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Population Status of the Razorback Sucker in the Middle Green River (U.S.A.) ,. TIMOTHY MODDE,* KENNETH P. BURNHAM,t AND EDMUND J. WICK *Colorado River Fish Project, U.S. Fish and Wfldlife Service, 266 W. 100 N., Vernal, UT 84078, U.S.A., email r6ffa_ver@maIl.fws.gov t Colorado Cooperative Fish and Wildlife Research Unit, National Biological Service, 201 Wagar Building, Fort Collins, CO 80523, U.S.A. $Colorado Cooperative Research Unit, National Park Service, 336 Aylesworth Hall, Fort Collins, CO 80523, U.S.A. Abstract: The razorback sucker, Xyrauchen texanus, in the middle Green River (U.S.A.) has been described as a static population consisting of old individuals that will eventually disappear through attrition. Capture data between 1980 and 1992 indicated a constant length frequency despite a slow butpositive growth rate of individual fish. Abundance and survival estimates indicated that the population of razorback sucker in the middle Green River is precariously low but dynamic. Although high variation existed among survival esti- mates, no significant decrease in the population between 1982 and 1992 could be detected. The low level of recruitment occurring in the razorback sucker population of the middle Green River was related to high flow years, indicating that floodplain habitats may be necessary for survival of the species. Fs estado poblacional de Xyrauchen texanus en el rio "Middle Green" (EEUU) Resumen: La probation de Xyrauchen texanus en el rio `Middle Green" (EEUU) ha sido descrita como es- tktica consistiendo en individuos viejos que eventaulmente van a desaparecer por atricion. Los datos de cap- tura entre 1980 y 1992 indicaron una frecuencia de tallas constante a pesar de una tasa de crecimiento indi- vidual lenta Pero positiva para los peces. Las estimaciones de abundancia y supervivencia indicaron que la poblaci6n de Xyrauchen texanus del rio `Middle Green" es precariamente Baja Pero dinkmica. Si bien existio una alta variation entre las estimaciones de supervivencia, no se puedo detectar un decremento poblacional significativo entre 1982 y 1992. Los bajos niveles de reclutamiento que ocurren en la poblacion de Xyrauchen texanus fue relacionada con anos de grandes caudales to que indica que los habitats de las planicies de inun- dation son necesarios para la supervivencia de la especie. Introduction The razorback sucker, Xyrauchen texanus, was listed as endangered under. the U.S. Endangered Species Act in 1991 (U.S. Fish and Wildlife Service 1991). This fish is endemic to the Colorado River Drainage. Once numer- ous in the mainstem and most major tributaries (Minck- ley et al. 1991), this species is now commonly found only in Lake Mohave and the middle Green River drain- age between the confluence of the Yampa and Duch- esne Rivers (Bestgen 1990). Decline of the razorback Paper submitted May S, 1995; revised manuscript accepted Septem- ber 26, 1995. 110 sucker has been attributed to physical changes in habitat associated with reductions in flow, alterations in tem- perature, and establishment of nonnative fishes (Carlson & Muth 1989). Today the middle Green River supports the largest population of razorback sucker occupying riverine habitat (Tyus 1987; Tyus & Karp 1990). This population was described by Lanigan and Tyus (1989) as a remnant stock of approximately 1000 individuals that lacks recruitment. Razorback sucker are known to congregate and spawn at two locations (Tyus & Karp 1990) in the mid- dle Green River (upstream of the town of Jensen, adja- cent to the Escalante Ranch, river kilometers [RKM] 492-501, and at the mouth of the Yampa River, Fig. 1), Conservation Biology, Pages 110-119 Volume 10, No. 1, February 1996