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Transactions of the American Fisheries Society 130:1095-1105, 7201 C Copyright by the American Fisheries Society 2001 Floodplain Wetland Suitability, Access, and Potential Use by Juvenile Razorback Suckers in the Middle Green River, Utah TIMOTHY MODDE* Colorado River Fish Project, U.S. Fish and Wildlife Service, 266 West 100 North, Suite 2, Vernal, Utah 84078, USA ROBERT T. MUTH1 U.S. Fish and Wildlife Service, 145 East 1300 South, Suite 404, Salt Lake City, Utah 84115, USA G. BRUCE HAINES Colorado River Fish Project, U.S. Fish and Wildlife Service, 266 West 100 North, Suite 2, Vernal, Utah 84078, USA Abstract. In the upper Colorado River basin, razorback suckers Xyrauchen texanus primarily occur as adults in low-gradient reaches of rivers or as juveniles or subadults in adjoining floodplain depressions. During the high-water years of 1995 and 1996, a number of habitat variables- zooplankton density, water temperature, quality, and depth, and vegetative cover-in a Green River floodplain depression were favorable to age-0 razorback suckers and other fishes. Conversely, main-channel habitats were not suitable as rearing sites due to lower temperatures, less food, and ineffective cover. During the spring runoff in 1995 and 1996, larval razorback suckers were found in the river after floodplains were isolated from the Green River. Although favorable nursery sites are located off-channel, the connectivity of the river and floodplain did not last long enough for all razorback sucker larvae to access these areas. Maintaining the connectivity of the floodplain to the river via levee removal and adequate flood duration will increase access to and the use of nursery habitat favorable to larval razorback suckers in the middle Green River. This study also suggests that larval razorback suckers are more likely to survive and grow in floodplain depression habitats than in main-channel habitats. Although age-0 razorback suckers grew and survived in an environment dominated by nonnative predators and competitors, the magnitude of nonnative fish impacts on razorback suckers in floodplain wetlands remains undefined. Floodplains are purported to be habitat used by both juvenile (Modde 1996) and adult (Tyus and Karp 1990) razorback suckers Xyrauchen texanus, which were once common throughout much of the Colorado River basin (Minckley 1983; Minckley et al. 1991). The razorback sucker has declined substantially and is listed as endangered by the U.S. Fish and Wildlife Service (U.S. Fish and Wildlife Service 1991). Its decline has been attri- buted to the lack of recruitment (McAda and Wy- doski 1980) resulting from habitat changes follow- ing the construction of dams on main-stem rivers and tributaries (Carlson and Muth 1993) as well as predation by nonnative fishes (Bestgen 1990; Minckley et al. 1991). In the upper Colorado River basin, the greatest numbers of razorback suckers have been captured in low-gradient or floodplain V. * Corresponding author: tini-modde@fws.gov 1 Present address: U.S. Fish and Wildlife Service, Post Office Box 25486 Denver Federal Center, Denver, Col- orado 80225-0486, USA. Received May 10, 1999; accepted May 29, 2001 reaches (Holden and Stalnaker 1975; Modde et al. 1995), with the largest population occurring in the middle Green River (Lanigan and Tyus 1989). Similarly, most razorback suckers in the lower Colorado River basin have been found in the low- er-gradient reaches below the Grand Canyon (Minckley 1983; Minckley et al. 1991). Tyus and Karp (1990) suggest loss of floodplain inundation as a factor in the lack of recruitment, and Modde et al. (1996) relate the recruitment of young ra- zorback suckers to high-flow years in the middle Green River. The reproductive biology of the razorback suck- er in the Green River is linked to spring flood events (Tyus and Karp 1989). This sucker repro- duces on the ascending limb of the hydrograph (Tyus 1987; Tyus and Karp 1990; Modde and Ir- ving 1998), which maximizes potential access to inundated floodplains by newly emerged larvae. The timing of spawning and the rapid growth in off-channel habitats (Osmundson and Kaeding 1989) support the premise that floodplain habitats are important to the early life stages of this fish. 1095