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50 STATUS OF ENDANGERED COLORADO SQUAWFISH 963 20 N=37 40 lower Reach 1991 o i ~L.I.~. . j ~ j ~~~II.~~ 50 1992 N= 33 1993 N=B2 1994 40 ...L N=55 20 o 250-299 350-399 450-499 550-599 650-699 750-799 850-899 300-349 400-449 500-549 600-649 700-749 80D-849 900-949 Length-class (mm) FIGURE I.-Length distributions of Colorado squawfish captured (N) in the lower reach of the Colorado River during 1991-1994. of three consecutive year-classes, which exhibited much overlap in fish lengths (Table 6). Subsamples from fish of this group captured in 1991 (N = 15, 259-383 mm) and 1992 (N = 22, 307-479 mm) were aged by using scale analysis (see Osmundson et al. 1997 for methods), and 22% had hatched in 1985,46% in 1986, 27% in 1987, and 5% in 1988. These results suggest a strong year-class was pro- duced in 1986 and perhaps also in 1985 and 1987. There were few fish present in the lower reach during 1991-1994 that were larger (older) or smaller (younger) than individuals from the 1985- 1987 year-classes (Figure 1). This group domi- nated the lower river subpopulation during the study period. This suggests that no similarly strong year-classes were produced for at least 3 years pri- or to 1985 and that none were produced for at least 3 years after 1987. These findings were consistent with those of McAda et al. (1994), who found that spring electro fishing catch rates had been rela- tively low in this portion of the river during 1986- 1990. In addition, relatively low catch rates back to 1986 suggest that no large pulse of young fish was produced since at least 1977. TABLE 6.-Ages of young Colorado squaw fish of vari- ous sizes as determined by scale analysis. Year Age collected Length (mm) N (years) Year-class 1991 259 1 3 1988 325-374 3 4 1987 332-383 9 5 1986 375-379 2 6 1985 1992 307 I 4 1988 326-452 7 5 1987 412-472 8 6 1986 430-479 6 7 1985 Size Frequency: Upper Reach Comparisons among subreaches.-Size distri- bution (combined 1991-1994 data) of upper-sub- reach fish (Figure 2) was significantly different from that of middle-subreach fish (P = 0.003, K- S test) and that of lower-subreach fish (P = 0.0002, K-S test); size distributions of middle- and lower- subreach fish were not significantly different (P = 0.11, K-S test). Comparisons of earlier years with recent years.-Only one significant difference was found in comparisons among length-frequency distribu- tions of recent and earlier years within the three 30 20 10 0 ....... '* 30 ....... >. 0 20 c Q) ::l 0- 10 ~ LL 0 30 20 10 Upper Subreach N=82 Lower Subreach N=58 o 250-299 350-399 450-499 550-599 650-699 750-799 850-899 300-349 400-449 500-549 600-649 700-749 800-849 900-949 length-class (mm) FIGURE 2.-Length distributions of Colorado squawfish captured (N) in the upper, middle, and lower subreaches of the upper reach of the Colorado River during 1991- 1994 (yearly data pooled).