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<br />
<br />DOUGLAS AND MARSH-ESTIMATES/MOVEMENTS OF GILA CYPHA 25
<br />
<br />cypha in the Yampa River. Interfaces between
<br />eddies and runs were similarly judged impor-
<br />tant in the Black Rocks area (below GrandJunc-
<br />tion, CO) (R. A. Valdez and B. A. Nilson, Proc.
<br />Am. Fish. Soc., Bonneville Chapter, 1982, un-
<br />pub!.). Adult G. cYPha are primarily nocturnal
<br />(fig. 5 of Valdez and Clemmer, 1982). During
<br />daylight hours in the LCR, they reside in deeper
<br />waters along cut banks with overhanging veg-
<br />etation (primarily reeds), along sheer rock out-
<br />crops, or in deeper pools away from shore; they
<br />are active during crepuscular hours and in late
<br />evening (C. O. Minckley, pers. comm.; M. E.
<br />Douglas and P. C. Marsh, pers. obs.).
<br />Greater numbers of G. c)pha were found in
<br />the Salt Canyon reach (Table 1) when compared
<br />to Powell Canyon reach. These data sustain at
<br />least two alternative hypotheses. Increased hab-
<br />itat complexity in the Salt Canyon reach, with
<br />greater numbers of large travertine dams, eddy /
<br />run interfaces, and deep pools, may increase
<br />residency of G. cYPha within this area. Alter-
<br />natively, those G. cYPha that move up the LCR
<br />may literally stack within the upper reach, due
<br />either to a physical barrier at RKM 14.9, or to
<br />a chemical one produced by high CO2, or to
<br />other chemical content.
<br />
<br />Glen Canyon Dam and Gila cYPha.- There is long-
<br />term residency by G. c)pha within the LCR, par-
<br />ticularly summer through winter (Table 3). In
<br />fact, many adults apparently overwinter within
<br />the LCR, effectively using it as a warm-water
<br />refugium. Two hypotheses are presented to ac-
<br />commodate these data. One suggests residency
<br />is a pre-dam component of G. cypha's life his-
<br />tory. The other proposes that it is a post-dam
<br />alteration. It is unclear which can be rejected;
<br />both are untestable in their present form.
<br />Long-term residency by adults may have al-
<br />ways been an aspect of G. cypha's life history.
<br />We know, for example, that it spawned within
<br />the pre-dam LCR during spring (Kolb and Kolb,
<br />1914: 127; Carothers and Brown, 1991 :93).
<br />However, its duration of stay was unknown. If
<br />residency has always been a component of G.
<br />C)pha's natural history, then our mark/recap-
<br />ture data simply define inherent behavior over
<br />evolutionary time.
<br />An alternative hypothesis is that the altered
<br />thermal regime of the mainstem has forced G.
<br />c)pha to adjust its life history. It now accom-
<br />modates lower mainstream temperatures pri-
<br />marily through avoidance (i.e., by increasing
<br />residency within the LCR). This hypothesis is
<br />anecdotally supported by three facts. First,
<br />movements into/from the LCR are primarily
<br />accomplished by larger (and presumably older)
<br />
<br />G. cYjoha (R. A. Valdez, pers. comm.). Gila cYPha
<br />attains great age (20 + years; Minckley, 1991:
<br />150); larger adults may thus represent main-
<br />stem-adapted individuals from pre-1968 co-
<br />horts (when Lake Powell filled). Second, larvae
<br />and juvenile G. cypha are often transported via
<br />flood into the mainstem, but adults smaller than
<br />200 mm TL are seldom taken there (R. A. Val-
<br />dez, pers. comm.). Kaeding and Zimmerman
<br />(1983:585) similarly noted that individuals larg-
<br />er than 145 mm TL were never taken in the
<br />mainstream above the confluence, even though
<br />mature fish were present there. Third, hydro-
<br />logic and thermal profiles of the LCR are con-
<br />sistem with the pre-dam Colorado River but
<br />differ markedly from the post-dam river.
<br />If G. cypha has altered its life history to ac-
<br />commodate dam-induced changes in the main-
<br />stem Colorado River, then its long-term persis-
<br />tence within the Grand Canyon is tied more
<br />intimately to the LCR than previously believed.
<br />The evolutionary effects of such a life-history
<br />alteration can only be speculated upon.
<br />One potential saving factor (Committee, 1991:
<br />4) is that ecosystem components are linked to
<br />one another and to flow regimes imposed by
<br />the dam. Flows can therefore be manipulated
<br />to manage the river and protect the environ-
<br />ment in GCNP. This offers the possibility that
<br />temperature, sediment load, and volume of dis-
<br />charge from the dam may eventually mimic a
<br />natural hydrograph, at least during parts of the
<br />year. This could enhance long-term survival of
<br />G. cypha (and may allow upriver movement of
<br />other introduced fishes from Lake Mead;
<br />Minckley, 1991: 146). In spite of such optimism,
<br />political and economic forces drive the system,
<br />even at the expense of cost efficiency (Leopold,
<br />1991). These forces likewise impact indigenous
<br />fishes and transform their conservation from
<br />the realm of science to that of politics.
<br />
<br />ACKNOWLEDGMENTS
<br />
<br />Numerous individuals were involved in data
<br />collection: G. Aldridge, B. Bagley, N. Brian,].
<br />Cook, G. Doster,]. Dunham, B. Dunnigan, A.
<br />Fegley, E. Gustafson, M. Horn, R. Larson, E.
<br />Montoya, C. Minckley, R. Mose, D. Oakey, D.
<br />Palmer, R. Reed, P. Ryan, R. Shepherd, R.
<br />Timmons, D. Valenciano, and R. Van Haver-
<br />beke. C. Weber entered and edited data. The
<br />dilligence and perseverence of all these individ-
<br />uals are to be commended. Grand Canyon Na-
<br />tional Park (GCNP) allowed research within the
<br />park's boundaries. GCNP and Arizona Game
<br />and Fish Department (AZGF) provided permits
<br />to collect fish at the confluence, whereas Navajo
<br />
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