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966 OSMUNDSON AND BURNHAM ever, were skewed by the large year-classes that materialized as sub adults in 1991 and dominated the 500-549-mm size-class by 1994. The dynamic nature of these large cohorts made definitive size- frequency comparisons between reaches difficult. However, larger fish were clearly more prevalent in the upper reach: fish longer than 600 mm were rarely caught in the lower reach, but they consti- tuted more than 30% of captures in the upper reach. Osmundson et al. (1997) indicated that length-frequency distributions for adults longer than 550 mm in the upper reach were relatively stable during 1991-1994. Though some reproduction and recruitment may occur every year, strong year-classes are evidently infrequent. The distinct pulse of subadult fish de- tected in the lower reach in 1991 was distributed throughout the river within a few years. This in- fusion of new fish into the population was appar- ently the result of one or more strong year-classes produced during the mid-1980s, a phenomenon corroborated by catch rates of larvae and young of year during those years. Given that high repro- ductive success later resulted in strong recruitment (at least to size-classes up to 550 mm), low num- bers of adult Colorado squawfish in the Colorado River may be a result of insufficient reproductive success during most years. The degree of reproductive success and first- year survival is probably determined by environ- mental conditions that vary yearly rather than by fluctuations in size of the spawning population (i.e., egg output), because apparent increases in adult numbers during the 4 years of this study did not result in concomitant increases in catch rates of larvae or age-O fish during the same time period. Conversely, weak year-classes beginning in 1988 followed the relatively strong year-classes of 1985 through 1987, while the number of adults probably changed little. A significant environmental factor that may affect reproductive success is the mag- nitude of spring flows: McAda and Kaeding (1989), Osmundson and Kaeding (1991), and McAda et al. (1994) found significant positive cor- relations between fall catch rates of age-O Colo- rado squawfish in the Colorado River and the mag- nitude of spring flows that preceded the spawning season. Exceptionally high flows may also be a precursor to successful reproduction because the strong year-classes of 1985-1987 were preceded by extremely high spring flows during 1983-1984. Demographic Trends Comparisons with historical abundance are dif- ficult because of the paucity of biological studies or surveys conducted prior to the 1970s. Jordan (1891) conducted surveys in the upper Colorado River basin in 1889 and later described (Jordan and Evermann 1896) abundance and distribution of Colorado squawfish as, "Colorado Basin, very abundant in the river channels as far north as the base of the Rocky Mountains in Colorado (Un- compahgre River at Delta)." Compiled accounts and photographs collected by Quarterone (1993) of Colorado River fish during the first half of this century also indicate much higher abundance of Colorado squaw fish in the upper Colorado River than observed in this study (or studies dating back to the early 1970s). Differences in abundance from these historic accounts and current estimates strongly suggest that the long-term population trend has been downward. Because of the longevity of Colorado squawfish and their pulsed recruitment, the significance of short-term trends in abundance is difficult to eval- uate. Our study coincided with a period when re- cruitment to the adult population was compara- tively high and overall numbers of adults appeared to increase. However, a longer time frame must be considered when evaluating trends of long-lived species, especially one with such a long lag-time between hatching and recruitment to the adult pop- ulation. Although the 1985-1987 year-classes might have recently increased the size of the adult population, reproductive success since 1987 has been relatively low, and the trend of increase in adults could be reversed in the near future. Major trends were not observed in length-fre- quency distributions of adult Colorado squawfish in the upper reach. However, a difference in length frequency of fish longer than 250 mm was ob- served in the middle subreach of the upper reach between the 1974-1976 period and this study (Fig- ure 4), suggesting a decrease in the proportions of juveniles and sub adults in the upper reach during the past 20 years. More than one-third of the fish in this subreach during 1974-1976 were 250-450 mm. The rarity of individuals shorter than 450 mm in the upper reach today, coupled with the stable length distribution of adults greater than 550 mm, suggests that upper-reach recruitment presently comes from the lower reach. In addition, most in- dividuals evidently do not move to the upper reach until they have attained a length of more than 450 mm (Osmundson et al. 1998). Two hypotheses may explain this change in the upper-reach, length-fre- quency distribution: (1) nursery habitat in the up- per reach was formerly of higher quality and quan- tity than in later years, and a smaller proportion