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<br />Based on a preliminary estimate of the pikeminnow population in 1998 there are 735 adults in two <br />reaches. Although these data suggest an increase over the baseline population estimate (600), this <br />would be considered a neutral population response, since it does not exceed 1100. <br /> <br />Adult Age-class Structure (based on length- frequency analyses) <br /> <br />The relationship between age and length can be used to determine the relative strength of different age- <br />classes of fish based on the frequency with which fishes of different lengths appear in samples. Length- <br />frequency data for Colorado pikeminnow were collected from 1986 through 1996 (Table 5, Figure 5). <br /> <br />Approximate age-classes and their corresponding average total lengths (mm) in parentheses are: <br />5 years (376); 6 years (424); 7 years (456); 8 years (496); 9 years (520); and 10 years (545). These <br />are only averages, however, and some overlap in size can be expected between adjacent age-classes. <br />Nevertheless, these data may allow the progress of individual year-classes to be followed from year to <br />year. For example, the exceptionally strong 400-450mm total length range (nominally the 1986 year- <br />class at age 6) in 1992 (n = 16) appears in 1993 as the 450-500mm length-class (7-year age-class, n <br />= 12). This year-class is represented by bold n values in Table 5 and white bars in Figure 5 from 1992 <br />(age 6) through 1997 (age 11). illdividual variability in growth rates and declining growth rates as fish <br />age make it more difficult to differentiate age-classes of older, larger fish. Therefore, after 1993 this <br />year-class becomes increasingly harder to distinguish from adjacent year-classes. <br /> <br />ill general, a population is considered more stable if it is represented by a variety of age-classes, which <br />is indicative of both recruitment and survival. On the other hand, if the vast majority of a population is <br />represented by only a few age-classes, the population is less stable because it is susceptible to mass <br />mortality when these classes reach their maximum life expectancy. This scenario is exacerbated if age- <br />class structure is skewed toward the maximum age of the species, which occurs when there has been <br />insufficient recruitment into the adult population. ill this case, age-class distribution shifts toward the <br />right side of the length-frequency graph as the average age of the population increases. <br /> <br />illterpretation oflength-frequency data can be highly subjective. Moreover, small sample sizes are <br />likely to under- or over-represent certain age-classes in the population and present a misleading picture <br />of age-class structure. However, if a population is expanding, we would expect sample sizes to <br />increase also, reducing the uncertainty of these data. <br /> <br />A positive response (Table 6, Figure 6) requires existing age-classes to be expressed in future years <br />and that strong, younger age-classes be recruited into the adult population. Under this scenario, the <br />number of age-classes represented in the sample increases over time, while mean length remains <br />relatively constant, with no discemable trends. We would expect the sample size to increase as the <br />overall population increases, thereby reducing the uncertainty of the results. <br /> <br />If the existing population continues to age, but there is little recruitment, the population would <br /> <br />5 <br />