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INVESTIGATIONS INTO POTENTIAL SPAWNING OF RBS AND CS IN LOWER GREEN 1994 and 1995 INTRODUCTION The razorback sucker , Xyrauchen texanus, was listed as endangered on October 23, 1991 (56 FR 54957). Razorback sucker populations have been declining for much of this century and are currently found only as remnant populations ui C_olorado River basin reservoirs and river reaches. c ' f!j~„~ ,~ The lazgest extant population is found in Lake Mohave, Arizona. The onl documented G~~/~' ~°~ - /(a Y ~.:.~- spawning, riverine population is found in the Green River neaz Jensen, Utah (Tyus 1997) where size of the population was most recently estimated at 524 individuals (Modde et al 1996) down considerably from the Latugan and Tyus (1989) estimate (N=948). In recent years, adult razorback suckers have been collected in relatively low numbers from the lower Green River T set al 1982, T set al 1987, and USFWS ISMP reports). Five of seven adult razof (yu yu rback suckers collected by the Utah Division of Wildlife Resowces (UDWR) since 1986 were captured between River Miles (RM) 104 and the mouth of the San Rafael River at RM 97.0..,;) 5v~~, F,; 5r:'wd ~ C ~. ~ ~ G ~:G ~ y~ On July 30, 1991, the Utah Division of Wildlife Resources collected two age 0 razorback suckers at Green River mile 55.6. (Gutermuth et al 1994), while conducting a study of Colorado squawfish (Ptychocheilus Lucius) nursery habitat. It was later determined that these fish were most likely spawned in the lower Green River, neaz the point of capture. A total of 1221arval razorback suckers were collected from nursery habitats on the lower Green River during 1993 (Muth 1995). Based on this eazly life history information and the incidental collections of adults between Green River, UT and the San Rafael confluence, the UDWR (in addition to personnel from the National Pazk Service and Larval Fish Lab at Colorado State University) was interested in concentrating some effort in this portion of the lower Green River to determine if,Fazorback 1Z(~s Cant-~, l suckers spawned The following project field objectives were developed determme:~ presence /a>~sence of larval and Age 0 Razorback suckers in the sprr~~nn~ from the town of Green River, U~`downstream to the Colorado River confluence. d b~ p sense /absence of awnin sP g adult razorback suckers from the town of Green River, Uf downstream to Red Wash at RM 95.0. The Colorado. s~uawfish was listed as endangered on March 11, 1967. Reproducing populations of Colorado squawfish aze found in each of the Colorado River sub-basins, with the most viable DRAFT