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Endangered Cui-ui of Pyramid Lake, Nevada

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Cui-ui (Chasmistes cujus) with dorsal radio tag. Courtesy G.G. Scoppettone, NBS
Cui-ui is one of three remaining species of the genus Chasmistes. Of the three, its habitat is most intact, and it thus has the best opportunity for recovery (Scoppettone and Vinyard 1991). Each spring, cui-ui adults, most of which mature at 8-12 years of age, migrate to the mouth of the Truckee River at the south end of Pyramid Lake, where they aggregate, awaiting environmental cues and sufficient stream flow to enter the river (Scoppettone et al. 1986). This behavior provides an excellent opportunity to capture the adults for estimating population numbers and year-class (year hatched) structure. In this article we report changes in adult cui-ui population number and year-class structure from spring 1983 to spring 1993.

Status and Trends
Each spring, cui-ui are captured, anchor-tagged, and released for recapture. The proportion of tagged to untagged fish is used to estimate population number. Virtually all mature adults enter the prespawning aggregate each year (Scoppettone, unpublished data); thus an estimate of the number of adults entering the aggregate is an estimate of the entire adult population. We provide data of 4 select years (1983, 1991, 1992, and 1993) to illustrate trends between 1983 to 1994.
Captures of cui-ui from the prespawning aggregate have been successful enough to give us reliable estimates of the adult population. In 1982 and 1983, 3,000 adults were captured and tagged. From 1989 through 1993, captures increased markedly because of a change in capture gear and increased population. More than 100,000 cui-ui have been captured, and tags were applied to 60% of these. By spring 1993, tag returns were close to 4% of the fish captured.
The adult cui-ui population has increased 10-fold from 1983 to 1993 (Fig. 1), an increase attributed in part to unusually wet years from 1980 to 1986. During these years more than 65,000 adults entered the lower Truckee River to spawn, and produced more than 250 million cui-ui larvae for Pyramid Lake. In contrast, virtually no spawning occurred in the Truckee River from 1988 through 1992, a fact that will probably be reflected later in this decade as a downward trend in the number of adults. Fig. 1. Estimated population of adult cui-ui in spring 1983, 1991, 1992, and 1993.
Adult Year-class Structure
Fig. 2. Year-class structure of adult cui-ui in spring 1983, 1991, 1992, and 1993.
To understand cui-ui demographics and why the species is still considered endangered, it is necessary to understand its year-class structure. In 1983 when there were about 100,000 adult cui-ui in Pyramid Lake, almost 90% were from a single year class produced in 1969; the second predominate class represented about 5% of the population and was hatched in 1950 (Fig. 2). From 1950 to 1968 and from 1970 to 1979, very little recruitment occurred. The situation has improved; in 1991, the 1981 year class replaced the 1969 in predominance, and it remained so through 1993. In 1993, 400,000 of the estimated 1 million adults were fish that had been hatched in 1981. The dramatic increase in the spawning population from 1991 to 1992 is assumed to be those fish that hatched in 1981, 1982, and 1983 and finally reached adulthood.
These improvements in population numbers and year-class structure are partly attributed to several extraordinarily wet years; similar conditions may not occur with sufficient frequency to assure species recovery or preclude extinction.
In addition to the prespawning aggregate, the adult and juvenile populations have been sampled around Pyramid Lake throughout the year. Our results suggest that few juveniles hatched after 1986, and thereby provide testimony to inconsistency in cui-ui recruitment.

Future Outlook
The cui-ui has an excellent prognosis for recovery. It has an approved recovery plan and supporting legislation (P.L.101-616), which provide for acquisition of water and water rights to elevate Pyramid Lake, improve fish passage over the delta, and enhance spawning flows. Plans to acquire water for Pyramid Lake and cui-ui are being developed. Cui-ui population trends over the past 10 years demonstrate the rebound potential of the species when it is provided with passage and sufficient water for reproduction. Because limited water is available for acquisition, however, Truckee River flows required for cui-ui recovery need to be precisely determined. Our monitoring of the adult cui-ui population is part of a cui-ui population dynamics study aimed at calibrating an existing Truckee River water-management model being used for cui-ui recovery. Monitoring will continue through the 1998 spawning season, at which time sufficient information should have been generated to calibrate the model.
Spawning cui-ui. Courtesy H.Burge, USFWS
For further information:
G. Gary Scoppettone
National Biological Service
Reno Field Station
4600 Kietzke Ln.
Bldg. C., Rm. 120
Reno, NV 89502
References
Scoppettone, G.G. 1988. Growth and longevity of the cui-ui and longevity of other catostomids and cyprinids in western North America. Transactions of the American Fisheries Society 117:301-307. Scoppettone, G.G., M. Coleman, and G.A. Wedemeyer. 1986. Life history and status of the endangered cui-ui of Pyramid Lake, Nevada. U.S. Fish and Wildlife Service Fish and Wildlife Res. 1:1-23.

Scoppettone, G.G., and G. Vinyard. 1991. Life history and management of four endangered lacustrine suckers. Pages 359-377 in W.L. Minckley and J.E. Deacon, eds. Battle against extinction: native fish management in the American West. The University of Arizona Press, Tucson.

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