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Conversely, these three native species have not maintained a historically abundant population or relative composition in the Yampa River. Miller (1982), Carlson (1979), Wick (1981 and 1986) and Prewitt (1977) reported that native suckers had become fairly rare in the Yampa River upstream of Maybell at River Mile (RM) 74. The most common species is the introduced white sucker (Catostomus commersoni). Elmblad (2003) reported native fish were only 24% and 35% of the catch during Interagency Standardized Monitoring Program (ISMP) surveys upstream of Maybell (River Mile 75). The Yampa River has experienced reduced base flows and has an established population of introduced nonnative predators, which obscure determination of the relationship between habitat and native fish density. The bluehead sucker is a large fish (up to 45 cm in length) that remains abundant in most west slope rivers. Not a lot of data is available from life-history studies of the bluehead sucker. However, biologists who sample in the Colorado drainage strongly associate this species with a single meso-habitat. This makes the bluehead sucker an excellent species for flow and habitat evaluation studies. Adult fish are typically found in moderate to deep riffle habitat, which is a combination of fast flowing water over cobble-rubble substrate (Woodling 1985; Sublette 1990). The bluehead feeds on invertebrates, which have their highest densities in riffles. Because the bluehead is an obligate riffle species, it's meso-habitat suitability can be defined by the depths and velocities of the riffle habitats the sucker occupies. The flannelmouth sucker is another large bodied fish that grows up to 60 cm long. The flannelmouth is abundant in most west slope rivers where hybridization with white sucker has not occurred. The flannelmouth is another species whose habitat requirements can be defined by depth and velocity combinations. Adults of generally occupy deeper run habitats (Woodling 1985). Runs are typically defined as areas with low to moderately fast currents. In the Colorado and Yampa Rivers, run habitat is generally more ubiquitous. Likewise, the flannelmouth tends to be more common in run dominated rivers. The flannelmouth's habitat overlaps with the bluehead's in the glide section of the river. Glides have similar depths .and velocities as runs, but glides are located between the tail of the pool and the head of a riffle where bed elevation is rising. The roundtail chub is a cyprinid that achieves a large body size (up to 45 cm in length) and is still fairly common in parts of its range in Colorado. The roundtail chub is a predator and has a much different life history and behavior than the two native suckers. These fish generally seek cover during the day and patrol or forage during the evening. These fish do not rely on one particular meso-habitat type; therefore, their habitat use can not be modeled by a simple range of depth and velocity criteria. This is a true multi- meso-habitat species. Factors that control their density may be more a function of forage availability than ofineso-habitat availability. The speckled dace (Rhinichthys osculus) is asmall-bodied cyprinid that may attain a length of 12-cm (Woodling 1985). Speckled dace are generally very abundant and like the bluehead sucker are ariffle-obligate species. Since dace are small-bodied fish they can occupy shallow riffles, which means during periods of reduced flows 14