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288 N Wyoming m .~ :.. .. Q .. ....... .......... Yampa Utah ~ White o ~o0 20o A B Colorado kilometers Gunnison , Jet ` Nevada P do ............ ....... a~a ....... ........ ... ........ o\ G San Juan C New Mexico Arizona Calif. Gila 8i~er D ,.:,.. :; ti::>•.;; ~ ... ........ . ......:...... Mexico Fig. 1. The Colorado River basin, showing rivers that historical- ly provided habitat for Colorado squawfish (light lines) and those that still support the species (heavy lines). (Modified from Seethaler 1978.) Sites of temperature-data collection are in- dicated in the Green River (A) and the upper (B), middle (C) and lower (D) Colorado River. range is generally agreed to be a result of the wide- spread and often profoundly evident effects of wa- ter-resources development, the introduction of non-native fishes, and poor land-use practices (Miller 1961, Minckley & Deacon 1968, Minckley 1973), the causes of its decline to the present low levels in the remaining habitat are not so clearly evident nor well understood. Among fishes, piscivores show an especially pro- nounced potential for rapid, early-life growth. This can be seen, for example, in two widely distributed piscivores, northern pike, Esox Lucius, and large- mouth bass, Micropterus salmoides (Fig. 2). Pre- sumably amechanism that has evolved to increase survival, such rapid growth reduces the period when fish are prey for other species and, in pisci- vores, it enables the use of a wider variety of prey (e.g. Keast 1985). Rapid, early-life growth is con- spicuouslylacking in the Colorado squawfish of the upper Colorado River basin, however (Fig. 2). Growth rate in fishes is largely dependent upon the interaction of water temperature and food availability (e.g. Weatherley 1972). That this inter- action might explain the slow growth of Colorado squawfish in upper-basin rivers became evident during a study of growth and survival of young Colorado squawfish in ponds by Osmundson (1987). In one test, 5-month-old Colorado squaw- fish 50-75 mm long stocked in a pond in which age-0 common carp, Cyprinus carpio, were abun- dant grew to an average length of 226 mm within one year; the largest was 304 mm long (Fig. 2). Osmundson attributed rapid growth in the pond to summer water temperatures nearly optimal for growth, plus abundant food. Such rapid growth had never before been reported for Colorado squawfish, and its observation led us to conclude that slow growth in upper-basin rivers was attribut- able to suboptimal conditions for growth in these upstream regions of the historic range. Among widely distributed species, such an effect is well known for populations that occur at high latitude and elevation where annual growth of fish can be much slower than the potential (e.g. Weatherley 1972). We develop the hypothesis that the interaction of slow growth and increased early-life mortality is an important cause of the decline of Colorado squawfish in the upper Colorado River basin. In so doing, we use agrowth-rate versus temperature relation for Colorado squawfish to compare tem- perature regimes of historic and present habitats, and we describe the relation between our measure of temperature-regime suitability and first-year growth of Colorado squawfish in upper-basin riv- ers. Simulation is used to show how the effects of increased early-life mortality can be especially sig- nificant onpopulations of slow-growing fishes, and we recommend management efforts to bring about the recovery of Colorado squawfish in its remain- ing habitat. ,.