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<br />4. Habitat Suitability Criteria <br /> <br />The response of a species during a particular life stage (e.g. Colorado <br />squawfish adult) to a streamflow dependant variable (e.g. velocity) is <br />mathematically described as a habitat suitability index (SI) curve. These <br />curves are based on the assumption that individual fish tend to inhabit the <br />most favorable microhabitat conditions from within the total range of <br />conditions present. They will use less favorable conditions with lesser <br />frequency and will eventually leave the area before microhabitat conditions <br />become lethal. The value of each variable represented by an SI curve ranges <br />from 0 to 1 with a rating of one assigned to the value which provides optimum <br />habitat conditions for the species. <br /> <br />51 curves for some life stages and for certain specified times of the year <br />have been developed for Colorado squawfish, humpback chub, and razorback <br />sucker for use in a PHABSIM analyses (Valdez et. al. 1987). Four curve sets <br />were available for this analysis. They include curves for Colorado squawfish <br />adult and depositional life stages from April through October and mid-June to <br />September, respectively, and for adult razorback suckers from May through June <br />and July through October (Figures 2 and 3). From the SI curve set for <br />Colorado squawfish, it is concluded that the adult life stage utilizes depths <br />around 5.18 ft with 0.00 feet per second (ft/s) velocities over sand <br />substrate, while the depositional life stage utilizes depths of 2.04 ft with <br />velocities around 2.27 ft/s over rubble substrate. Adult razorbacks utilize a <br />depth of 5.08 ft, with a velocities between 0.00 and 0.34 ft/s over sand <br />substrate from May to June, and depths of 3.44 ft with 1.68 ft/s velocities <br />over sand substrate from July to October. To verify these conclusions <br />obtained from the SI curves the theoretical optimum depths and velocities were <br />compared to the mean microhabitat values reported in Tyus, Jones and Trinca <br />(1987; Table 4). According to these authors, adult Colorado squawfish occupy <br />a variety of habitats with mean depths and velocities of 1.8 meters (m) (5.9 <br />ft) and 0.25 meters per second (m/s) (0.82 ft/s), respectively. They are <br />found predominately in eddies and runs along shorelines, over sand and silt <br />substrates. In contrast, the SI curve suggests a theoretical optimum velocity <br />of 0.00 ft/s. The breeding adults deposit and fertilize eggs on cobble bars <br />with mean depths and velocities around 3.09 ft and 1.58 ft/s (Tyus et. al <br />1987). This is slightly deeper and slower than optimums predicted by the SI <br />curves (2.04 ft and 2.27 ft/s; Figure 2). The razorback sucker uses main <br />channel habitat in depths ranging from 0.6 to 3.4 m (1.97 to 11.16 ft) and are <br />most common in runs near shore in spring, and on main channel sand bars <br />composed of unconsolidated coarse sand in summer. Water depths on the sand <br />bars were usually less than 2 m (6.56 ft) with an average water velocity of <br />0.5 m/s (1.64 ft/s). In general these habitat descriptions coincide with the <br />depth/velocity ratings represented by the SI curves for razorback suckers. <br /> <br />8 <br />