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c:) c-) ...... co 1." ,0 razorback sucker hybrids have never been reported, but they may be possible because of the hybridization between the closely related flannehnouth sucker, Occurrences of razorback sucker x flannehnouth sucker hybrids are fairly well documented (Hubbs 1955, Holden and Stalnaker 1975) but not common, Ifwhite sucker become more common in the San Juan River, hybridization with razorback sucker, as well as fIannehnouth sucker and bluehead sucker, may become a greater concern (Dowling and Minckley 1993). Hybridization between razorback sucker and flannehnouth sucker likely occurs since flannehnouth sucker often use the same spawning bar as razorback sucker, Flannehnouth sucker also spawn on many different bars, whereas razorback sucker show fidelity to a few spawning areas, Razorback sucker begin spawning slightly earlier than flannehnouth sucker, but their spawning times overlap. With no wild razorback sucker population to attach to, or wild spawning site to use, stocked razorback sucker have the potential to spread out spawning rather than concentrating it in one or two areas. This could result in increased use of bars also used by flannehnouth sucker and increased chance of hybridization. Management Implications Because of the concern for nonnative fish interactions, one of the original goals of the SJRIP was to reduce numbers of nonnative fishes, One reason for the reoperation of Navajo Dam in 1992 was the hypothesis that high flows, or a more-natural flow regime, may reduce numbers of red shiner, channel catfish, and other nonnative species. This hypothesis was proposed by Minckley and Meffe (I 987) and Meffe and Minckley (1987) after reviewing fish populations in portions of Arizona where native species numerically dominated streams with natural flow patterns and magnitude, and nonnative species dominated regulated streams, These authors hypothesized that nonnative species are not well adapted to the flooding characteristic of Southwestern streams, where native species evolved. Several studies documented declines in nonnative minnows (red shiner, sand shiner, and fathead minnow) during high-flow years in the Upper Basin (McAda and Kaeding 1989, Osmundson and Kaeding 1989, Valdez 1990, Muth and Nesler 1993, Lentsch et al, 1996, McAda and Ryel 1999), McAda and Ryel (1999) also documented an increase in native fishYOY during higher spring flow years in the Colorado River, although this relationship was not always significant. These various studies on the Green and Colorado rivers support the hypothesis that higher spring flows, or mimicry of the natural hydrograph, improves conditions for native species and reduces conditions for nonnative species, at least within a year or two of the event. Numbers of channel catfish and common carp in San Juan River primary channels increased since the reoperation of Navajo Dam, whereas other nonnative species either remained relatively stable or the trends are not clear (Brooks et al. 2000, Ryden 2000a). Propst and Hobbes (2000) saw similar trends in secondary channels, except in 1997 when numbers of all species were reduced in the autumn, During autumn, red shiner density in secondary channels was more related to sununer flows than spring runoff flows (Propst et al. 1999). High sununer flows, typically resulting from thunderstorms rather than dam releases, were associated with reduced numbers of red shiner in autumn. Archer and Crowl (2000a) studied San Juan River backwater nursery habitats in August and September. Young of native species were more abundant during high and late spring runoff years, and nonnative abundance was lower, But Archer and Crowl (2000a) noted that part of this September 2000 3-50 Program Evaluation Report