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, GROWTH AND SURVIVAL OF COLORADO SQUAWFISH 695 TABLE 3.-Survival rates of Colorado squawfish 550 rom total length and longer for which stable length distri- bution did not differ from measured distribution (Kolmo- gorov-Smirnov one-sample test, P > 0.05). Year N Survival rate 1982 48 0.82-0.92 1990 15 0.60-0.91 1991 34 0.82-0.94 1992 41 0.83-0.87 1993 49 0.74-0.86 1994 34 0.74-0.91 1995 44 0.75-0.88 1991-1994 158 0.83-0.87 All years 265 0.82-0.87 in the Green and Yampa rivers, and Hawkins (1992) reported 1O-l5 mm for fish longer than 500 mm TL from various rivers. Direct comparisons are difficult because annual increments vary with fish size and because the proportion of earlier sam- ples consisting of faster-growing small adults is unknown. However, when we pooled increments from all our fish 450 mm TL and longer (N = 121), mean annual growth was 14.4 mm (SE = 1.2 mm); for fish 500 mm and longer (N = 104), mean annual growth was 11.9 mm (SE = 1.0 mm). Thus, estimates from earlier studies that used Car- lin-tagged fish were similar to those that used PIT- tagged fish, and suspicions of negative effects on growth from dangler tags seem unfounded. An av- erage annual growth increment of 30 mm esti- mated from back calculation of scale radii (Haw- kins 1992) appears too high, except for fish less than 500 mm, suggesting an upper size limit for using scales to estimate growth of Colorado squawfish. It is important to note that our estimates of growth rates of adults were mostly made from fish living in the upper reach; adults that remain in the lower reach could experience higher growth rates (provided food is sufficient) because of warmer water temperatures. Using our method, we noted a decline in growth rate after fish reached about 550 mm and an in- crease after fish reached about 650 mm (Figure 3). The difference was not statistically significant, which may have resulted from small sample size of larger recaptured fish. Hawkins (1992) also not- ed increased growth rate at about 650 mm TL in both back-calculated scale and recapture data. If this phenomenon is real, as we suspect, it suggests an increase in food availability at this size; this could result from increased gape size or more ef- fective foraging and handling ability allowing use of larger and more abundant prey. Alternatively, TABLE 4.-Survival rates of Colorado squaw fish 550 rom total length and longer for which stable length distri- bution did not significantly differ from measured distri- bution for years 1991-1994 (Kolmogorov-Smirnov [K-S] one-sample test, P > 0.05, N = 158) for annual population growth rates from -10 to 10%. "Best estimate" is the survival rate with smallest K-S D-statistic and greatest P-value. Growth rate (%) Survival rate Best estimate 10 0.93-0.97 0.95 5 0.88-0.92 0.90 0 0.83-0.87 0.85 -5 0.80-0.83 0.81 -10 0.76--D.80 0.77 differential growth and survival rates between sex- es could explain an increased growth rate for fish about 650 mm. However, because sexing Colorado squawfish in the field is unreliable, assessing growth rates by sex was not feasible. Although variation among individuals exists, most fish do not grow consistently faster or slower than average in successive years, and average growth rates do not differ greatly among years. Nevertheless, there can be large differences among individuals in time to a given length, e.g., 20-year range of potential ages at a given length for fish greater than 800 mm. Although length is therefore a poor indicator of age, we conclude that the pop- ulation's largest fish are quite old, For individuals 900 mm TL, 34 years might be a minimum age, but an average would likely be 47-55 years (Fig- ures 3, 4a). Such fish are very rare. Of about 1,080 subadult and adult Colorado squawfish captured from the Colorado River during 1979-1995 only three (0.28%) were greater than 900 mm; two of these were less than 905 mm (c. McAda, USFWS, unpublished data). The largest, captured in 1990 by UDWR biologists, was 960 mm. Exceptional longevity is probably a life history strategy that allows this and some other western cyprinid and catostomid species to survive periods of limited reproduction or recruitment when adverse envi- ronmental conditions are prolonged (Scoppettone and Vinyard 1991). Although exhibiting declining growth rates with length, adults recaptured in our study did not stop growing. Of 69 fish greater than 550 mm later recaptured, only three exhibited no growth be- tween years, and all of the largest fish captured (850-899 mm) exhibited growth between cap- tures. If Colorado squawfish do stop growing at a certain age or length, cessation of growth may not