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Last modified
7/14/2009 5:02:33 PM
Creation date
6/1/2009 11:55:18 AM
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UCREFRP
UCREFRP Catalog Number
8175
Author
Converse, Y. K., L. D. Lentsch and R. Valdez.
Title
Evaluation of Size-dependent Overwinter Growth and Mortality of Age-0 Colorado Pikeminnow.
USFW Year
1999.
USFW - Doc Type
Salt Lake City, Utah.
Copyright Material
NO
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<br />are referred to as adults. Adult Colorado pikeminnow were collected from the Green River and <br />two tributaries, the Yampa and White rivers, using a variety of gear types including angling, <br />electro fishing, fyke nets, gill nets, trammel nets and seines (C. McAda, database manager, <br />USFWS, Grand Junction, Colorado; personal communication). <br /> <br />Scale collection and reading <br /> <br />Scales were collected from age-O Colorado pikeminnow from the Green River during one <br />fall period, 19 and 21 September and 2 October, 1991, to represent fish entering their first winter. <br />These fish were collected from the middle and lower Green River areas (Figure 1). Scales were <br />collected from all adult Colorado pikeminnow (as described above). In all cases, scales were <br />removed with forceps from an area between the lateral line and anterior of the dorsal fin offish <br />with visible scales. For fish without visible scales, the area was gently scraped with a knife to <br />remove scales. Scales or skin scrapes were then placed in wax paper and into envelopes <br />indicating date of capture, river and river mile, and fish total length (mm). <br />Scale analysis for this study followed traditional methods (Jearld 1983; Summerfelt and <br />Hall 1987). Newman and Weisberg (1987) and Smale and Taylor (1987) reported that within- <br />fish variation was not a major source of variation in scale analysis; therefore, data in this study <br />comprised one scale per sample. Circuli are raised or visible rings, that represent periods of <br />growth, formed on some regular time interval. In this study, a growth check was recognized as <br />either closely spaced circuli or as a change in circuli patterns, which generally represents a <br />change in the environment and metabolism of the fish, such as slow or no growth during winter. <br />As growth rates resume in spring, circuli are again formed at a regular interval and the slow <br />growth period appears as thickened, closely spaced circuli or change in circuli pattern. <br />Total number of circuli from focus to edge of scale was recorded for age-O, one and two <br />year old fish. The number of circuli from focus to the first growth check (this number henceforth <br />referred to as first-year circuli or C 1) and number of circuli from first to second growth check <br />(this number henceforth referred to as second-year circuli or C2) was recorded for adults. <br /> <br />Size-selective overwinter growth v. mortality <br /> <br />To discern between mechanisms of size-selective overwinter growth and overwinter <br />mortality, we examined deviations between slopes of plotted quantiles from fall age-O and spring <br />age-l length-frequency distributions based on the technique described by Post and Evans (1989). <br />Fish numbers were categorized by length into percentiles of the total number (1,5, 10,25,50, <br />75,90,95 and 99%) referred to as a quantile. For example, the 50th quantile would be the length <br />at which 50% of the fish collected were less than. A quantile-quantile (QQ) plot describes the <br />fall quantile lengths on the x-axis and spring quantile lengths on the y-axis. In the Post and <br />Evans (1989) study, quantile-quantile plots ofage-O to age-l length-frequency distributions <br />under different scenarios of overwinter size-selection were evaluated. No change in fish lengths <br />over winter resulted in a I: 1 line on the QQ plot. Normal growth resulted in an upward shift of <br />the 1: 1 line with no change in slope. A slope less than 1 that began above the 1: 1 line but <br />intersected it at the upper end was evidence for size-selective ovenvinter mortality favoring <br /> <br />4 <br />
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