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<br />
<br />DOUGLAS AND MARSH-ESTIMATES/MOVEMENTS OF GILA CYPHA 23
<br />
<br />species-composition; Committee, 1991). All have
<br />severely impacted the natural ecosystem; some
<br />are irreversible.
<br />Indigenous fishes inhabiting Glen, Marble,
<br />and Grand canyons were impacted following
<br />closure of Glen Canyon Dam (Holden and Stal-
<br />naker, 1975; Suttkus and Clemmer, 1977;
<br />Minckley, 1991). Many (including G. c)pha:
<br />Holden and Stalnaker, 1975; Anonymous, 1980)
<br />persisted in Lake Powell but were unable to
<br />reproduce (Holden, 1973:4). Downstream from
<br />the dam, the fish community shifted from pre-
<br />dominantly warm-water native and introduced
<br />fishes to one dominated by either cold-water
<br />fishes [i.e., rainbow trout (Oncorhynchus mykiss)
<br />and brown trout (Salmo trutta)) or those with
<br />broad temperature tolerances. Within GCNP,
<br />five of eight indigenous fishes still persist in low
<br />to moderate numbers. These are usually re-
<br />stricted to warmer habitats such as tributaries
<br />and backwaters. Although terrestrial species in
<br />GCNP adapted to the post-dam Colorado River
<br />ecosystem (Carothers and Brown 1991: 147;
<br />Johnson, 1991), indigenous fishes found it dif-
<br />ficult or impossible (Kaeding and Zimmerman,
<br />1983:592).
<br />
<br />Little Colorado River as habitat.- Temperataure
<br />and flow conditions in the LCR are similar to
<br />those of the pre-dam Colorado main stem and
<br />thus suit habitat requirements of indigenous
<br />fishes shaped over evolutionary time. Kaeding
<br />and Zimmerman (1983) argued that G. cYPha
<br />persisted within the canyon, whereas other en-
<br />demics were eliminated, because a portion of
<br />its population spawned within the LCR. They
<br />also argued that, given post-dam temperature
<br />disparities between LCR and mainstem, signif-
<br />icant reproductive success for G. cYPha hinged
<br />upon reproduction within the LCR. Thus, se-
<br />lection should be strong for development of a
<br />spawning migration (Kaeding and Zimmerman,
<br />1983). Critical though these observations are to
<br />the ecology and conservation of G. cypha, they
<br />have yet to be substantiated. Although data pre-
<br />sented herein do not address movements of G.
<br />C)Pha from the mainstem into the LCR, they do
<br />suggest that staging occurs at the confluence.
<br />Our data do demonstrate that adult G. cypha
<br />actively move up the LCR in spring (primarily
<br />to reproduce) and often remain within the LCR
<br />for long periods, possibly the entire year. These
<br />observations are based both on monthly pop-
<br />ulation estimates by reach (Fig. 3) and on sea-
<br />sonal recaptures of tagged G. cypha (Table 3).
<br />Before each of these results is discussed, how-
<br />ever, it is important to briefly review population
<br />models and their assumptions.
<br />
<br />Open vs closed population models.-Modelling of
<br />capture history is defined by the idea of popu-
<br />lation closure. An open population is one in
<br />which study organisms enter and leave (via birth,
<br />death, immigration, emigration, or ontogeny).
<br />A closed population does not change compo-
<br />sition during the course of the study (Nichols,
<br />1992). Although open populations are the norm
<br />in wildlife investigations, closed models ap-
<br />proximate the short-duration realities of nature
<br />(Skalski and Robson, 1992). In fact, Pollock
<br />(1982) recommended as an ideal survey design
<br />a sequence of intense trapping sessions each fol-
<br />lowed by a longer period of cessation of trap-
<br />ping. Data from each session would be analyzed
<br />separately using closed models (as done herein).
<br />Survival rates derived from the time-duration
<br />between trapping sessions could then serve as
<br />input for open-population models (M. E. Doug-
<br />las and P. C. Marsh, unpub!.).
<br />However, three assumptions are crucial to
<br />closed-population studies: closure is substanti-
<br />ated; organisms do not lose marks during the
<br />course of the experiment; and all marks are
<br />correctly recorded at each trapping occasion.
<br />The most critical is the first. Closure for the
<br />duration of a trapping session allows the re-
<br />sulting estimate to represent a "snapshot" of
<br />the population at a given point in space and
<br />time. In the present study, sampling each month
<br />was brief, and movements between reaches were
<br />negligible during sampling (Table 4). Thus, clo-
<br />sure both by reach/month and by month for
<br />the entire LCR is indeed supported, and the
<br />resulting population estimates appear robust.
<br />
<br />Past and present population estimates in the LCR.-
<br />Population estimates for G. C)Pha in the LCR
<br />are presented in Table 5. In May of 1992 (Ap-
<br />pendix 1), the confluence was estimated to con-
<br />tain 1320 adult G. C)Pha. This is a reduction of
<br />27% and 54%, respectively, from estimates of
<br />1800 and 2900 individuals in May of 1987 and
<br />1988 (Table 5). An estimate for the entire 14.9
<br />km length of the LCR during May of 1992 was
<br />4346 (summed estimate for the three reaches
<br />= 4602; Appendix 2). This contrasts with the
<br />estimate of 25,000 chub in 1989 (Table 5).
<br />The best-fitting population estimate for our
<br />entire 19-month study (4508 individuals; Table
<br />2) was obtained using Pollock and Otto's esti-
<br />mator (Mbh). This model is one of the most re-
<br />alistic and useful for a mark-recapture experi-
<br />ment, in that it allows for individual variance
<br />in behavioral response to capture (Otis et a!.,
<br />1978). Its estimate is larger than two average
<br />estimates for the 19-month study [i.e., 2992
<br />(monthly summed over reaches) and 2434
<br />
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