8208 - Laserfiche WebLink

Home Browse Search

reflective of the needs or preferences of the species under optimum conditions more likely to allow the population to thrive. Thus, our recommendations are based on the habitat-selection behavior of adult squawfish observed during the higher, somewhat more normal flow conditions. Ranking Flow Levels by Weighted Area of Preferred Summer Habitats (Objective No. 3) Among the moderate discharge levels (1,240, 1,530, 1,630 and 2,870 cfs) a flow of 1,630 cfs provided both the highest total area and the highest total weighted area of the habitat types preferred at those flow levels (Fig. 16). Total weighted area at 1,630 cfs was 26% higher than at the discharge level with next highest TWA value (1,530 cfs). The highest weighted area occurred at a discharge of 557 cfs. However, because these habitats are not preferred when the water is that low, weighted area of backwaters, eddies and pools is irrelevant at the lower flow levels. Based on this analysis, 1,630 cfs is the best flow level during summer. For slow and fast runs, preferred at low flow levels, total area and total weighted area increases with increasing discharge. However, as before, weighted area at moderate discharge levels is irrelevant because slow and fast runs are preferred only at the lower flow levels. Of the discharge levels studied here, 810 cfs provided a 20% increase in weighted area over that at 557 cfs (Fig. 16). We recommend a minimum of 810 cfs during drought years. Winter Habitat Use and Availability For winter, no comparison could be made between habitat use during low-flow and moderate-flow conditions. All winter, habitat-use data were collected during moderate-flow levels because in winter no water is withdrawn for irrigation, and upstream dams often release additional water to increase reservoir storage capacity in anticipation of spring runoff. When habitat-use data were collected (November-March 1986-1988), flows in the 15-mile reach were 1,654-3,452 cfs (Appendix Table IX). Adult Colorado squawfish used fewer habitat types during winter than during summer (Fig. 17). In the study sites where fish were located during winter, no rapids or gravel-pit ponds were available. Also, unlike summer, fast runs and riffles were not used in winter. Colder water temperatures resulting in lower metabolic rates during winter may account for avoidance of high velocity sites. During winter, Colorado squawfish were located largely in pools (53%) and backwaters (27%). Again, to determine availability, percent total area of each habitat type was averaged across those flows that most closely approximated those that occurred at the time when habitat use data were collected. Maps made at 1,630 and 2,870 cfs were used to calculate mean percent area Again, slow runs (51 %) and riffles (23%) accounted for about 74% of all water surface area at this flow range. Winter Habitat Preference (Objective No. 2) As in summer, backwaters, eddies and pools were the preferred types of habitat in winter (Fig. 18). Unlike summer, however, pools rather than eddies were the most preferred type. The preference rating (mean difference between use and availability) for pools was twice that for backwaters and five times higher than that for eddies. Slow runs were also used but were not preferred. 33