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4. Habitat Suitability Criteria <br />The response of a species during a particular life stage le.g. <br />Colorado squawfish adult) to a streamflow dependant variable (e.g. <br />velocity) is mathematically described as a habitat suitability index (SI) <br />curve. These curves are based on the assumption that individual fish tend <br />to inhabit the most favorable microhabitat conditions from within the <br />total range of conditions present. They will use less favorable <br />conditions with lesser frequency and will eventually leave the area before <br />microhabitat conditions become lethal. The value of each variable <br />represented by an SI curve ranges from 0 to 1 with a rating of one <br />assigned to the value which provides optim~un habitat conditions for the <br />species. <br />SI curves for some of the life stages of Colorado squawfish, humpback <br />chub, and razorback sucker have been developed for use in a PHABSIM <br />analysis (Valdez et. al. 1987). Four curve sets are used in this <br />analysis. They include curves for Colorado squawfish adult and <br />depositional life stages from April through October and mid-June to <br />September, respectively, and for adult razorback suckers from May through <br />June and July through October (Figures 2 and 3). From the SI curve set <br />for Colorado squawfish, it is concluded that the adult life stage selects <br />depths around 5.18 ft with 0.00 f/s velocities over sand substrate, while <br />the depositional life stage prefers depths of 2.04 ft with velocities <br />around 2.27 f/s over rubble substrate. Adult razorbacks prefer a depth of <br />5.08 ft, with a velocities between 0.00 and 0.34 f/s over sand substrate <br />from May to June, and depths of 3.44 ft with 1.68 f/s velocities over sand <br />substrate from July to October. <br />10 <br />