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Last modified
7/14/2009 5:02:33 PM
Creation date
5/17/2009 11:15:08 PM
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UCREFRP
UCREFRP Catalog Number
8177
Author
Valdez, R. A., B. R. Cowdell and L. Lentsch.
Title
Overwinter Survival of Age-0 Colorado Pikeminnow in the Green River, Utah, 1987-1995\
USFW Year
1999.
Copyright Material
NO
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<br />DISCUSSION <br /> <br />The extent and magnitude of overwinter survival of age-O Colorado pikeminnow on the <br />population of the Green River cannot be accurately and defInitively determined from existing data. <br />Seine catch rate densities cannot be validated to determine if these accurately reflected numbers of <br />age-O fIsh in the system. However, testing of hypotheses and associated analyses reveal important <br />rdationships and linkages in understanding the magnitude of mortality during the winter period and <br />possible causal relationships. Overwinter survival was compared to fIsh length in fall, flow <br />variability, river temperature, backwater depth, and densities of non-native fIshes. <br /> <br />Evaluation of Hypotheses <br /> <br />HJpotllesis 1: Length of Age-O Colorado Pikeminnow in Fall <br /> <br />The hypothesis that overwinter survival is signifIcantly higher for larger age-O Colorado <br />pikeminnow than for smaller individuals showed a poor fIt of data to a linear model, but a positive <br />slope suggesting that in some years, this relationship held true. Six of the nine average fIsh lengths <br />in fall considered in this analysis for Reach 3 equaled or exceeded 37 mm TL; however, survival was <br />not signifIcantly different from year classes with average fIsh lengths less than 37 mm TL. Converse <br />et al. (1999), using a quantile-quantile plots of age-O lengths in fall and in spring for the same years, <br />found a relationship between fIsh length in fall and overwinter survival with four of seven winters <br />showing a dominant effect. McAda and Ryel (1999) also found a relationship between age-O length <br />in fall and overwinter survival for fIsh in the Colorado River. Thompson et a1. (1991) suggested that <br />fish larger than about 37 mm TL in fall have a sufficient fat content to survive better than smaller <br />fish. indicating a threshold effect. <br /> <br />HJPotllesis 2: Flow Variability <br /> <br />The hypothesis that overwinter survival is signifIcantly higher with less flow variability was <br />accepted. Although the data fIt the linear model poorly, a negative slope (P=0.285) indicates that <br />survival index decreased with increasing flow variability. It was not clear if the causative factor in <br />wi nter was flow variability or the combination of flow variability and ice formation processes, which <br />l~ads to ice jams, ponding of the river, and flow-through of nursery backwaters with erosion of <br />r~taining sand bars. Flow variability in winter as an influence to survival of native fIsh is clearly a <br />variable in need of further investigation. Tyus (1991) also hypothesized a loss of recruitment due <br />to high discharges that flood ephemeral backwater habitats and flush fIsh from these critical nursery <br />areas. <br /> <br />H}pothesis 3: River Temperature <br /> <br />The hypothesis that overwinter survival is signifIcantly higher in years of very warm or very <br />cold water temperature was not accepted. The data fIt poorly to a linear model and the analysis <br />showed no relationship (i.e., slope = 0). In this analysis, we attempted to use river temperature as <br />an index of ice formation. This may not be a good, reliable relationship, since there are clearly other <br />factors that influence ice formation (e.g., water volume, flow fluctuation, distance from Flaming <br />Gorge Dam). Nevertheless, other studies (Valdez and Cowdelll999) have found that ice formation <br /> <br />18 <br />
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