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WSP03748
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Last modified
1/26/2010 12:51:54 PM
Creation date
10/11/2006 11:56:22 PM
Metadata
Fields
Template:
Water Supply Protection
File Number
8240.200.33.I
Description
15-Mile Reach (UCRBRIP)
State
CO
Basin
Colorado Mainstem
Water Division
5
Date
2/12/1999
Title
Draft Colorado River Programmatic Biological Opinion (PBO) Endangered Fish Populatio Status Assessment
Water Supply Pro - Doc Type
Publication
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<br />. <br /> <br />. <br /> <br />8 <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 <br />strength of different age-classes of fish based on the frequency with which <br />fishes of different lengths appear in samples. Length-frequency data for <br />Colorado pikeminnow were collected from 1986 through 1996 (Table 5, Figure 5). <br /> <br />Approximate age-classes and their corresponding average lengths are: 5 years <br />(376mm): 6 years (424mm): 7 years (456mm): 8 years (496mm): 9 years (520mm): <br />and 10 years (545mm). These are only averages, however, and some overlap in <br />size can be expected between adjacent age-classes, Nevertheless, these data <br />may allow the progress of individual age-classes to be followed from year to <br />year, For example, the exceptionally strong 400-450mm (nominal 6-year) class <br />in 1992 (n ~16) appears again in 1993 as the 450-500mm (7-year) class (n ~12), <br />This age-class is represented by the bold n values in Table 5 and the white <br />bars in Figure 5 from 1992 (age 6) through 1997 (age 11), Individual growth <br />variability and declining growth rates as fish age make it more difficult to <br />differentiate age-classes of older, larger fish. Therefore, as age-class <br />boundaries become less distinct after 1993, this age-class becomes more <br />difficult to differentiate from adjacent age-classes. <br /> <br />In general, a population is considered more stable if it is represented by a <br />variety of age-classes, which is indicative of both recruitment and survival, <br />On the other hand, if the vast majority of a population is represented by only <br />a few age-classes, the population is less stable because it is susceptible to <br />mass mortality when these classes reach their maximum life expectancy. This <br />scenario is exacerbated if all the age-classes are skewed toward the maximum <br />age of the species, which occurs when there has been insufficient recruitment <br />into the adult population. In this case, the age-class distribution shifts <br />toward the right side of the length-frequency graph as the population ages. <br /> <br />Interpretation of length-frequency data can be highly subjective. Moreover, <br />small sample sizes are likely to under- or over-represent certain age-classes <br />in the population and present a misleading picture of age-class structure, <br />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 that existing age-classes <br />continue to be expressed in future years and that strong, younger age-classes <br />be recruited into the adult population, Under this scenario, the number of <br />age-classes represented in the sample increases over time, while mean length <br />remains relatively constant, with no discernable trends, We would expect the <br />
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