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0 13 decreased to 0 C in early January. Highest flows occurred in March when maximun river temperatures rose to 1 C and ice break-up occurred (Appendix C, preliminary USGS records provided by Bob Boulger, Meeker, Colorado). Weekly average ice thickness recorded in habitats used by radiotagged squawfish for the eight sampling trips ranged from 0.2 to 1.5 feet (Table 4). Ice was present in habitats used by squawfish throughout the winter study except the last scheduled trip March 16-21 when the entire river was ice free in the two study areas. Ice formation and habitat use Ice formation was noted on the Yampa River as early as November 11. Squawfish were observed by radiotelemetry to move under the ice at the large backwater at RMI 95.7 (Figure 4) and at the large embayment at RM1 81.1 (Figure 5) at this initial ice formation. On the first scheduled trip December 1-5, the main river channel was clogged with floating ice and slush during the early morning hours. Low velocity shoreline areas were frozen over. All nine squawfish located on that trip were in habitat areas that were iced over even though the main channel would clear of ice by late afternoon. The initial ice over pattern outlined and identified habitat types that were used by squawfish throughout the rest of the winter. Because the lower velocity waters were the first to freeze, the ice actually served as a velocity 3Njf_sh were apparently attracted to these frozen-over low indicator. Sgu velocity areas fgr- the cover and security they provided. In addition,.large schools of Mj=Laus were observed through the thin clear ice near shore, so food may have been an additional attraction. Micro-habitat measurements Out of the 590.5 total observation hours, 399.0 hours had associated depth velocity and substrate measurements which could used to develop habitat use information for these variables. A fish had to utilize a map location for a least 30 minutes for habitat measurements to be taken. Total observation times for each fish ranged from 36.6 to 78 hours. This met our goal of 36 hours total observation time for each fish. Observation hours with associated habitat measurement for each fish ranged from 22.2 to 57.6 hours. General habitat, use During the? winter, four distinctive habitat types were frequented by radiotagged fish. These habit s were eat identified because of the pattern of initial ice formation occurring in these low velocity habitats-. h'`r squawfish from Group A (A8, A10, and Ail) utilized a large backwater at RMI 95.7 (Figure 4) almost exclusively throughout the winter (Table 3). This large winter macro-habitat backwater (50x100 meters) was the only one of its kind used during the winter study. Backwaters of this size are not common on the upper Yampa, River during low flow periods. Of 590.5 total observation hours on 10 radiotagged squawfish, 211 hours (36%) were from this backwater (Table 5). Observation hours tended to accumulate in this habitat because fishes A7, A10, and All used this habitat concurrently and observations were made simultaneously on all squawfish present. These three fish were often very close (<2 meter's) to each other. Most often they would be located near a large 0