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942 MODDE ET AL. of 2.0% (Modde et al. 1996) of the wild popula- tion. Although our captures undoubtedly under- estimated the total fish in the area, both hatchery- reared and wild fish were proportionately repre- sented. Because both hatchery and wild male fish showed secondary reproductive characteristics (tubercles) and expressed milt, it is probable these fish were engaged in spawning activity. Among nine hatchery fish captured, two were females and one was the last radio-tracked fish to leave the spawning area. Five radio-tagged fish (one wild, four hatchery) were detected in the study area dur- ing the spawning period. Only hatchery fish were detected on the spawning bar during the 24-h te- lemetry monitoring surveys, but the one wild fish was detected in the spawning area for 11 d, which overlapped with three hatchery fish. Because both wild and hatchery fish were collected simulta- neously in the same locations with ?electrofishing gear and fish were collected in the same locations as in previous years (senior author's personal ex- perience), it is likely that hatchery fish were oc- cupying the same spawning areas as wild fish. Hatchery-reared razorback suckers have sur- vived and successfully spawned in the Gunnison (D. Osmundson, U.S. Fish and Wildlife Service, Grand Junction, Colorado) and San Juan rivers (Ryden 2004). In these rivers, hatchery razorback suckers have located suitable spawning sites and successfully produced larvae. Hatchery-reared ra- zorback suckers have only been stocked into two extant populations, Lake Mohave (Marsh et al. 2005) and the Green River. In both areas, hatchery razorback suckers have occurred at documented spawning sites simultaneously with wild fish. Hatchery-reared catostomids (i.e., June suckers Chasmistes horus (Keleher et al. 1998) and robust redhorse Moxostoma robustum; Jimmy Evans, Georgia Department of Natural Resources, per- sonal communication) have been observed with secondary and primary sexual characteristics as- sociated with wild fish during the spawning sea- son. Thus, hatchery-reared suckers have shown the ability to seek out wild or other stocked fishes and. have successfully spawned and produced off- spring. In this regard, stocking has the potential to be an important tool in the recovery of razorback suckers, at least in several upper basin rivers and lower basirFimpoundments. Sholtz et al. (1992) reported that razorback suckers experienced thyroxin peaks at the hatching and swim-up life stages and suggested that this pattern is consistent with imprinting. Experimental trials of subadult fish showed significant differ- ences in fish movement toward imprinted sub- stances (Sholtz et al. 1996). Even though the hatchery-reared fish observed in our study were not imprinted to the spawning area, they still ag- gregated with wild fish at rkm 504 before peak flows. A recent evaluation of razorback suckers stocked in the middle Green River suggested the majority of stocked fish remained in the area they were stocked and that a large number were cap- tured in the area of the spawning aggregation near rkm 504 (Modde et al. 2004). Although stocked fish were found at the primary spawning aggre- gation without being imprinted, imprinting may increase the number of hatchery-reared fish mov- ing to established spawning aggregations that pro- vide a survival advantage for age-0 fishes they produce. That is, wild razorback suckers in the Green River spawn in the upstream areas of al- luvial reaches (Muth et al. 1998), which allows drifting larvae to access inundated floodplains downstream that provide important nursery habitat (Modde et al. 2001). Therefore, the larvae of stocked fish engaged in spawning activity below these nursery sites, would have a higher risk of mortality. Thus, although imprinting may not pre- clude razorback suckers from spawning, it has the potential to increase effective spawning site se- lection and ensuing larval survival. Razorback suckers have successfully repro- duced in rivers during flood flows (Muth et al. 1998), large reservoirs (Minckley et al. 1991), and small impoundments (Mueller et al. 2003). Hatch- ery razorback suckers observed in our study ap- peared to have the ability to locate and presumably participate in wild spawning aggregations. They responded to natural cues, aggregated in a primary spawning area, and presumably engaged in spawn- ing activity. Thus, it is likely that hatchery-reared razorback suckers are capable of contributing to recovery. In this regard, the challenges associated with recovery relate more to implementation of stocking programs that include the most efficient means of attaining survival (i.e., size and timing of stocking and, particularly, the adherence to a broodstock maintenance and mating system that maximizes genetic diversity; Meffe 1986). Genetic diversity is particularly important in that the small extant populations can be swamped or replaced through repatriation efforts (USFWS 2002). Dowl- ing et al. (1996) reported the loss of genetic di- versity in refugia stocks of razorback suckers. Giv- en the potential to quickly increase the abundance of declining or extirpated populations of razorback suckers with large viduals, the challen proving the factors cline of the species, ment due to lack of a de et al. 2001; interactions with n among early life st; Minckley and Deao Bestgen, K. R. 1990. sucker, Xyrauchet sity, Larval Fish Collins. Dowling, T. E., W. L. Goldstein. 1996. the endangered r anus): analysis of for captive propa€ 120-127. Holden, P. B., P. D. . Razorback sucker 1997. Annual Re Department of Re Authority, Las Ve Irving, D. B., and B. L inventory and pri tial bottomlands it 1993-1994. Final Implementation P Species in the UI Fish and Wildlife Keleher, C. J., L. D. 1998. Evaluation sucker (Chasmiste Division of Wildli Lake City. Lanigan, S. H., and H and status of the r; er basin, Utah and nal of Fisheries Tv Marsh, P. C., B. R. Ke patriation as a m? critically imperilt Journal of Fished Marsh, P C., and C. native and nonnat Restoring native 1 interactions of n: Fish and Wildlife ico. Meffe, G. K. 1986. Cc agement of endan 23. Minckley, W. L., P. C.1 and B. L. Jensen covery of the raz L. Minckley and against extinction