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14
<br />razorback sucker hybrids in the present collection does
<br />not necessarily indicate fewer hybrids in the system,
<br />but may be due to variations in sampling locations or
<br />other factors. Misidentification of younger hybrids is
<br />possible because of their close resemblance to the
<br />flannelmouth sucker. However, large hybrids are
<br />readily identifiable by their small dorsal ridge and
<br />intermediate lip structure.
<br />Future of the Razorback Sucker in the
<br />Colorado River Basin
<br />The Yampa River plays a major role in the continued
<br />survival of the razorback sucker and other native
<br />fishes. The Green River begins to resume its natural
<br />temperature regime as it gains warmer water from the
<br />Yampa River. The ameliorating impact of the Yampa
<br />River on the Green River is believed to be important to
<br />the razorback sucker and the other native fish in the
<br />river system (Holden and Stalnaker 1975b~. Vanicek et
<br />al. (1970) documented the reduction in native fish
<br />populations of the Green River above the mouth of the
<br />Yampa River after the closure of Flaming Gorge
<br />Reservoir. The four rare native fish (Colorado squaw-
<br />fish, razorback sucker, humpback chub, and bonytail
<br />chub) were eliminated from the Green River above the
<br />confluence with the Yampa at that time. Their decline
<br />was associated with alterations in temperature,
<br />streamflow, turbidity, and other characteristics of the
<br />river. The Yampa River partly restored the historical
<br />water conditions in the Green River, sufficiently to
<br />allow the fish to survive below the confluence of the
<br />two rivers.
<br />The Yampa River is the largest free-flowing river
<br />that remains in the Colorado River basin. In this
<br />basin, about one-fourth (22%) of the river length is
<br />inundated by reservoirs (Utah Water Research
<br />Laboratory 1975) and most of the remaining reaches
<br />have been adversely affected by water releases from
<br />these reservoirs. Alterations to the natural tem-
<br />perature regime of the Yampa River could be detri-
<br />mental to the native fauna, particularly to the four
<br />threatened or endangered fishes. The Green River,
<br />from the confluence with the Yampa River to its
<br />mouth in Canyonlands National Park, contains most
<br />of the remaining large-river habitat in the Colorado
<br />basin. Any changes in water temperature or stream-
<br />flow that occur upstream, such as the Yampa River,
<br />could significantly reduce the remaining habitat of
<br />these four species.
<br />The decline of the razorback sucker is well correlated
<br />with the development of the Colorado River basin.
<br />However, the specific changes that have caused the
<br />decline are not yet clear. Sections of the Green and
<br />Yampa rivers are relatively unaltered, but contain
<br />only small numbers of razorback suckers, whereas
<br />other native species are still abundant. Reproductive
<br />failure appears to be a major factor in the decline of the
<br />species. However, the reason for this failure is difficult
<br />to determine because a closely related fish-the
<br />flannelmouth sucker-which appears to have similar
<br />spawning requirements (McAda 1977, is still doing
<br />well.
<br />Acknowledgments
<br />We thank G. Kidd, formerly of the Colorado Divi-
<br />sion of Wildlife, who participated in several collec-
<br />tions; K. Seethaler, D. Cox, J. Henderson, L. Maclain,
<br />and C. Reger, who assisted in the field; J. King, C.
<br />Larson, K. Priest, K. Rudd, and J. Wydoski, who
<br />helped with the laboratory work.
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