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In addition to the greater depth, chubs in this study were found in backwaters with greater length, width, surface area, and volume. However, these preferences did sometimes shift between seasons as might be expected as chubs age. Increased turbidity was also a characteristic of backwaters used by chub, and showed greatest significance in August and September, months. in which rain events and flash floods were common. Some storms were so severe that entire backwaters were changed by the influx of debris. Valdez and Ryel (1995) and Converse et al. (1998) reported that chub use of backwaters was dependent on high turbidity. It was also reported that chubs were most numerous at night (Kaeding and Zimmerman 1983). These behaviors may indicate that chubs prefer habitats with low visibility. However, it important to note that these studies were conducted in the Grand Canyon where the-river is colder and clearer than the Green River. Temperature has been mentioned as a preference character for YOY Colorado pikeminnow (Haines and Tyus 1990, Tyus and Haines 1991). Chubs in this study varied in their response to temperature. Temperatures in chub backwaters were higher than the main channel in some months, and lower in others. Sometimes chubs used backwaters warmer than those not used, but the reverse was also noted. In April and August samples, chubs were found in backwaters with higher mean temperature (18.8°C) than September (17.8°C}. Backwaters used in September were warmer than the main channel where they were connected, but cooler than unused backwaters. It seems probable that some combination of factors was responsible for habitat selection. Gila spp. recruitment Day et al. report that captures of age-1+ chubs were highest in June 1994, i.e. recruitment of the 1993 YOY cohort. Similar results were reported by Chart and Lentsch in their fish community monitoring (primarily electrofishing) data set. Juvenile chubs were also collected in 1995, which was likely a combination of Age 1+ (the 1994 cohort; represented well as YOY in the nursery habitat study) and Age 2+ (the 1993 cohort) fish. Sampling in Westwater Canyon of the Colorado River indicated survival, as determined through electrofishing catch rate analysis, was high once chubs reached 100 mm TL (Chart and Lentsch 1998b). In addition, sampling in Westwater indicated that a relatively weak cohort of YOY chubs produced in 1994 recruited best to the Age 1+ class. The good overwinter survival for 1994 YOY chubs in Westwater was attributed primarily to their size (mean TL - 45 mm on 18 August 1994) in the fall. The 1993 cohort of Age 0 chubs in Deso/Gray averaged 37.4 mm TL in late August. However, length frequency of the 1993 cohort was bi-modal with the second (larger) peak at 45 mm. Day et al. reported excellent, long-term conditions for growth in 1994; as evidenced by YOY chub mean total length of 60.8 mrn by mid-September. It is our feeling these fish should have overwintered and recruited to the Age l+ class better than was observed in this study, which suggest some other limiting factors may be operating. Nonnative fish are much more abundant in Deso/Gray than in Westwater Canyon and likely affect survival of young native species there (see Report C). There was a greater ' abundance of 200-300 mm TL channel catfish (potential competitor /predator) in Deso/Gray in 1994 than 1993, but overall densities were actually very similar between these years. Overwinter flows likely have an effect on early survival rates as well. Green River flows from i 5 November through O1 March 1994 averaged 3,339 cfs (Std Dev - 493) and averaged 2,293 cfs (SD - 400) the next winter. Perhaps the higher winter (`93-'94) flows had some beneficial effect. Flows xiii