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404
<br />PERSONS AND BULKLEY
<br />ervoir filled to maximum elevation (1,128 m
<br />above mean sea level) for the first time during
<br />1980.
<br />During the 1981 sampling period, the reser-
<br />voir level was about 1,122 m and the river flow
<br />averaged less than 380 m3/sec. As a result of the
<br />lower reservoir level and reduced river flow, the
<br />mixing zone was greatly altered: mud and silt
<br />bars that formed during 1980 were exposed in
<br />1981; the river channel was narrower and shal-
<br />lower, current velocity was greatly reduced; and
<br />there were fewer and smaller side eddies (Persons
<br />et at. 1981).
<br />The second sampling site, Spanish Bottom
<br />(river km 343), lies above the upper rapid of
<br />Cataract Canyon and just below the confluence
<br />of the Green and Colorado rivers. The river is
<br />narrow and relatively straight for 5.6 km from
<br />the confluence to Spanish Bottom. Cataract Can-
<br />yon consists of a series of relatively short rapids.
<br />At Rapid 23 or Big Drop, the most severe in the
<br />canyon, the rate of fall is 7 m/km (Baars and
<br />Molenaar 1971).
<br />METHODS
<br />Fish were collected at Gypsum Canyon during
<br />the period April 26-June 18, 1980 and May 24-
<br />July 22, 1981 with multifilament nylon gill nets
<br />31 in long x 3 m deep and with 6.4-cm mesh
<br />(bar measure). One end of the net was attached
<br />by rope to the shore and the net was fished in
<br />water 1-15 m deep. Nets were checked every 4-
<br />5 hours during 1980 and every 0.5-3 hours in
<br />1981. Catch per gill-net hour is reported in terms
<br />of ore 31-,,. net. Nets also we.;, fisl.ed near Span-
<br />ish Bottom on May 7-21, 1980, and on May 26-
<br />June 2, 1981, to determine if striped bass moved
<br />upriver through the Cataract Canyon rapids.
<br />All striped bass captured were weighed and,
<br />measured (total length). Stomachs of most adult
<br />striped bass collected in 1980 were preserved in
<br />10% buffered formalin. Stomachs were examined
<br />in the field during 1981. Gonads also were exam-
<br />ined to determine stage of maturity and were
<br />preserved for later study when maturity was
<br />questionable. Gonads were classified as imma-
<br />ture, maturing, mature, or spent-based partly
<br />on work by Scruggs (1957) and Persons et al.
<br />(1981).
<br />Other workers have effectively collected eggs
<br />in plankton nets to determine striped bass repro-
<br />ductive success (Johnson and Koo 1975; Turner
<br />1976; Combs 1979). Plankton nets (0.5-m diam-
<br />eter mouth, 0.565-mm mesh) were fished daily
<br />in the main river channel at bottom, middle, and
<br />sub-surface depths for 3-5 minutes at each depth.
<br />Nets became clogged with debris if towed for a
<br />longer period. Tows also were made in quiet water
<br />and slow back eddies near Gypsum Canyon. Ich-
<br />thyoplankton samples were preserved in 5-8%
<br />buffered formalin containing rose bengal dye for
<br />later examination for the presence of fish eggs or
<br />larvae.
<br />River discharge in the sampling area was esti-
<br />mated to be the sum of the discharge of Green
<br />River measured at Green River, Utah, 188 km
<br />upstream from the confluence, and the discharge
<br />of the Colorado River measured at Cisco, Utah,
<br />156 km upstream from the confluence (Unpub-
<br />lished data from the U.S. Geological Survey, 1980
<br />and 1981). Surface temperature was recorded
<br />daily at the sampling sites.
<br />RESULTS
<br />During the 1980 sampling season, the river
<br />discharge ranged from 680 to 1,800 (average,
<br />1,280) m'/sec. Discharge during the same period
<br />in 1981 ranged from 200 to 650 (average 380)
<br />m-1/sec (Fig. 1).
<br />Water Temperatures
<br />Surface water temperature at Gypsum Canyon
<br />varied from 12.8 to 17.2 C during the 1980 sam-
<br />pling period (Fig. 1), and was within the range
<br />of temperatures normally present during striped
<br />bass spawning (16-18 C) during May 54, May
<br />20-24, and after June 9, 1980. Water tempera-
<br />tuue at Gypsum Canyon ...IS -W-Ilth::c or above he
<br />range suitable for spawning throughout the 1981
<br />sampling period. Temperatures at Spanish Bot-
<br />tom were suitable for spawning during the 1981
<br />sampling period but not during that of 1980.
<br />Catches
<br />Common carp (Cyprinus carpio), striped bass,
<br />walleyes (Stizostedion vitreum), and channel cat-
<br />fish (Ictalurus punctatus) were the most fre-
<br />quently captured species during about 8,200 hours
<br />of sampling with gill nets in 1980 and 1981 (Per-
<br />sons et al. 1981). The striped bass was the most-
<br />frequently collected species in 1980; common
<br />carp and walleyes were most abundant in 1981.
<br />Specimens of two endangered species were cap-
<br />tured: 6 razorback suckers in 1980 and 10 in
<br />1981; 45 Colorado squawfish in 1980 and 6 in
<br />1981.
<br />GYPS
<br />U 25-
<br />Z_ SPAN
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<br />20
<br />w
<br />g 15?..
<br />W
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<br />APRIL
<br />Figure 1. F
<br />.water temp
<br />1981.
<br />Catches of
<br />the same ge
<br />temperature
<br />early May, a
<br />mid-June (F
<br />two peaks, o
<br />June. Other
<br />four peaks
<br />fated with
<br />levels (Hare
<br />Peak strip
<br />0.2 fish per
<br />per unit of
<br />1980 (Fig. 2
<br />during 1981
<br />ing populati?
<br />of an additi
<br />Records ind:
<br />brood stock
<br />higher in 19
<br />1981).
<br />Although
<br />1980, and N
<br />Bottom, no
<br />bass eggs wr
<br />striped bass
<br />Cataract Ca;
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