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1375 STATUS OF GREEN RIVER COLORADO PIKEMINNOW . 24.7% of the Colorado pikeminnow captured were recruit-size. Given that the apparent average annual mortality rate of the average-size fish in the Green River basin from 1991 to 1999 was about 18% (1 - 0.82 = 0.18 X 100), recruit abundance should have equaled or exceeded losses to mortality. Recruitment rates higher than mortality could have resulted in the apparently expanding population of Colorado pike- minnow in the Green River basin from 1991 to 1999, based on ISMP CPUE data. Similarly, the lower abundance of recruits in 200D-2003 (including 3 years in which there were none), coupled with higher mortality rates, evidently played a major role in the lower abundance of adult Colorado pikeminnow in the Green River basin in 200D-2003. Because Colorado pikeminnow are long-lived and recruitment may be episodic, comparisons of recruit- ment in the periods 1991-1999 and 200D-2003 may not be valid. This could be the case in our study because we did not observe significant recruitment during the shorter period; if significant recruitment had occurred, it may have balanced out mortality. Howev- er, recruitment of Colorado pikeminnow has occurred with reasonable frequency in the Green River basin (e.g., Figure 10), because moderate (n = 3) or high (n = 4) recruitment events occurred in 7 of the 9 years from 1991 to 1999. In contrast, based on the size structure of Colorado pikeminnow in the Green River basin, recruitment was low in all years of this study, 2000 to 2003. The population rate-of-change analysis also supported that assertion because the value of Ie was less than 1, sometimes substantially so, in all years. Thus, size-structure metrics and the population rate-of- change analysis jointly support the thesis that reduced recruitment in recent years was a main factor in the decline of Colorado pikeminnow abundance during 2000-2003 and that regular and relatively high recruitment is needed to sustain populations. Although it was not a part of this study, we believe that relatively weak year-classes of young-of-year Colorado pikeminnow produced in nursery areas was the reason for the relatively low abundance of recruit- size fish in 200D-2003 (Figure 11; Muth et al. 2000; McAda 2002; Utah Division of Wildlife Resources, unpublished data). The abundance of age-O Colorado pikeminnow in backwaters in autumn has declined rather dramatically since the early 1990s, especially in the middle Green River. Relatively strong year-classes in the middle and lower Green River were last produced in 1993. That year-class would have first contributed adults to the Green River Colorado pike- minnow population in 2000, the year when estimates of the abundance of adult Colorado pikeminnow in the middle Green River were the highest observed during . 3500 3000 2500 ......lower Green River ..... middle Green River ~ ~ 2000 E i 1500 .. 1000 500 o 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 Year FIGURE 11.-Abundance of age-O Colorado pikeminnow in seine samples from backwaters in the middle and lower Green rivers, Utah, 1986-2003. this study. This recruitment scenario was based on the growth rates of Colorado pikeminnow presented by Osmundson et al. (1997), whereby an average age-O fish in a given year-class (e.g., 1993) was expected to grow to an average of 425 mm TL (a recruit-size fish) 6 years later (Osmundson et al. 1997). That same group of fish would then grow to minimum adult size (450 mm) 1 year later at age 7 (e.g., 2000). Subsequent age- o year-classes (1994 to 1996), which would have contributed adults to the Green River population of Colorado pikeminnow from 2001 to 2003, were very weak in the middle Green River and weak or only moderately strong in the lower Green River. In the Colorado River, strong recruitment of adult Colorado pikeminnow year-classes was also linked to strong year-classes of early life stages (Osmundson and Burnham 1998). At present, we are conducting an analysis that includes information on the production of larvae (Bestgen et al. 1998), abundance of juveniles in backwaters (Haines and Tyus 1990; Tyus and Haines 1991; Haines et al. 1998; McAda 2002), and biotic and abiotic factors that may influence year-class strength and recruitment to the adult size-class (Bestgen 1996; Bestgen et al. 1997; Bestgen and Bundy 1998; Osmundson and Burnham 1998; Bestgen et al. 2006a), so that the complexities of the recruitment process for Colorado pikeminnow in the Green River can be better understood. Probabilities of Capture The relationships between capture probabilities and fish length suggest that small and very large Colorado pikeminnow had very low probabilities of capture, which can be, in part, independent of abundance at those life stages. The linear term for capture probability as a function of TL indicated that fish up to about 580 mm were progressively easier to capture, but the