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294 <br />theoretical populations to bear additional mortal- <br />ityyet remain serf-sustaining. As such, the capacity <br />of the slow-growing population experiencing 99% <br />early-life mortality is quite low. <br />Synthesis <br />The decline of Colorado squawfish and that of <br />other native fishes of the southwestern U.S. has <br />been attributed to alteration of discharge and tem- <br />perature regimes downstream from dams and di- <br />versions, conversion of riverine ecosystems to la- <br />custrine ones in the reservoirs upstream, introduc- <br />tion of non-native fishes, and altered water quality <br />(Miller 1961, Minckley & Deacon 1968, Minckley <br />1973, Holden & Wick 1982). However, aside from <br />the obvious detrimental effect of severe reduction <br />in river discharge on Colorado squawfish, only the <br />effect of unseasonably cold, hypolimnetic waters <br />from dams has been demonstrated. Marsh (1985) <br />showed that survival of Colorado squawfish em- <br />bryos is appreciably reduced by low water temper- <br />atures such as those evident below mainstream <br />Colorado River dams. However, this effect alone <br />does not account for the Colorado squawfish de- <br />cline to the present low numbers in its remaining <br />habitat. Recent analyses of USGS records showed <br />that dam operation did not reduce temperatures in <br />most Colorado and Green river reaches still inhab- <br />ited by Colorado squawfish (Robert Green, un- <br />publisheddata). Nonetheless, the species is rare in <br />these areas, and factors other than alteration of <br />temperature regimes must therefore have brought <br />about the presumed dramatic reduction of the <br />stock in these river reaches. Our analyses are useful <br />for developing the hypothesis that the interaction <br />of slow growth and increased early-life mortality is <br />an important cause of the decline of Colorado <br />squawfish in the numerous upper-basin river <br />reaches whose temperature regimes have not been <br />importantly affected by the operation of upstream <br />dams. <br />In their review of literature on Colorado squaw- <br />fish, Behnke & Benson (1983) noted the slow <br />growth of Colorado squawfish in upper-basin riv- <br />ers and speculated that it was a recent phenom- <br />enon. They observed that the largest Colorado <br />squawfish found today weigh about 7 kg, whereas <br />early in this century squawfish weighing more than <br />20 kg apparently were not uncommon. Behnke & <br />Benson (1983) hypothesized that a replacement of <br />large, native prey fishes by introduced species that <br />attain only small body size had caused a decline in <br />Colorado squawfish food availability and growth <br />rate. But fish need not grow rapidly to attain large <br />size; they might also be slow-growing but long- <br />lived. Moreover, to us Behnke & Benson's hy- <br />pothesis seemed an unlikely explanation because <br />prey species of a variety of sizes are abundant in <br />upper-basin rivers (Holden & Stalnaker 1975, Tyus <br />et al. 1982), although their actual availability to <br />Colorado squawfish is unknown. <br />We believe a more plausible explanation is that <br />the slow growth of Colorado squawfish in the up- <br />per basin is both historic and the result of sub- <br />optimal conditions for growth in these upstream <br />regions of historic range. Temperature regimes in <br />the former middle and lower Colorado River were <br />more favorable to Colorado squawfish growth than <br />are those of its present habitat (Fig. 3), although <br />the availability of food and the rate of growth in <br />these former habitats is unknown because squaw- <br />fish were eliminated from these areas before such <br />life-history information could be collected. None- <br />theless, as judged by the growth rate of Colorado <br />squawfish in Osmundson's (1987) pond (Svhich had <br />abundant food and a temperature regime with suit- <br />ability of 3.6) it is reasonable to assume that annual <br />growth in these former downstream areas was rap- <br />id. <br />We believe that in the historic Colorado River <br />basin there was a marked longitudinal effect on <br />growth of Colorado squawfish, most important on <br />that of the age-0 fish. This belief is supported by the <br />strong relation between size of age-0 Colorado <br />squawfish in fall inupper-basin rivers and our mea- <br />sure of the relative suitability of the temperature <br />regime for growth (Fig. 4). The actual cause of this <br />relation might include a direct effect of temper- <br />ature on Colorado squawfish metabolism, on the <br />production of food organisms, on the time of <br />spawning and length of the subsequent first-year <br />growing season, or perhaps a combination of these <br />~l <br />