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TEMPERATURE REQUIREMENTS OF RAZORBACK SUCKERS 605 ~ t to 14 C is that cold-acclimated (< 15 C) fish initially may overshoot their final preferendum, Other workers have reported that by the third or fourth day of testing the final preferendum of cold-acclimated fish equals that of warm-ac- climated fish, and the effect of overshoot is no longer evident (Beitinger and Magnuson 1976; Reynolds 1978), Our razorback suckers were in the shuttlebox for only 2 days, and the pref- erence for high temperature shown by fish ac- climated to 14 C might have proven transient had the tests continued. The estimate of final preferendum of razorback suckers acclimated to 8 C was also higher than preferenda of fish acclimated to 20 C and 26 C (Fig, 1), suggesting overshoot, but the difference was not statisti- cally significant, perhaps due to the small sam- ple size of fish acclimated to 8 C. Mathur et at (1981) found strong geographic and taxonomic similarities in acute temperature preferences for several freshwater fishes, Our work with the razorback sucker suggests that these similarities may hold for the final pref- erence temperature as well. Reynolds and Cas- terlin (1978) reported that the final preferen- dum for the white sucker Catastamus commersani was 24.1 C. This value falls within the 94% con- fidence limits of estimates for the razorback sucker based on fish acclimated at 8, 20, and 26 C. In other work in our laboratory, Bulkley et al. (1981) estimated the final preferenda of three other native Colorado River fishes from their acute temperature preference (see Fry 1947; Richards et al. 1977; or Stauffer et al. 1975 for calculation procedures); their estimates agreed closely with the 24,8 C geometric-mean esti- mate for the final preferendum of the razorback sucker: Colorado squawfish ptychacheilus lucius, 25.4 C; humpback chub Gila cYPha, 24.0; bony- tail Gila elegans, 24,2 C. Coutant (1975), who surveyed the literature on temperature avoidance and preference of different species of fish, concluded that upper avoidance temperatures appeared to be about 2 C above the final thermal preferenda for most species, Data on the white sucker collected by Reynolds and Casterlin (1978) agreed with Cou- tant's observations, Upper avoidance temper- ature for the white sucker was 2.6 C higher than its preferred temperature, In contrast, esti- mates of the upper avoidance temperature of razorback suckers in our study averaged 3,7 C (range, 2.4-5.3 C) above the estimated final pre- ~ .... ferendum. The reason for this greater differ- ence between upper avoidance and final pref- erence temperature for razorback suckers is attributed to our method of estimating avoid- ance. Had we estimated upper avoidance tem- perature during the second day of testing, rath- er than the first, its deviation from the final preferendum would agree with Coutant's gen- erality because of the much narrower range of temperatures experienced by razorback suckers as they closed in on their final preferendum, Hence, our more liberal estimates of upper and lower avoidance should fully encompass the temperature range tolerated by adult razorback suckers in nature, The behavior of razorback suckers acclimat- ed to 8 and 14 C was similar to that of other species with prior exposure to low temperatures (Mathur et al. 1981). Many of the razorback suckers held at low temperatures sought in- creasingly higher temperatures until they died, or at least they did not avoid lethally high tem- peratures. That is, their upper avoidance tem- perature exceeded the critical thermal maxi- mum as described by Reynolds and Casterlin (1979). Other fish were responsive but less pre- cise than fish acclimated to a higher tempera- ture and overshot the final preferendum, A physiological explanation for this fatal behavior is that temperature reduction slows down met- abolic rate, including rate of cerebral activity of fish (Stascheit 1979), Acclimation rate for behavior obviously differs from that of temper- ature tolerance, Threadfin shad Darasama pete- nense exposed to low temperatures lost orien- tation and became insensitive to external stimuli (Griffith 1978). Doudoroff (1942) described such behavior as the primary chill coma. Humans suffering from hypothermia similarly lose men- tal acuity and ability to respond appropriately to external stimuli (Guyton 1966). The lower avoidance temperature and ther- mal preferenda presented here support state- ments that temperature change was involved in disappearance of razorback suckers and the three other native endangered species from the tail waters of Flaming Gorge Reservoir on the Green River, Utah (Holden 1978; Behnke and Benson 1980; McAda and Wydoski 1980), After construction of the dam in 1963, mean summer Qune-September) water temperature in the riv- er below the dam dropped from about 18 C to 6.8 C (Vanicek and Kramer 1969). Annual post-