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<br />
<br />ember-October 1979-1981; C = fish
<br />
<br />B[OLOGY OF YOUNG COLORADO SQUA WFISH
<br />
<br />8
<br />
<br />Main channel Side channel
<br />C E C E
<br />0.6 II 0.0 9
<br />0.0 I 3.0 [I
<br />0.3 6 1.5 10
<br />
<br />waters in which the fish were absent and present.
<br />respectively. Backwater sizes in the lower Green
<br />River were smaller, and no size selection was not-
<br />ed (Table 4). We detected no relationship between
<br />the presence or absence of young and backwater
<br />substrate (Table 4). There were no strong corre-
<br />lations (r 2 < 0.1) among the temperature, depth,
<br />or size of backwaters.
<br />
<br />I River (strata A and B) and in
<br />r river (strata E and F).
<br />.Iorado squawfish occupied back-
<br />warm, deep (t-tests, P < 0.05),
<br />:sts, P < 0.0 I, Table 4). Back-
<br />>rado squawfish averaged about
<br />armer than the a verage of those
<br />es (Table 4).
<br />wfish were usually captured in
<br />""S (mean depth, 38 cm), but the
<br />mdant in larger backwaters only
<br />n River (t-test, P = 0.05), where
<br />: 269 m2 and 1,158 m2 for back-
<br />
<br />Growth and Survival
<br />
<br />Age-O Colorado squawfish averaged about 40
<br />mm in total length in October (range, 29.0-47.3
<br />mm). Average total lengths were similar in the
<br />upper and lower Green River (upper: mean = 39.7
<br />mm, range = 29-46.3 mm; lower: mean = 39.9
<br />mm, range = 30-47.3 mm; Table 5), although the
<br />fish were larger in the upper area in four of the
<br />nine years (t-tests, P < 0.0 I; Table 5).
<br />The relative abundance (log,.fish/lOO m2) of
<br />age-O Colorado squawfish captured in October was
<br />inversely correlated with late-summer flows in the
<br />upper (r = -0.80, P < 0.01 for strata 0, E, F)
<br />and the lower (r = -0.86, P < 0.0 I for strata A,
<br />B, C) Green River (Figure 3). Total length was
<br />also inversely correlated with late-summer flows,
<br />particularly in the upper Green River (r = -0.81,
<br />P < 0.01 for upper; r = -0.57, P = 0.11 for lower;
<br />Figure 3).
<br />Spring catches of age-O Colorado squawfish
<br />(range, 0-29 fish/lOa m2) were greater than au-
<br />tumn catches for four of eight comparisons. They
<br />ranged from 0.2 to 2. I times higher than autumn
<br />catches in the lower Green River and from 0.0 to
<br />9.2 times higher in the upper river. Age-I Colo-
<br />rado squawfish caught in spring were larger (mean,
<br />45.2 mm, N = I, 193) than the same cohort cap-
<br />tured the previous autumn (mean, 41.5 mm, N =
<br />1,2 I 2). When fish were partitioned into upper and
<br />
<br />of Colorado squawfish in the Green
<br />ackwaters. The study area in lower
<br />in upper Green River ranged from
<br />:rences between means for species-
<br />'.01**; NS = not significant.
<br />
<br />:s present
<br />
<br />SO
<br />
<br />N
<br />
<br />Test
<br />
<br />4.4
<br />26.5
<br />622.6
<br />
<br />
<br />[ [8
<br />147
<br />49
<br />
<br />(*
<br />
<br />1**
<br />
<br />I. NS
<br />G**
<br />
<br />4
<br />15
<br />38
<br />
<br />G,NS
<br />
<br />126
<br />21
<br />I
<br />I
<br />
<br />lower Green River groups, the mean totallengt
<br />of fish in March and April (upper: mean = 45.
<br />mm. N = 191; lower: mean = 45.0 mm, No
<br />1.002) were greater than those of fish sampled i
<br />the same areas the previous October (upper, mea!
<br />= 39.8 mm, N = 137; lower, mean = 43.2 mm
<br />N = 1,075) in five of six comparisons (t-tests, j
<br />< 0.05).
<br />
<br />Discussion
<br />
<br />Distribution and Hatching
<br />
<br />Larval Colorado squawfish dispersed down.
<br />stream soon after hatching and occupied ephem-
<br />eral shoreline embayments (backwaters) by mid-
<br />summer. Our drift-net and seine samples in 1987
<br />illustrated this downstream transport from the
<br />Yampa River spawning site. We did not capture
<br />larvae in the lower Yampa and upper Green rivers
<br />between June 29 and July 3, 1987, but they were
<br />present from the Yampa River spawning site
<br />downstream to RK 362 on the Green River from
<br />July 13 to 24. Drift-net catch at the mouth of the
<br />Yampa River showed that peak larval abundance
<br />occurred July 17, when the mean back-calculated
<br />age of the larvae was II d (average TL, 9.0 mm).
<br />If we assume that the fish hatched at the midpoint
<br />of the spawning area (RK 29), and that the fry
<br />emerged about 5 d after hatching (Hamman 1981),
<br />the larvae drifted 29 km in approximately 6 d. By
<br />mid-August, age-O postlarvae were not captured
<br />in the Yampa River, but they were taken imme-
<br />diately below, in the Green River.
<br />[n October, postlarval Colorado squawfish were
<br />rare in the Green River for about 80 km below
<br />the Yampa River confluence. Rapid downstream
<br />transport was documented by Nesler et al. (I988),
<br />who noted that young Colorado squawfish were
<br />transported downstream and out of the Yampa
<br />
<br />TABLE 5.-Mean total lengths ofage-O Colorado squawfish in the lower and upper Green River in October 1979-
<br />1985 and 1987-1988. N = number of fish. Asterisks denote significance at P s 0.0 I"; NS = not significant (P >
<br />0.05).
<br />
<br /> Lower Green River Upper Green River
<br />(** Total length (mm) Total length (mm)
<br />(**
<br />,.* Year Mean SO N Mean SO N I-test
<br />0-* [979 44.5 7.02 639 47.3 7.07 267 **
<br /> 1980 36.9 7.02 810 43.1 8.47 424 **
<br /> 1981 46.3 8.34 47 45.2 9.33 267 NS
<br /> 1982 33.2 4.52 127 35.7 5.03 398 **
<br />G,NS 1983 29.0 5.33 27 32.8 6.50 4 NS
<br /> 1984 43.0 11.31 2 30.0 4.76 23 NS
<br /> 1985 41.4 7.54 67 35.5 6.06 59 **
<br /> 1987 43.6 6.35 [55 43.1 7.17 74 NS
<br /> 1988 41.3 6.97 1,329 45.0 8.65 288
<br />
<br />4.6 100
<br />25.3 128
<br />638.4 30
<br />
<br />o
<br />7
<br />14
<br />
<br />
<br />86
<br />43
<br />o
<br />o
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