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Modde et al. 70 60 50 ~ 40 G d 7 a L 30 20 10 Status of Razorback Sucker 117 Total length (mm) Figure 6. Length frequency (total length) of male and female razorback suckers collected from the middle Green River between 1975 and 1992. population of razorback sucker consists of a precari- ously small but dynamic population that appears to be stable or declining slowly. A stable or slowly declining population contrasts with the dramatic declines re- ported for razorback sucker populations in the upper Colorado River (Modde et al. 1995) and lower Colorado River Basin following impoundment of mainstem rivers (Minckley et al. 1991). Historically, the population of razorback sucker was more abundant than at present (Minckley et al. 1991). The primary changes in the Colorado River system have been the introduction of nonnative fishes and changes in the natural hydrograph (Minckley & Douglas 1991). Since the closure of Flaming Gorge Dam, the peak spring flows of the Green River between the mouths of the Yampa and Duchesne rivers have been significantly reduced (Stanford 1994). Reductions in flows, together with diking practices associated with floodplain agricul- ture, have severely reduced the frequency of overbank flooding. Behnke (1980), Tyus and Karp (1990), and Modde et al. (1995) suggest that flooded bottomlands are important habitats linked with early life-history needs of razorback sucker. The positive response of small adults to high flows supports this contention. The reduction of floodplain connectivity caused by flow re- ductions has been linked to major declines of several species of fishes occupying a large acid river, the Murray- Darling River in Australia (Walker & Thoms 1993). In- creased numbers of nonnative fishes have also been linked to population declines of native species in the Murray-Darling system. There have likewise been in- creases in nonnative fishes in the Colorado River Basin during the last several decades that have been associated with declines in native fish populations (Tyus et al. 1982). Some specific information also supports the hy- pothesis that razorback suckers have been affected by nonnative fish species. The successful individual growth and survival of razorback suckers in isolated coves and backwaters adjacent to Lake Mohave (Marsh & Lang- horst 1988) has been attributed to protection from pre- dation by exotics and increased prey availability (Minck- ley et al. 1991). Predation on razorback sucker larvae, under experimental conditions, by adult red shiners, ju- venile green sunfish (Lepomis cyanellus), and juvenile channel catfish (Ictaluris punctatus) was mediated by the presence of alternate invertebrate prey (Tubifex worms) (Robert Muth, personal communication). Inver- tebrate densities in the Green River between June and August appear to be low: 24 zooplanktors/L in riverine backwaters compared to 402 zooplanktors/L in an adja- cent wetland on the Ouray National Wildlife Refuge (Old Charley Wash) (Maybe 1993). Conservation Biology Volume 10, No. 1, Febntary 1996