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Background Status of humpback chub Humpback chub is endemic to the Colorado River basin (Miller 1964, Minckley 1991), with origins extending as far back as Miocene (Miller 1959, Minckley et al. 1986). In Grand Canyon, humpback chub occupy unusual habitat relative to other populations in the watershed, largely inhabiting the LCR, a saline tributary to the mainstem Colorado River. The Grand Canyon population is comprised of individuals that live some portion of their life history in the mainstem Colorado River and migrate to the LCR for spawning purposes. Humpback chub was listed as endangered in 1967 (U.S. Office of the Federal Register 32:48 [1967]: 4001). In Grand Canyon, the species faces threats, including habitat loss (Suttkus and Clemmer 1979, Minckley 1991), watershed mismanagement (Abruzzi 1995), cumulative effects of environmental variation (see Gilpin and Soule 1986), parasite loads (Clarkson et al. 1997), and predation by introduced non-native fishes in the mainstem Colorado River (Valdez and RyeI1995). Even though multiple causes of population decline have been identified, no known progress has been achieved in elevating population numbers since listing in 1967. Early accounts of the abundance of humpback chub in Grand Canyon, while sparse, suggest a much higher historical population. The Kolb brothers witnessed humpback chub spawning in the mouth of the LCR in numbers so large that they described the striking of their tails upon the surface waters as sounding like "a slide of shale" (Kolb and Kolb 1914). This simple description suggests very high densities of fish, something not currently observed in the LCR. The two brothers referred to the fish as "bony tails," but photographs show them to be humpback chub. Another photograph taken by the Rust expedition shows numerous large humpback chub captured during a day of angling in the mainstem Colorado River just above Bright Angel Creek (RM 87.5; Figure 1). Since Glen Canyon Dam has been in place (nearly forty years), only a few humpback chub have been captured in this vicinity with high intensity effort. Minckley et al. (2003) calculated that there were about 200,000 adult humpback chub inhabiting the Colorado River basin during historic pre-dam times. Recent population estimates indicate that during the past twenty years, humpback chub in Grand Canyon have declined. Population estimates have dropped from around 7,500 fish >200 mm total during the late 1970s (Kaeding and Zimmerman 1982), to -4,500 fish >150 mm during spring in the LCR in the early 1990s (Douglas and Marsh 1996). Recent closed estimates from the LCR for humpback chub ~150 mm have been 2,082 fish (SE = 242) in spring 2001 (Van Haverbeke and Coggins 2003), 2,666 fish (SE = 463) in spring 2002 (Van Haverbeke 2003), and 3,419 fish (SE = 480) in spring 2003 (Van Haverbeke 2004). Modeling based on the database of humpback chub in Grand Canyon has confirmed this overall declining trend (Coggins et al. 2003). This evidence indicates that there has been a decline in the abundance of humpback chub in Grand 8