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t North American Journal of Fisheries Management 18:318-326, 1998 American Fisheries Society 1998 Use of Multiple Spawning Sites and Seasonal Movement by Razorback Suckers in the Middle Green River, Utah TIMOTHY MODDE* AND DAVID B. IRVING Colorado River Fish Project, U.S. Fish and Wildlife Service 266 West 100 North, Suite 2, Vernal, Utah 84078, USA Abstract.-Radiotelemetry data through three successive spawning events (1993-1995) and cap- ture data for 15 years (1975, 1979-1993) were used to describe movement patterns and fidelity to spawning sites by male razorback suckers Xyrauchen texanus in the middle Green River, Utah. Movement to spawning areas was influenced primarily by discharge. The greatest distances traveled by male razorback suckers were in a downstream direction and occurred just before and shortly after spawning. Three of six surviving males implanted with radio transmitters were located on more than one spawning site between 1993 and 1995. Thus, although most razorback suckers in the middle Green River spawned in a single area between river kilometers 492 and 501 (from the confluence of the Green and the Colorado rivers), other spawning areas were probably used. Tag recapture and telemetry data supported the hypothesis that razorback suckers in the middle Green River represent a single reproductive population. .~ Razorback suckers Xyrauchen texanus occupy large riverine habitats and are endemic to the Col- orado River basin. As with many large endemic species occupying the main tributaries of the Col- orado River basin, the razorback sucker is a long- lived species that reaches ages older than 40 years (McCarthy and Minckley 1987). Populations of razorback sucker in the lower basin of the upper Colorado River once supported a commercial fish- ery but declined dramatically following the com- pletion of major impoundments (Minckley et al. 1991). This decline was attributed to a lack of successful recruitment (Minckley et al. 1991). Lack of recruitment has been attributed to a loss of floodplain nursery habitat resulting from sub- stantially reduced peak flows (Tyus and Karp 1990; Modde et al. 1995) and negative interactions with exotic fishes that have become established in altered riverine environments (Marsh and Brooks 1989; Minckley et al. 1991). Because of major environmental changes, riverine razorback suckers have nearly disappeared from the lower Colorado River basin, and only a small population exists in the upper Colorado River basin (Minckley et al. 1991; Modde et al. 1996). In response to popu- lation declines and habitat loss,. the razorback sucker was listed by the U.S. Fish and Wildlife Service as an endangered species in 1991 (USFWS 1991). The largest population of razorback suckers oc- cupying exclusively riverine habitat in the Colo- rado River basin is located in the middle Green * Corresponding author: tim_modde@fws.gov ~~L~ River between the confluence of the White and Yampa rivers (Lanigan and Tyus 1989). Population size, estimated to be less than 500 individuals (Modde et al. 1996), declined following construc- tion of Flaming Gorge Dam (1964) on the Green River, 110 km upstream of the confluence of the Yampa River. The influence of Flaming Gorge Dam has reduced both variation and magnitude of flows in the Green River (Tyus and Karp 1989) and reduced the historic river channel width and sediment transport (Andrews 1986). Because ra- zorback suckers migrate to specific spawning areas on the ascending limb of the hydrograph (Tyus 1987; Tyus and Karp 1990), changes in the natural hydrograph may affect the ability of fish to form spawning aggregations. Because of its small population (Modde et al. 1996), augmentation efforts have been proposed for the middle Green River population (Biology Committee 1995). Concerns related to augmen- tation activity require information on potential ge- netic isolation (Modde et al. 1995). Tyus and Karp (1990) suggested that razorback suckers show fi- delity to two spawning areas, the Escalante (Jen- sen, Utah) and Yampa River sites. If fish show fidelity to both spawning sites, genetic isolation occurs and broodstock should be developed for both populations. If fidelity were not a factor, then a single population would exist and broodstock for introduction efforts could be mixed. This study describes razorback sucker move- ment before and after spawning. Movement of six male razorback suckers was monitored by means of radiotelemetry between three spawning events and compared with temperature and dischazge. In 318