9300 - Laserfiche WebLink

Home Browse Search

l "L L? S ? Vk (A-Vo Gl S C. +) MPORTANCE OF THE 15-MILE REACH TO COLORADO RIVER POPULATIONS OF ENDANGERED FISH by Douglas B. Osmundson July, 1996 U. S. Fish and Wildlife Service Colorado River Fishery Project, Grand Junction, Colorado The `15-mile reach' is that stretch of the Colorado River between the Grand Valley Diversion Dam at Palisade, Colorado and the Gunnison River confluence near downtown Grand Junction (Fig. 1). This stretch of river, as the name implies, is roughly 15-miles long and runs through the upper half of the Grand Valley, a large alluvial valley containing the largest urban area on the western slope of Colorado and one of the largest farming areas. The Grand Valley, including the 15-mile reach, is home to two endangered fish species, the Colorado squawfish (Ptychocheilus lucius) and the razorback sucker (Xyrauchen texanus). As recently as the late 1980's, razorback sucker were still occasionally captured in the 15-mile reach, primarily during their spawning season which coincides with the high-flow period of spring runoff. In recent years, only four razorback sucker have been captured in the Colorado River and these were found in the reach immediately downstream of the 15-mile reach. Colorado squawfish, though rare, can still be routinely captured year round in the 15-mile reach. Though more is known about the life history of Colorado squawfish than that of the razorback sucker, it is evident that both species require large stretches of river to fulfill their respective life history needs. As individuals go through different stages in life, the requirements for specific habitat types change. For instance, young Colorado squawfish are reared in downstream reaches where nursery habitat is most suitable, but adults prefer upstream regions and large tributaries where food availability is highest; this is true in both the Colorado and Green river systems. Because they are the top predator in the riverine food chain, and subsist almost exclusively on a diet of other fish, adult Colorado squawfish prefer areas with abundant supplies of young native fish. In the Colorado River, the greatest fish production and consequently the greatest numbers of Colorado squawfish occur upstream of Westwater Canyon, especially in the 15-mile reach (see Figs. 1 and 2). Catch rates of native suckers and roundtail chub were 24 times higher in the 15- mile reach than in the adjacent 18-mile reach downstream, depending on the gear type used. Catch rates were 34 times higher in the 15-mile reach than in the Loma-to-Westwater reach, and 50.100 times higher than in the Moab-to-the confluence reach. In the Green River, the majority of adult Colorado squawfish are also found in upstream reaches as well as in the Green River's two major tributaries, the Yampa and White rivers. g3Uv Upper reaches generally have greater numbers of forage fish because food for these fishes (algae and aquatic insects) is in turn higher than in downstream reaches. Higher water clarity, and therefore deeper light penetration, allow greater production of phytoplankton (free-floating: algae)