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76 NESLER ET AL. factors serve as cues synchronizing the internal reproductive cycles of the individuals in a popu- lation (Lam 1983). Timing of annual spawning has evolved to ensure that the optimal environmental conditions are present and that young hatch and commence feeding at the optimum season for survival (Nikolsky 1963; Schwassman 1971; Lam 1983; Bye 1984). A temporally appropriate re- sponse to external cues maximizes reproductive success by achieving an optimal balance between survival of adult spawners and progeny (Stacey 1984). Bye (1984) suggested that any recumng environmental variable within the sensory compe- tence of an organism can act as a timing cue. To extend this concept further, multiple environmen- tal cues may be integrated in a fish species' reproductive cycle, either interactively or in se- quence, to synchronize gonad maturation (game- togenesis), migration, and spawning (Liley 1969; Lam 1983; McKeown 1984}. A general consensus is evident in the literature that photoperiod and temperature interact as important regulatory fac- tors in the timing and process of gonad maturation in many fish species and especially cyprinids (DeVlaming 1972; Magnuson et al. 1979; Lam 1983; Bye 1984; McKeown 1984; Stacey 1984). However, both Lam (1983) and Stacey (1984) indicated that although these environmental fac- tors ensure that the gonads are mature at the appropriate season, other specific stimuli may be required to initiate the final phase of gonad matu- ration (ovulation and spermiation) and the release of gametes. Potential stimuli include current ve- locity, water quality, substrate, barometric pres- sure, or pheromone release due to aggregation of potential mates. Vanicek and Kramer (1969), Wick et al. (1983), Haynes et al. (1984, 1985), Tyus and McAda. (1984) and Tyus et al. (1987) have concluded that Colorado squawfish spawn as river flows decrease in early summer and that the timing may be largely influenced by water temperatures. Thresh- old temperatures of 20-22°C and the number of degree-days above 18°C have been proposed as key factors. This conclusion is supported by evi- dence from hatchery operations. Both Toney (1974) and Hamman (1981) reported temperatures of 20-22°C as important for spawning. Observa- tions by Hamman (1981) under hatchery condi- tions indicated that Colorado squawfish spawned only after injection with gonadotropin when the temperature was 18°C, but they spawned sponta- neously at temperatures of 20°C and- above. Evidence that temperature is important for Col- orado squawfish spawning is substantial, but we propose a different perspective of its role. The water temperature data in Haynes et al. (1985) indicate that some spawning activity in 1983-1984 occurred when temperatures in the Yampa River were less than 20°C. In 1985, the mean tempera- ture during peak spawning was only 16.1°C (Tyus et al. 1987). Temperature data at the Maybell gage showed that a drop of 4°C (18 to 14°C) occurred on June 25-27, concurrent with the flow spike and peak spawning activity by the Colorado squawfish (Nesler 1986). This association of events was clearly divergent from the spawning temperature threshold hypothesis that had been formulated prior to 1985. This relationship was also directly opposite that observed by Beamesderfer and Con- gleton (1981) for northern squawfish Ptycho- cheilus oregonensis in the St. Joe River, Idaho. They reported a dramatic decline in adult num- bers and spawning activity coinciding with a 4°C drop in river temperature (15 to 11°C) due to a rainstorm that raised the river level approximately 22%. The estimated period of peak spawning in the Yampa River in 1985 occurred at water tem- peratures under which hatchery fish would re- quire injection to induce spawning, which also supports the flow-cue hypothesis. Our data sug- gest that temperature may fulfill a secondary or supplementary role in the spawning requirements of Colorado squawfish in the Yampa River. Dif- ferences in spawning behavior between Colorado and northern squawfish may be related to stark differences in their respective habitats. Colorado squawfish have adapted to desert river ecosys- tems in more southerly latitudes receiving 25 cm annual rainfall, whereas northern squawfish have adapted to Coldwater rivers in northern latitudes that receive over 77 cm annual precipitation. Some spawning was calculated to dates that preceded any flow cue. Several explanations may account for these early outliers. Relative to as- sumptions made earlier, wild progeny may expe- rience variable environmental conditions that af- fect growth, and their growth curve may be unlike the hatchery-based growth curves used here. From a genetic perspective, it is likely that a segment of each year class of Colorado squawfish larvae has inherent growth characteristics unlike those of Willow Beach hatchery fish. Further, not all adult fish may respond equally to an environ- mental cue for migration or spawning. In discussions of environmental cues leading to ovulation and spawning, Stacey (1984) highlighted differences between salmonids and cyprinids.