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7/14/2009 5:02:29 PM
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UCREFRP
UCREFRP Catalog Number
7066
Author
Behnke, R. J. and D. E. Benson.
Title
Endangered and Threatened Fishes of the Upper Colorado River Basin.
USFW Year
1980.
USFW - Doc Type
Bulletin 503A,
Copyright Material
NO
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in the lower basin reservoirs, but no young fish have <br />been found. With a long evolutionary background in a <br />river environment, young razorback suckers might lack <br />the instincts necessary to avoid predation in a lake <br />environment. Reservoirs have predatory species in <br />abundance. Schools of feeding carp have been ob- <br />served in the lower basin reservoirs over areas where <br />the razorback sucker had spawned. <br />Most observations of spawning have been in reser- <br />voirs. Spawning is reported to occur at temperatures <br />of 540 F to 680 F, in water 1 to 20 feet deep. In <br />river environments, groups of spawning razorback <br />suckers have been observed on gravel bars in the <br />Colorado and lower Yampa rivers when the water tem- <br />perature reached about 620 F. Ripe and spent fish <br />found in off-channel ponds suggests that spawning <br />also occurs in such habitats. Along the Colorado <br />River in Colorado, razorback suckers are most fre- <br />quently found in ponds, created by gravel excavation, <br />adjacent to and connected with the river. <br />Past and Present Distribution <br />The original range of the razorback sucker was <br />approximately that of the squawfish and bonytail <br />chub, in the large-river environments from Mexico to <br />Wyoming. Historically, it was more common in the <br />lower than in the upper Colorado River basin. In the <br />lower basin large populations built up in the reser- <br />voirs during the early years of impoundment, but they <br />gradually declined and now mainly consist of old, <br />large fish. <br />In the upper basin, razorback suckers disappeared <br />from the Green River above the mouth of the Yampa <br />River after the completion of Flaming Gorge Dam and <br />the release of cold water. Some razorback suckers <br />persist in the Green River below its confluence with <br />the Yampa River, and are occasionally found in the <br />lowermost reaches of the Yampa. In the Colorado River <br />in Colorado, razorback suckers occur upstream to De <br />Beque, about 30 miles above Grand Junction. In 1977 <br />an estimated 250 razorback suckers were found <br />stranded when a small irrigation reservoir, connected <br />to the San Juan River near Bluff, Utah, was drawn <br />down. <br />Causes of Decline <br />For the razorback sucker, like the other species <br />discussed, dams and impoundments can be pointed to as <br />the major cause of decline. Land-use and water-use <br />practices, changing flow regimes, and river channel <br />characteristics that eliminated the lagoon or <br />backwater type habitat can also be blamed. This <br />seems evident from the intensive use of artifi- <br />cially created, off-channel pond habitat by razorback <br />suckers. Non-native fishes such as carp, largemouth <br />bass, and green sunfish also typically thrive in <br />these pond areas, and they can effectively suppress <br />successful reproduction of razorback suckers by pre- <br />dation on the eggs and young in such habitat. <br />In seeking clues bearing on the reasons for the <br />decline of the razorback sucker, interpretation from <br />an evolutionary perspective can be made. It is known <br />that before environmental changes occurred and before <br />non-native fishes became widely established in the <br />Colorado River basin, two species of large suckers, <br />the razorback and the flannelmouth, were both abun- <br />dant. This means that the razorback sucker and <br />flannelmouth sucker must have different niches. <br />That is, the two species avoided direct competition <br />with each other; because of differences in their life <br />histories and ecologies, the food and space resources <br />of their environment were divided in such a way that <br />both maintained abundant populations. <br />The ecological distinctions between flannelmouth <br />and razorback suckers can be interpreted from the <br />differences in the way the two species are put <br />together -- the differences in body shape, lip <br />structure, and gillrakers. These distinctions in <br />body parts are a reflection of the different evolu- <br />tionary pathways followed by the two species to make <br />maximum use of a certain part of their environment <br />and to avoid direct competition when populations of <br />the two species occupy the same waters. <br />The flannelmouth sucker still maintains abundant <br />populations under the present altered environmental <br />regime, but the razorback sucker is rare. Obviously, <br />then, the evolutionary specializations adopted by <br />the razorback sucker to best use its historical <br />niche have placed the species at a severe disadvan- <br />tage in the modified environment of the Colorado <br />River basin. What factors in the original environ- <br />ment characterized optimum habitat for razorback <br />suckers? How have these factors been lost, impaired, <br />or modified? <br />As the razorback sucker became rarer, the inci- <br />dence of hybridization with flannelmouth suckers <br />apparently increased. Almost half of the specimens <br />captured, mainly in the Green River, from 1967 to <br />1973 were identified as hybrids. The proportion of <br />hybrids taken from the Colorado River in recent years <br />24
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