Our Living Resources — Interior West 



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Cui-ui (Chasmistes ciijiis) is a large plank- 

 ton-feeding fisli that only occurs in 

 Pyramid Lake, Nevada. It was put on the feder- 

 al endangered list in 1967 based on declining 

 piipulaiion and absence of reproduction. A lake 

 dweller, cui-ui is a stream spawner. Most of this 

 century, this sucker species was unable to 

 access the Truckee River, Pyramid Lake's only 

 perennial tributary, to reproduce. Water diver- 

 sion from the Truckee River, as a result of the 

 nation's first Bureau of Reclamation project 

 (Newlands Project), reduced the lake elevation 

 and, in most years, caused an impassable delta 

 to form at the mouth of the Truckee River Cui- 

 ui live more than 40 years; it is this longevity 

 that has allowed the species to persist for as 

 many as 19 years with virtually no recruitment 

 (see glossary) to the adult population 

 (Scoppettone 1988). 



Cui-ui is one of three remaining species of 

 the genus Chasniistes. 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) struc- 

 ture. In this ailicle we report changes in adult 

 cui-ui population number and year-class struc- 

 ture from spring 1983 to spring 1993. 



Status and Trends 



Each spring, cui-ui are captured, anchor- 

 tagged, and released for recapture. The propor- 

 tion of tagged to untagged fish is used to esti- 

 mate population number. Virtually all mature 

 adults enter the prespawning aggregate each 



Cui-ui iCIwsinisles eiijus) witli dorsal radio tag. 



year (Scoppettone. unpublished data): thus an 

 estimate of the number of adults entering the 

 aggregate is an estimate of the entire adult pop- 

 ulation. 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 cap- 

 ture 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, vir- 

 tually 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. 



Adult Year-class Structure 



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 estimat- 

 ed 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 condi- 

 tions may not occur with sufficient frequency to 

 assure species recovery or preclude extinction. 



In addition to the prespawning aggregate, 

 the adult and ju\enile populations have been 

 sampled around Pyramid Lake throughout the 

 year. Our results suggest that few juveniles 

 hatched after 1986, and thereby provide testi- 

 mony to inconsistency in cui-ui recruitment. 



Endangered 

 Cui-ui of 

 Pyramid 

 Lake, Nevada 



by 



G. Gary Scoppettone 



Peter H. Rissler 



National Biological Senuce 



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Fig. 1. Estimated population of 

 adult cui-ui in spring 1983. 1991, 

 1992, and 1993. 



