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... '- 98 The Southwestern Naturalist vol. 30, nil ! 14.C 26.C i x 30 I 30 I~ ~ I . 20 20 0"- I )0- 10 10 u Z W => 0 0 0 15 20 W 25 30 a:: \.L. W 20.C "*' > 30 l- e::{ 20 ...J UJ a:: 10 0 15 20 25 30 TEMPERATURE oc FIG. 2.-Relative frequency distribution o( temperatures selected bv yearling Colorado sqll" Iish. Data pooled (or third hour. one observation per minute [or (ish acclimated to 140. 20'. M' 260C. Fi!(ures above each hlSlogram represent the range o( individual modes and the mo"" . individual modes:i 2 standards errors (darkended box) and:i I standard deviation (clear box). Means of individual modes for 14c, 200, and 260C-acclimated juvenik. were 21.90, 27.60, and 23.70C, respectivel~' (Fig. 2). Al\ other measures of [(I. tral tendency agreed c1osel~' due to little skewness in the data (Table 1). Acute preferenda of fish acclimated to different temperatures w<;re (011' pared by analysis of variance on the individual modes. Acute prefereno. were significantly different (P = 0.01). Preferenda at different acclimalioi temperatures were further analyzed with Fisher's LSD test. Preferenda a', each acclimation temperature were significantly different from each other (I' = 0.01 or 0.05); 20"C-acclimated fish selected the highest temperattW (27.60C), followed by 260C-acclimated fish (23.70C) and l4OC-acclimated [i,! (21.90C). TABLE 1.- Temperature distribution statistics of yearling Colorado squawfish. Acc1ima.lion R2n~ of Pooled 0310 TemperaLUre lndi,,"adual - oc Mooes Mean of Mode~' Mode Mean SD Skt'wn~\ -- 14 18 17.27 21.90 :i 0.74 210 22.10 2.97 0.4 20 20 24-32 27.60 :i 0.44 280 27.40 2.46 -{),3 26 19 21-31 23.70 :i 0.64 230 24.10 3.46 O.~ - , . :t 'lO!>danl error 01 .be mean I a. Prarsan's cotHici~n1 of skew-ReS" = (mean _ model/standard drviallon. A negalive value indicues a longer tail on ,tIC' \l- t.emptralue end of the distribution curve. JlIICI11985 Black and Bulkley-Preterrea Jempt"liUUIC Vl ..;:l''''i'"u..............u () o 26 UJ 0: ::> ~ 0: UJ D- :E w t- O UJ 0: 0: W u. W 0: D- 20 14 14 20 26 ACCLIMATION TEMPERATURE oC FIG. 3.-Estimation method (or Iinal preferendum o( yearling Colorado squaw(ish. Fmal preferendum of juvenile Colorado squawfish was visually estimated lrom a plot of preferred temperature against acclimation temperature lllichards et aI., 1977). The final preferendum, that point at which the pre- imed temperature equals the acclimation temperature, was estimated at 25 C(Fig.3). Other endemic fishes of the Colorado River drainage have been tested for Ihtnnal preferences, and had acute and final preferences similar to those o[ 'Karling Colorado squawfish (Bulkley et aI., 1981). Estimates o[ final perfer- mda of adult Colorado squawfish, razorback suckers (Xyrauchen texanus), and humpback chubs (Gila cypha), were 250C, 23-240C and 240C, respec- u\'ely. Based on their laboratory thermal preferences, these fishes would avoid cold tailwaters of a reservoir. This avoidance probably explains at Gst in part the disappearance of native species from the Green River below flaming Gorge Reservoir. Since the closure of Flaming Gorge Dam, native Colorado fishes have not been found from the dam downstream to the con- nllenee of the warmer Yampa River (Holden, 1978; McAda and Wydoski, 1980). The decrease from historical summer water temperatures throughout -uch of the Colorado drainage appears to have effectively reduced the habi- III that meets the thermal requirements of [ish endemic to the drainage such -lbe Colorado squawfish. ~nk Richard Pimental (or care o( laboratory (ish and assistance in construction o( test _ Funds (or this project were provided by U.S. Fish and Wildli(e Service ConITact 14-16- ~06I.-A2. The project was administered by the Utah Cooperative Fishery Research Unit, tltU IS J~mtly sponsored by the Utah Division o( Wildli(e Resources, Utah State Univenity, and '; .s. FISh and Wildli(e Service.