Laserfiche WebLink
the reproductive potential of the population because it takes longer for the <br />fish to reach the size of sexual maturity. Squawfish at times select <br />habitats that are warmer than the main channel (Table 7). This may be an <br />example of behavioral thermal regulation, whereby Colorado squawfish seek <br />temperatures that are closer to their physiological optimum (about 25 C <br />according to Black and Bulkley 1985a and 1985b). In the 15-mile reach, such <br />habitats include backwaters and flooded gravel pits, which are used <br />extensively in spring and may be as much as 10.5 C warmer than the main <br />channel. Slow growth also makes young fish vulnerable to predation for a <br />longer period, thereby decreasing the likelihood of their surviving to <br />maturity and reducing the overall reproductive capacity of the population. <br />Temperatures considered minimal for squawfish spawning (20 C) occur in the <br />15-mile reach, though not until mid summer. Thus the first growing season of <br />the young produced there is especially short. <br />Water Clarity <br />Colorado squawfish and razorback sucker evolved in the turbid environment of <br />the Colorado River. Water clarity in the 15-mile reach may affect habitat <br />use by adult squawfish. We regressed depth at fish location against water <br />clarity and found a significant, though weak, positive relationship <br />(r = .39; P < .01; Fig. 8). Although squawfish do not always use deep water <br />when turbidity is low, the relationship shown in Figure 8 suggests that they <br />are more likely to do so then. The reduced light intensity at greater <br />depths may provide cover for fish, much as turbid conditions otherwise might <br />do. The periodic high clarity of the 15-mile reach during the summer may <br />require squawfish to use greater depths to achieve needed cover. Spates <br />from summer thunderstorms cause the river's silt load to dramatically <br />increase and its clarity to decline (Fig. 9). <br />26