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72
<br />LANIGAN AND TMS
<br />from two additional specimens and counted 16
<br />and 26 growth rings. It is probable, however, that
<br />the last three fish may be somewhat older, because
<br />McCarthy and Minckley (1987) demonstrated that
<br />fish aged by annular vertebral centra were consis-
<br />tently lower than those aged by otolith annuli.
<br />They aged nine razorback suckers using both ver-
<br />tebrae and otoliths, and found the otoliths aver-
<br />aged 8.6 more rings than vertebrae. Ages of Green
<br />River fish are still several years less than the 24-
<br />44 years (in 1980-1981) reported by McCarthy
<br />and Minckley (1987) for Lake Mohave fish. If ra-
<br />zorback suckers in the Green River have a lon-
<br />gevity similar to Lake Mohave fish (40+ years),
<br />the Green River adults probably will experience
<br />low mortality.
<br />The assumptions of demographic and geograph-
<br />ical closure are critical to population estimates,
<br />but because a model allowing time-specific changes
<br />in probability of capture was used, no valid sta-
<br />tistical test of closure can be constructed on the
<br />basis of only the capture-recapture data (White et
<br />al. 1982). Movement data collected from tag re-
<br />turns and radio-tagged fish (Tyus 1987) supported
<br />our hypothesis of a geographically closed popu-
<br />lation in the upper Green River with no recruit-
<br />ment and low adult mortality. We consider the
<br />few adult razorback suckers reported from the re-
<br />mainder of the upper Colorado River basin (Tyus
<br />et al. 1982) to be remnant populations.
<br />Tag loss was considered negligible during this
<br />study. Although we did not specifically test for tag
<br />retention, we were able to verify the loss of only
<br />4 tags (i.e., the tag was missing but the line that
<br />once held the tag was still present). Thus, tag re-
<br />tention of recaptured fish could be estimated at
<br />about 94.4%.
<br />Status and Future Prospects
<br />The razorback sucker does not receive protec-
<br />tion under the Endangered Species Act. Our find-
<br />ings emphasize the need for development of new
<br />strategies for the preservation of this species and
<br />further consideration for federal listing. The peril
<br />of this species is perhaps best appreciated when
<br />one considers that our population estimate is for
<br />the largest stock of razorback suckers surviving in
<br />the species' native riverine environment (Tyus
<br />1987). The results of many years of razorback
<br />sucker study in the upper and lower Colorado Riv-
<br />er basin demonstrate a lack of recruitment and
<br />mandate a coordinated effort to prevent its extir-
<br />pation. To date, no comprehensive plan has been
<br />implemented. In the interim, these old fish will
<br />die without replacement and the size of the Green
<br />River population will diminish until recovery of
<br />this population may no longer be possible.
<br />Acknowledgments
<br />These studies were funded in part by the U.S.
<br />Bureau of Reclamation, the U.S. Fish and Wildlife
<br />Service, and the U.S. National Park Service. C.
<br />W. McAda, B. D. Burdick, W. B. Harned, J. J.
<br />Krakker, R. M. McNatt, and L. A. Trinca were
<br />principals among the many U.S. Fish and Wildlife
<br />Service employees who helped gather data. We
<br />thank R. D.• Radant, M. O. Moretti, and J. S.
<br />Cranney for providing Utah Division of Wildlife
<br />Resources capture data. K. M. Paulin aided in data
<br />retrieval. D. L. Otis and D. R. Anderson provided
<br />the software for program CAPTURE and offered
<br />suggestions for its use. We thank K. M. Paulin, C.
<br />A. Karp, D. L. Otis, W. L. Minckley, R. Wydoski,
<br />and two anonymous reviewers for their sugges-
<br />tions about an earlier draft manuscript.
<br />References
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<br />Carlson, C. A., and E. M. Carlson. 1982. Review of
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