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experimentally controlled temperature to determine effects on percent hatch of razorback sucker eggs; of six temperatures studied, 20 C resulted in highest hatch success, followed by 25 C. There was a significantly lower hatching success at 15 C, and complete mortality of eggs at 5, 10, and 30 C. Results for Colorado squawfish were almost identical. One major differ- ence between the two species, however, is that upper basin Colorado squaw- fish spawn in the summer and razorback suckers spawn in the spring. Though Kidd (1977) did not note whether the razorbacks he captured were ripe, subsequent researchers did; of 157 captured during studies by McAda and Wydoski (1980), Valdez et al. (1982), USFWS (unpublished data), and Osmundson and Kaeding (1989), 42 were in spawning condition (38 from flooded gravel pits, one from a backwater, and two from a main channel shoreline). Of the 42 running ripe fish, 40 (95%) were caught between 24 May and 17 June (Fig. 13); the two remaining ripe razorbacks were males and were captured between 3 and 10 April in the Walker Wildlife Area. Colorado squawfish spawn when main-channel temperatures reach 19-22 C after spring flows have decreased; when this occurs varies greatly among years (late June to early September) depending on river and weather condi- tions. However, razorback suckers always come into spawning condition in spring even though main-channel temperatures are far from the optimum required for maximum egg hatching success. The explanation that we offer for this is that the razorback sucker has evolved a reproductive strategy that differs from that of the Colorado squawfish. Both are warmwater species and spawning is thus timed such that resulting young are hatched under conditions that favor rapid growth. However, whereas Colorado squaw- fish wait for main-channel temperatures to rise to the optimum level before spawning occurs, razorback suckers seek out off-channel habitats 39