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Last modified
7/14/2009 5:02:36 PM
Creation date
5/20/2009 10:00:23 AM
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UCREFRP
UCREFRP Catalog Number
9546
Author
Nesler, T. P.
Title
Interactions Between Endangered Fishes and Introduced Gamefishes in the Yampa River, Colorado, 1987-1991 - Draft Final Report.
USFW Year
1995.
USFW - Doc Type
91-29,
Copyright Material
NO
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<br /> <br />i <br />ILJ <br />t <br />t <br /> <br /> <br /> <br />A <br /> <br />F <br />t <br /> <br />Lengths of fish prey were recorded from 149 specimens. Included were 50 redside <br />shiner, 19 white sucker, 17 speckled dace, 16 roundtail chub; 6 each of flannelmouth sucker and <br />unknown sucker species; 2 each of bluehead sucker, mottled sculpin, and common carp, 1 each <br />of plains killifish, fathead minnow, green sunfish, and sand shiner; and 25 unidentified fishes. <br />Length frequency distribution of the fish prey ingested by northern pike showed 90 % of the prey <br />were less than 140 mm in length, and 82 % ranged in length from 21 to 80 mm total length. This <br />prey base was comprised of all fish species identified (Figure 13). Fish prey larger than 160 <br />mm in length were comprised mostly of roundtail chub and white sucker. Eleven of the 15 large <br />fish prey recovered from northern pike stomachs were roundtail chub (73 %). A comparison of <br />fish prey length with northern pike length suggested a curvilinear relationship (Figure 14). <br />Northern pike up to 600 mm in length appeared heavily reliant on fish prey less than 100 mm <br />in length. Fish prey larger than 200 mm in length appeared in the stomachs of northern pike <br />larger than 600 mm in length. The predominant fish prey species for these larger northern pike <br />was roundtail chub, ranging in length from 210 to 388 mm in length. <br />Vulnerability of fish to northern pike predation <br />Indices of prey vulnerability to predation by northern pike could be expressed as a <br />physical potential based on the northern pike's gape area relative to the cross-sectional area of <br />the fish prey's body at its greatest thickness between the head and the dorsal fin insertion; or it <br />could be expressed as a percentage of the prey population whose individual lengths are less than <br />or equal to the maximum prey size observed in northern pike gut contents in the Yampa. The <br />first index would constitute an upper limit to predation vulnerability based on the physical ability <br />to swallow an object. The second index would reflect vulnerability based on the reality of <br />empirical data. A rule of thumb for northern pike predation is that northern pike will take other <br />fish up to 40 % its own length. In this study, the 388 mm roundtail chub prey was 43.6 % of the <br />size of the 890 mm northern pike predator, and was the largest fish documented to be taken as <br />a prey item. Expressing prey length as a percentage of northern pike length, the average fish <br />prey size was only 14 % of the size of the northern pike predators. The maximum was 48 % and <br />the minimum was 3 %. Using 43.6 % as an upper level criterion based on the largest prey item <br />recorded and the size of the largest northern pike captured in the Yampa, the maximum prey size <br />of northern pike in the Yampa would be 487 mm. Length frequency distributions of other fish <br />species associated with northern pike, using lengths of fish sampled in the study area from 1986- <br />1991, were examined using this maximum prey size threshold to determine the percent of the <br />sample population of each species vulnerable to northern pike predation. The samples of <br />associated fish species used for these length frequency distributions included 180 Colorado <br />squawfish, 124 roundtail chub, 75 common carp, 154 flannelmouth sucker, 163 white sucker, <br />and 17 bluehead sucker. <br />In 1991, gape measurements were taken from 125 northern pike, ranging in length from <br />191 to 913 mm. To determine the range of fish prey physically vulnerable to capture and <br />ingestion, body measurements were also taken from four native fish species and two non-native <br />fish species as potential northern pike prey. The sample of prey species included 39 Colorado <br />March <br />DRAFT <br />March 12,19% <br />40
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