7860

• UCREFRP Catalog Number: 7860
• Author: Douglas, M. E. and P. C. Marsh
• Title: Population Estimates/Population Movements of
• USFW Year: 1996
• Copyright Material: YES

22

COPEIA, 1996, NO.1

DOUGLAS

TABLE 4. MOVEMENT (AS DETERMINED BY TAG/RECAPTURE) OF INDIVIDUAL Gila cypha BETWEEN THREE REACH-
ES OF THE LITTLE COLORADO RIVER DURING EACH OF 18 DIFFERENT SAMPLING TRIPS OF 1991/1991. Trip =
month/year; N = Total Number; C = Confluence; P = Powell; S = Salt; Tot-UP = Total recaptured upstream;
Tot-DN = Total recaptured downstream.

species-composition; 0
severely impacted the
are irreversible.
Indigenous fishes il
and Grand canyons \
closure of Glen Canyo
naker, 1975; Suttku
M inckley, 1991). Ma
Holden and Stalnaker,
persisted in Lake Po
reproduce (Holden, I!
the dam, the fish com
dominantly warm-wat
fishes to one domina
fishes [i.e., rainbow tl
and brown trout (Sal
broad temperature t(
five of eight indigenOl
to moderate number
stricted to warmer hi
and backwaters. Alth(
GCNP adapted to the
ecosystem (Carother:
Johnson, 1991), indig
ficult or impossible (1<
1983:592).

Trip N C-IO-P C-lo-S P-IO-S S-lo-P S-lo-C P-to-C Tot.UP TOI.DN
07/91 500 0 0 0 0 0 0 0 0
08/91" 955 2 0 0 9 3 0 2 12
08/91" 794 5 0 0 I I 3 5 5
09/91 376 2 0 2 0 I 3 4 4
10/91 255 1 0 0 0 0 I I I
11/91 254 0 0 1 0 0 0 I 0
12/91 138 0 0 0 0 I 0 0 1
TOTAL 3272 10 0 3 10 6 7 13 23
01/92 125 0 0 0 0 0 0 0 0
02/92 299 I 0 0 0 0 0 I 0
03/92" 292 0 0 0 0 0 0 0 0
03/92" 275 0 0 0 0 0 0 0 0
04/92 933 0 0 0 0 0 0 0 0
05/92 341 0 0 0 0 0 0 0 0
06/92 841 0 0 0 0 0 0 0 0
07/92 258 0 0 0 0 0 0 0 0
08/92 115 0 0 0 0 0 0 0 0
09/92 90 0 0 0 0 0 0 0 0
10/92 278 0 0 0 0 0 0 0 0
11/92 183 0 0 0 0 0 0 0 0
TOTAL 4030 0 0 0 0 0 0
. Early month sampling.
b Lale monlh sampling.

Table 4). In 1992, only one of 4030 fishes was
recaptured during the same trip in a reach up-
stream from their initial capture, whereas none
was recaptured in downstream reaches (i.e., n
= 1; 0.0003% of total; Table 4).

DISCUSSION

Colorado River as habitat.-During historic times,
temperature and flow regimes of the Colorado
River fluctuated greatly; seasonal flooding
transported heavy sediment loads whereas low
waters carried vast amounts of dissolved salts to
the Sea of Cortez (Carlson and Muth, 1989). In
flood, the Colorado was a wild, swift, turbulent
river, the result of extreme flow, a channel con-
strained for most of its length by steep cliffs,
and a 3700 m drop in altitude from headwaters
to sea (Fradkin, 1984).

Dams and impoundments.- Dam construction and
chronic dewatering for agriculture and urban
development precipitated major changes in the
Colorado River ecosystem. Temperature and
flow regimes as well as salt and sediment loads
of the river are now greatly ameliorated. The
2400 km of riverine habitat suitable for large-

river fishes has been reduced to 965 km (Miller,
1982).
Those sections of the Colorado River that
were converted into lakes Mead and Mohave
(following closure of Hoover and Davis dams in
1935 and 1954, respectively) clearly possessed
the river's unique fish fauna, to include G. cYPha
(Miller, 1955). These fishes [except for relictual
bony tail chub (Gila elegans) and razorback suck-
er (Xyrauchen texanus)J are now extirpated (see
also Minckley, 1983). They were also eliminated
from the Green River above the mouth of the
Yampa River when Flaming Gorge Dam be-
came operational in 1962 (Vanicek et aI., 1970;
Fig. 2A).

Little Colorado River w
and flow conditions i
those of the pre-dam
thus suit habitat req
fishes shaped over ev
and Zimmerman (19
persisted within the c
demics were eliminat
its population spawn!
also argued that, giv1
disparities between L
icant reproductive su
upon reproduction \I
lectionshould be sm
spawning migration (J
1983). Critical thougl
the ecology and cons
have yet to be subs tan
sented herein do not
cYPha from the mainsl
suggest that staging
Our data do demon
actively move up the
to reproduce) and oft
for long periods, poss
observations are bas,
ulation estimates by
sonal recaptures of t
Before each of these
ever, it is important t<
models and their assl

Glen Can)'on Dam.- The operation of Glen Can-
yon Dam precipitated major changes in the
Marble/Grand Canyon ecosystem of the Col-
orado River (Marzolf, 1991 :33). Some occurred
immediately upon closure of Lake Powell in
1963 (e.g., decreased water temperatures; re-
duced sediment loads; diminished salinity; al-
teration of flow regimes). Others developed over
a much longer time frame (e.g., geomorphic
adjustment of channel; secondary succession of
terrestrial vegetation; modification of aquatic