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a tributary to the Little Snake River in south-central Wyoming. Our goal was to examine the effects of human-made instream structures on movements of the three species in the system. Fish were captured and implanted with passive integrated transponder (PIT) tags in three segments of Muddy Creek formed by human-made structures during fall 2004. Movements over structures between April and August 2005 were evaluated using fixed locality monitoring stations that recorded tagged fish upon passage. Reaches throughout the study area were sampled in July and August 2005 by electrofishing. Monitoring stations detected movements of 393 fish through the stations and sampling by electro fishing recovered 200 fish. Bluehead suckers appeared to be the most mobile species. Fish moved downstream, but not upstream, at two instream structures, and both upstream and downstream at a third structure. Probable bluehead sucker and flannelmouth sucker spawning runs up an ephemeral tributary of Muddy Creek were identified in one segment. PIT -tag technology was useful in detecting fish movements, all three native fish species were mobile, human-made structures formed barriers to upstream movements, and source/sink dynamics may be occurring among segments of Muddy Creek formed by human-made structures. 8. Closed Population Estimates of Humpback Chub (Gila cypha) in the Little Colorado River, Grand Canyon, AZ Author: DAVID R. VAN HA VERBEKE, U.S. Fish and Wildlife Service, 323 N. Leroux, Suite 401, Flagstaff, AZ 86001 Abstract: From 2000 to 2005, a series of two-pass, closed mark-recapture efforts were conducted in the Little Colorado River (LCR) to determine the abundance of humpback chub (Gila cypha). Since 2001, the annual spring abundance estimates of humpback chub ~ 150 mm TL have ranged from 2,082 (SE = 242) to 3,419 (SE = 480). Spring spawning abundance estimates for humpback chub ~ 200 (age-4+ adults) have ranged from 1,421 (SE = 209) to 2,002 (SE = 463). A comparison of these numbers with closed mark- recapture studies performed in the early 1990s indicates that there has been a decline in the spring spawning abundance of humpback chub in the LCR since the early 1990s. This trend has been independently confirmed by open population model assessments. Since 2000, the annual fall abundance estimates for humpback chub ~ 150 mm have ranged from 1,064 (SE = 33) to 2,774 (SE = 209). These efforts suggest that the annual fall abundances of humpback chub ~ 150 mm in the LCR have remained relatively stable since mark-recapture efforts conducted in the early 1990s. Annual fall abundance estimates for humpback chub ~ 200 mm since 2000 have ranged from 483 (SE = 48) to 897 (SE = 105). Taken together, the data suggest that the observed declines in humpback chub abundance in the LCR since the early 1990s may be largely in the portion of the population that migrates between the mainstem Colorado River and the LCR for spawning activities. Abundance estimates for 2005 are provisional until approval by Grand Canyon Monitoring and Research Center. 10