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increase is real, as we suspect, it suggests an increase in food availability as Colorado squawfish reach this size. Increased gape size, or more effective foraging and handling abilities may allow utilization of larger and more abundant prey. While there is much variation in growth among individual fish, it appears that in successive years most fish do not grow consistently faster or slower than the average and average growth rates do not differ greatly among years. Nevertheless, there can be large differences among individuals in time it takes to reach a given length: there was a 20-year range of potential ages for a given length for fish > 800 mm TL. Although length is therefore a rather imprecise indicator of age, we can conclude that the largest fish present in the population are quite old. For individuals 900 mm long, 34 years might be a minimum age, but an average age would likely be 47-55 years (Fig. 3 a and b). These fish are very rare: of approximately 1,080 subadult and adult Colorado squawfish captured from the Colorado River during 1979-1995 only three have been > 900 mm TL (0.28%) and two of these were < 905 mm TL. The largest, captured in 1990 by UDWR biologists, was 960 mm TL, considerably larger than any other reported from upper basin rivers (McAda et al. 1994). There are numerous anecdotal accounts of Colorado squawfish historically reaching lengths of 1,200-1,800 mm TL (e.g., Quarterone 1993), far larger than any found during the past 20 years. Two explanations for the disappearance of such large individuals have been proposed: 1) growth rate has declined (Behnke and Benson 1983), and 2) survival rate has declined (Gilpin 1993). Behnke and Benson (1983) suggested that extirpation of the bonytail (Gila elegans) impacted the food supply of Colorado squawfish. Gilpin (1993) showed, through simulations, that average and maximum fish size would significantly increase if adult survival rate was increased to 0.95, and suggested that rates may have declined due to angling mortality. However, neither hypothesis adequately explains the disappearance of very large individuals. Kaeding and Osmundson (1988) concluded that slow growth in the upper basin was the historic norm because temperature regimes of mainstem rivers have, with few exceptions, remain unchanged and potential foods, Gila spp. and native suckers, remain plentiful. Also, the size structure of Gilpin's simulated population with a survival rate of 0.95 indicated that the largest individuals would still be < 1,000 mm long. Though the preceding two explanations may explain in part the reduction in average and maximum size, we offer a third hypothesis: very large fish may have attained their size while residing in the Lower Colorado River Basin (downstream of Lee's Ferry) and moved upstream later in life where they were eventually captured. Kaeding and Osmundson (1988) demonstrated that longer growing seasons and warmer temperatures in the lower basin during historic times would have provided 1.5-2.3 times the annual thermal units for growth than that in upper basin reaches. Additionally, Colorado squawfish are capable of making long-distance movements; radio-tagged adults have been documented traversing the entire length of their current range in the upper Colorado River (313 km) in less than 3 months (McAda and A-17