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37 mechanism may be highly sensitive to temperature. Comparison of temperature data in Figs. 9 and 10 shows that the temperatures of fish 5 and 6 are very similar. Fish number 5 (608 mm) migrated, fish number 6 (565 mm) did not. These fish were located very close together from mid-April to mid-June; then fish 5 moved downstream a few kilometers and finally entered a large backwater in early July. Then a large temperature fluctuation occurred with fish number 5, and this temperature change may have triggered migration. It is interesting to note that fishes 2 and 6 of the nonmigrant group were never found in backwater habitats (Appendix B). The temperature of fish number 2 was consistently cooler by several degrees than that of the environment. This might have been caused by an original calibration error or a sensing-unit failure. Fish number 8, located in the farthest upstream group used backwaters quite extensively at times but was never located on the spawning area. Fish number 8 showed cooler temperatures in late April and again in mid- to late June than fish numbers 9 and 10, which were migrants and originally tagged in the same upstream area. Cooler temperatures of fish number 8 reflect its location near the mouth of the. tributary stream backwater,' where cooler river temperatures were an influence. These observations suggest a delicate relationship between habitat use and temperature that may or may not trigger spawning in individual fish in a given year and given river reach. The upper sections of the study area now represent the upstream distributional range for Colorado squawfish. During high-water years temperatures can be colder for extended periods in the main channel than in low-water years, when temperatures can rise quite rapidly. During higher, colder water years, use of warm backwater habitat may be important in triggering gonadal