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In addition to spawning limitations, the upstream reduction in temperature has a large effect on <br />potential growth rates of Colorado squawfish. Controlled laboratory experiments have shown <br />that growth rate of Colorado squawfish is maximized at 25 C; at 20 C, growth rate is 54% of the <br />maximum; at 15 C, growth is only 18%, and at 13 C, Colorado squawfish cease to grow. To <br />understand the effect of low temperatures on habitat suitability, annual, accumulated, degree days <br />available for growth can be calculated for various reaches of river. A degree-day is defined as one <br />degree of temperature (C) above the developmental zero (13 C in this case) for a period of one <br />day. Daily temperatures are summed and a correction factor applied by subtracting 13 C from <br />each daily temperature. For example, one day with a mean temperature of 25 C would equal 12 <br />degree days (25-13 = 12). At Cameo, annual degree days for the four years studied ranged from <br />52 to 68% of the total calculated for the state-line site. Accumulated, annual degree-days at <br />Rulison were only 49% (1994) and 52% (1995) of that at the state line. In contrast, the lower <br />Grand Valley site (18-mile reach) had 80% of the state-line degree-day total in 1995. <br />Providing passage to upstream reaches above Palisade should be pursued as part of recovery <br />efforts for the razorback sucker and Colorado squawfish. Razorback sucker managed to persist <br />in this reach up until recent times and its thermal requirements may be slightly different than that <br />of the Colorado squawfish. Also, the life history strategy of the razorback sucker may allow it <br />maintain a population further upstream by exploiting warm, off-channel ponds and flooded areas <br />to offset the temperature limitations of the main channel. The Palisade-to-Rifle reach might also <br />be seasonally used by adult Colorado squawfish from the Grand Valley as an additional feeding <br />area during the warm months of late summer. <br />In summary, individual Colorado squawfish may utilize the entire 185 miles of the upper Colorado <br />River during the course of their lives, moving among reaches to meet the demands of their <br />changing life history needs. Habitat site selection made at the landscape scale involves tradeoffs <br />between the relative availability of various resources. More optimum thermal regimes are located <br />in lower portions of the river, but substrates there are unsuitable for spawning and food <br />availability for adults is low. Further upstream, steeper gradient and local geological conditions <br />allow for coarser substrate and more habitat-rich channel morphology resulting in better physical <br />conditions for spawning. These features combined with greater nutrient inputs and light <br />penetration result in greater food production in upstream reaches. However, for warm-water <br />species, the further upstream an individual ventures into cooler and cooler waters the more the <br />potential for growth declines. The greatest concentrations of adult Colorado squawfish are found <br />in the Grand Valley, ostensibly because the benefit of high food availabilityoutweighs the <br />negative effects of a reduced thermal regime. Further upstream, food availability remains high but <br />temperatures continue to decline to the point where food availability can no longer offset the <br />negative effects of low temperatures and thereby limits the range of the species. Thus, below the <br />Grand Valley, particularly below Westwater, food is the limiting resource; above the Grand <br />Valley, temperature is the limiting resource. Within the Grand Valley exists the best available <br />tradeoff between these two competing resources. Downstream, some adults are found, and <br />upstream some adults would be found if passage over the existing barriers were provided; but <br />these will continue to be suboptimal regions for adults. <br />10