationship between spawning and water temperature has been widely discussed. For example, Vanicek and Kramer (1969) suggested that spawning occurred in the Green River when water tempera- tures reached approximately 2IOC, after temperatures have exceeded 180C for about one month. Toney (1974) reported that squawfish spawned under hatchery conditions when water temperatures reached approximately 220C. Hamman (1981) observed the spawning of captive squawfish at 20-210C and suggested that the optimum temperature for spawning is probably between 20 and 220C. For the present study, the earliest dates during 1979-81 when water temperatures reached or exceeded 18, 20 and 220C, along with our earliest estimated spawning dates, are summarized in Table 4. In those years for .which temperature records are available, except 1979, spawning is closely correlated with the earliest date of record when the maximum water temperature reached or exceeded 220C. In 1979, spawning possibly occurred as early as June 29, coinciding with the earliest 180 date but nearly a month before tempetatures reached 220C and about two weeks before temperatures reached or exceeded 200. This suggests that, since the earliest estimated spawning dates in the Colorado River during 1979-81 vary only by 12 days (18-29 June ), either Colorado squawfish respond to an overall general warming and spawn at approximately the same time each year, or water temperatures were warmer in at least one immediate spawning area and, at this locality, reproduction followed the <<20-220 rule." Although the above observations suggest a positive cause and effect correlation between reproductive success and flow-temperature patterns, the strongest evidence comes from observations on the Green River in Colorado and Utah that has flows regulated by damming. Flaming Gorge Dam, in northeastern l]tah, became operational in 1962. A fish eradication program was conducted prior to dam closure and native fishes, including Colorado squawfish, were greatly reduced, if not totally eliminated, for 105 km below the dam and replaced with a trout fishery. Continuing cold tailwater releases and the depression of normal seasonal flows, along with this <<rough fish" eradication program, have been implicated as operating to limit upstream squawfish distribution in the Green to a reach approxi- mately 30 km above the Yampa confluence (Vanicek and Kramer, 1969; Vanicek, 1970; Holden and Crist, 1978; Holden, 1979; Tyus et al., 1982). Holden (1980) concluded that reproduction in the Green River has been limited largely to areas downstream from Jensen, Utah (approximately 69