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~- ~~ K ~q~5 bCJ TIsMYERATURE AS A RESOURCE LIMITING COLORADO SQUAWFISH IN THE UPPER COLORADO RIVER BASIN Lynn R. Kaeding United States Fish & Wildlife Service Colorado River Fishery Project 551 25-1/2 Road Grand Junction, Colorado 81505 Douglas B. Osmundson and Charles R. Berry, Jr. Utah Cooperative Fish & Wildlife Research Unit Department of Fistxeries and Wildlife Utah State University Logan, Utah 84322 Introduction Early records suggested Colorado squawfish PtyChocheilus lucius were formerly abundant throughout the main rivers and most. of the major tributaries of the Colorado River basin (Jordan 1891, Evermann and Rutter 1895, Gilbert and Scofield 1898, Jordan and Evermann x896); however, they occur today only in the upper basin where they are threatened with extinction (Tyus et al. 1982, Holden and Wick 198'L) (Figure 1). The existence of Colorado squawfish, as well as that of numerous other Southwestern fishes, is jeopardized by development of water resources and other activities of man (Miller 1961, Minckley and Deacon 1968, Behnke and Benson 1983, Ono et al. 1983). Agencies charged with management of endangered species must: determine wliy the Colorado squawfish is nearing extinction and txow this trend might be reversed. The factors limiting the size of the Colorado squawfish population ,are not well understood, however (Holden and Wick 1982, Behnke and Benson 1983). In the present report, we argue that the short growing season of the upper basin results in slow growth and late age of first reproduction of Colorado squawfish. We then demonstrate that a consequence of these life--history characteristics is greatly reduced potential for population growth. Temperature, as it affects length of growing season, should therefore be considered a limiting resource when programs intended to maintain or increase the Colorado squawfish population of the upper Colorado River basin are developed. Colorado Squawfish of the Upper Basin Colorado squawfish is the major native piscavore of the Colorado River basin (Minckley 1973). Growth rates were similar for Colorado squawfish near the confluence of the Green and Yampa rivers and from the Colorado River near Grand Junction, Colorado (Vanic:ek and Kramer 1969, Seethaler 1978) (Figure `l). Our observations on the growth of age-0 Colorado squawfish in the upper Colorado, and on the growth of recaptured adult-size fish there, corroborate the growth rates reported by these earlier workers (United States Fish and Wildlife Service, unpublished data). 147