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y -...~!~rJ. t78.tS v~!~n~e Mf1~qdilOria1 Limusa, S. A. Arcos de Belen I'>. 1.1.. . I'W:::rJ. STEBBINS. R. C. 1958. A new alhgalOr hzard from the Panamint Mountains In\' C '- ').~ fornia. Amer. Mus. Novitales (1883): 1-27. . . () '()Un".l- TIHEN. J. A. 1948. A new Gerrhonotus from San Luis POlosi. Trans. Kans' 51 :302-305. . as .~, d'; 1949. The genera of gerrhonoline lizards. Amer. Midland Nal. 41:580-601. WADDICK, J. W. AND H. M. SMITH. 1974. The significance of scale characlers in Ih. . of the lizard g-enera GrrrhOllotlLS. Elf{aria. a~d Bori.sza. Greal Basin Nal 3.1JV~ ~'\"alu..;." '-- . "t...Ji-2hfl Address of autpors: Dept. of Biology, Sui Ross Stale Un,,'. Alpine. TX 79832 , ;;;SOUTHWESTERN NATURA,LIST jU\Ij::n-Iuv , ::;--- / ((>-) /7f~ I PREFERRED TEMPERATURE OF YEARLING COLORADO SQUAWFISH TAMARA BLACK AND Ross V. BULKLEY . ItIIfU'cr.- Tem~ralure preference of yearling Colorado squawfish, Ptychocheilus lucius, was ~ ..,-i~ in a honzontal gradIent trough. FIsh were acchmated to 14, 20, and 26 C, and twenty ~ ... ~ lested from each acclimation temperature. Acute preferenda were 21.9, 27.6, and 23.7 C .14.!O. and 26 C -acclimated fish, respectively. Final preferendum was estimated as 25 C. The Colorado squaw fish (Pytochocheilus Lucius), endemic to the Colorado I,J\Tr drainage, is the largest native minnow in North America. Historically , was found throughoUl the Colorado River basin, primarily in the main- .-am channels of the Colorado and Green rivers and in the large tribUlar- ., such as the Gila, San Juan, Gunnison, and Yampa (Behnke and Benson, MIOl. Large dams, built in the 1930's through the 1960's, have presumably ~ a rapid decline in Colorado squawfish abundance and distribution to _ point it is now classified as an endangered species. The species is no ~er found in the lower Colorado River basin, but is restricted to areas of .. upper basin where detrimental eHects of dams and of land- and water- __ practices are less. Changes in the annual water temperature pattern imposed by these dams ".1' cited as one possible cause for decline of this species. Temperature is JttOgTIized as an important factor in the life history of all fishes (Fry, 1947; a.rtholomew, 1977). Brett (1971) considered temperature as the "ecological Imler factor" among the abiotic environmental factors. Giatinna and Gar- !I:lll (1982) emphasized the importance of determining preferred temperatures d \ish, not just lethal temperatures. Preferred temperatures should reHect tilrrmal optima for certain biological processes. These processes may be llmctional over the entire range of normal habitat temperatures for a species, Dot would function best at or near the preferred temperature of that species. for a particular fish species, reduction of desirable habitat could occur iuough changes that effectively reduce the area of water having tempera- ~ within the zone of thermal preference. The objectives of this study were to determine acute temperature preferen- m of yearling Colorado squawfish, the influence of recent thermal history lIII preference, and to estimate first thermal preferendum for this life stage. Fish presented with a suitable range of temperatures in a restricted space "ill tend to congregate in, or spend most of the time in, a relatively narrow Q8gt' of temperatures (Fry, 1947). Such behavior is termed temperature pref- tItn~ or behavioral thermoregulation; and the temperature in which the Olngregation occurs, or in which the most time is spent, is the preferred -perature or thermal preferendum (Reynolds and Casterlin, 1979). ~nding on the length of time spent in a thermal gradient. the preferred -perature.might or might not be inHuenced by the recent thermal history . the fish. Acute temperature preference is usually determined within two /('I/V1frv/J (.-c . ~ 8~~ Ef/lk)CJ/ '\ Jq~. r.--. '-.J"'; C' .!:- ~.7\ ,..s::::, n. ~- \f\ i.\J