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35 migratory movement are similar to those observed in man-made backwater and flooded tributary habitats on the Yampa River. Temperature Observations Most radiotagged fish in the migrant group began to move downstream in late June and early July, when their temperatures rose to 160C. These fish were present in the spawning area when their temperatures approached 19°C. However, spawning behavior was noted only after their temperatures exceeded 19°C. Hamman (1981) observed captive Colorado squawfish (wild fish captured in the Upper Colorado River system and not induced by hormonal injection) spawning at water temperatures of 20-21°C. In addition, 1982 Yampa River data indicate that several fish in the migrant group moved below the known spawning site on the Yampa River and then returned upstream to spawn. The same pattern was observed by the USFWS in 1981 (Miller et al., 1982b,c). These movements could be related to selection of physical conditions (temperature and spawning habitat) that are suitable for spawning. Fish may migrate downstream from the Upper Yampa in search of suitable spawning habitat or a familiar scent. Once a site is located, migrating fish may go into a holding pattern until proper temperatures physiologically prepare them for spawning. This type of pattern was noted in fish numbers S, 9, and 10. Fish also recognize spawning habitat from downstream as evidenced by migrants from the Green and White rivers. This suggests a mechanism of scent imprinting similar to that of salmon (Hasler, 1971). As larvae, squawfish may imprint the odor of the Yampa spawning area and later, as migrating adults, recognize this odor and detect it in downstream waters (upstream migrants) or as they pass through the spawning area (downstream migrants).