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962 OSMUNDSON AND BURNHAM TABLE 4.-Number of Colorado squaw fish captured (in- cluding recaptures) in the upper reach of the Colorado Riv- er by occasion and year, the number of different individ- uals captured by year (My), and estimated annual abun- dance (Ny); ft is used to calculate Ny and is explained in the text. Occasion Year or variable 1991 1992 1993 1994 Captures I 23 21 31 27 2 17 25 31 37 3 25 23 33 38 Mean 21.7 23.0 31.7 34.0 Quantities My 59 65 78 93 fir 0.093 0.102 0.112 0.123 p(My) 0.254 0.276 0.300 0.325 NI' 232 236 260 286 a Mean Ny = 254 for 1991-1994. tion size and capture probabilities in the upper reach, annual abundance (Ny) was computed by assuming increases in both abundance and capture probabilities during 1991-1994. If My is the num- ber of distinct fish captured by year, the probability of catching a fish (once or more) present in a year is p(My) = 1 - (1 - py)3, where py is from the open model {, py}. The best estimator of abun- dance by year allowing this trend in capture rates is thus Ny = My/p(My); these estimates, averaging 254 but reflecting an increase in abundance from 1991 to 1994, are shown in Table 4. Lower reach.-Estimates in the lower reach in- cluded not only adults, as in the upper reach, but also sub adults as small as 250 mm. ,However, the vast majority of captures were of individuals lon- ger than 300 mm (see section below on size fre- quency). Size bias from switching to a finer-mesh net after 1991 in the lower reach was probably minimal: only two fish smaller than 259 mm (the smallest captured with the larger-mesh net) were subsequently captured with the finer net. Results from CAPTURE using model M, for the 3 years with two within-year sampling passes were N = 224 (SE = 155) for 1992, N = 512 (SE = 228) for 1993, and N = 297 (SE = 126) for 1994; the average estimate for the 3-year period was N = 344 (SE = 101, 95% CI = 196-604). Program RECAP provided an estimate of average abun- dance over the 3-year period (1992-1994) in the lower reach of N = 425 (SE = 144, 95% CI = 196-732). However, because RECAP assumes that time intervals between occasions are all equal (within-year intervals were similar to each other TABLE 5.-Mean catch per unit effort (CPUE) of Col- orado squawfish captured in trammel nets (by year) in the upper-reach study area of the Colorado River; CPUE = number of fish caught per net; N = number of nets set; TL = total length. Fish less Fish 550 Year and than 550 mm(TL) statistic All fish mm (TL) or longer 1991 N 139 Total fish 45 10 35 Mean CPUE 0.324 0.072 0.252 SE 0.065 0.022 0.061 1992 N 117 Total fish 57 16 41 Mean CPUE 0.487 0.137 0.350 SE 0.086 0.036 0.074 1993 N 121 Total fish 75 26 49 Mean CPUE 0.619 0.215 0.405 SE 0.112 0.053 0.082 1994 N 105 Total fi sh 72 37 35 Mean CPUE 0.686 0.352 0.333 SE 0.142 0.094 0.075 but were not similar to among-year intervals), some non optimal weighting occurred when the program averaged across years. A better weighting produced an estimate of N = 385 (SE = 130,95% CI = 202-733). From RECAp, there was indica- tion of recruitment over the 3-year period (average B = 118, SE = 74), but abundance estimates sug- gest that sizable recruitment occurred only be- tween 1992 and 1993 with an increase of several hundred fish. Survival estimates for the lower reach were cal- culated from SURGE ( = 0.57, 95% CI = 0.34- 0.95) and RECAP ( = 0.43, 95% CI = 0.15- 0.78). Because of wide standard errors, neither es- timate is particularly useful; however, both esti- mates indicated a considerably lower apparent sur- vival rate for the lower reach than for the upper reach. This would be consistent with fish emi- grating from the lower reach if their actual physical survival was similar to that of fish in the upper reach, or it could actually represent higher mor- tali ty. Size Frequency: Lower Reach A relatively large number of subadults was found in the lower reach during 1991, and growth and distribution of this group was subsequently tracked through 1994 (Figure 1). Scale analysis indicated this pulse of subadult fish was composed '. .