Populations of the endangered razorback sucker (Xyrauchen texanus) in the middle Green River have declined since closure of Flaming Gorge Dam in 1962. The apparent cause for the decline is a lack of successful recruitment. Recruitment failure has been attributed to habitat alteration and competition and predation by exotic fishes on early life stages of razorback sucker. This study was conducted to evaluate two of the potential reproductive bottlenecks that might limit recruitment of razorback sucker in the Green River Drainage; (1) reduced larvae production due to sediment deposition on spawning areas, and (2) reduced survival of larvae or juveniles due to lack of timely access to food-rich backwater and floodplain habitats. A paired-site comparison of channel dynamics, hydraulics, temperature, and physical habitat characteristics between known spawning areas in the Green and Yampa Rivers was conducted. Hydrologic data from pre-and post Flaming Gorge time periods was used to assess long-term changes in relevant hydrologic characteristics and changes in the frequency of floodplain inundation. Repeated hydrographic surveys were used to examine patterns in sediment movement and to develop hydraulic models that were used to simulate habitat conditions at both study sites. Channel change was evaluated during water year 2000 and habitat conditions were simulated for runoff during 2000 and 1984. Telemetry was used to evaluate razorback sucker use of the spawning area during the ascending limb of the 2000 runoff cycle in the Green River. Habitat conditions over time for 2000 and 1984 at both study sites were evaluated using maps and duration statistics to represent the period of time during the telemetry study (May 2000) and a period of time associated with known fish occupation of the bar that led to limited recruitment (May 1984). Hydrologic analyses showed that since closure of Flaming Gorge Dam base flows were higher and less variable and the magnitude of peak flows was reduced. Reductions in the magnitude and duration of peak flows have reduced the frequency of floodplain inundation at suspected rearing areas downstream from the Green River spawning bar. Water temperatures near the Green River spawning area were within an acceptable range during the spawning period and probably did not directly limit reproduction during 2000. Repeated surveys showed variable patterns of scour and fill at Green and Yampa River cross sections including deposition of sediment in the Green River spawning area at flows less than 207 m3/s. Cross section surveys suggested that lenses of sand move through both study areas over a range of discharges and that patterns of fill during high flow and scour during recession occur in both the main and side channels of the Green River site. Habitat comparisons showed that the Green River site had a proportionally larger area of slow current velocity habitats over a range of flows than the Yampa River site due largely to differences in the size and elevation of channel features. Availability of shallow, moderate velocity habitats typical of those used by spawning razorback sucker in previous studies was higher during May 2000 than in May 1984 at both study sites. Conditions at spawning areas during 1984 were dominated by relatively deep and fast current velocity habitats. Year 2000 telemetry data showed that fish used a range of habitats during the spawning period but were often located in depths of less than 1.0 m and velocities between 0.4 and 1.2 m/s. Razorback sucker use of the Green River spawning area under very different conditions in 2000 compared to 1984 suggests that physical variables or cues other than depth and velocity influence use of the Green River spawning area. Use of the Green River spawning area by hatchery fish stocked previously at various locations in the Green River was documented by telemetry and indicates that mechanisms other than natal imprinting influence site selection. Razorback sucker used the Green River spawning area during very different flow and habitat conditions during 2000 and 1984. This result is consistent with other studies showing flexibility in razorback sucker spawning behavior including the possible use of multiple spawning sites. Conversely, previous studies indicate that environmental requirements for survival of larvae are fairly narrow and must include access to food-rich backwater habitats. Although sediment deposition in spawning areas could limit recruitment during some years, it appears that larval access to floodplain habitats is potentially a more restrictive reproductive bottleneck over long periods of tim
