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Last modified
7/14/2009 5:02:36 PM
Creation date
5/20/2009 9:59:55 AM
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UCREFRP
UCREFRP Catalog Number
9591
Author
Nesler, T. P.
Title
Interactions Between Endangered Fishes and Introduced Gamefishs in the Colorado River, Colorado, 1986-1991.
USFW Year
2002.
USFW - Doc Type
91-29,
Copyright Material
NO
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<br />4 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />species susceptible to capture by the 25mm inner panel of the trammel net or capture via <br />electrofishing. This sampling approach was used on all backwaters available from RM 185-128. <br />All centrarchid and icta1urid fish species collected were enumeIated and measured for total length <br />in mm and weight in gms. All centrarchid fishes and a representative subsample of icta1urid fishes <br />based on length categories were killed for stomach content analyses. <br /> <br />In 1991, large bodied fish sampling was limited to nighttime sampling using blocking nets <br />in two backwaters on July 9 and 23-24 to assess daytime sampling bias. Two backwaters were <br />selected on the basis of vehicle access and camping to conduct nighttime seine and gillnet sampling <br />for centrarchids and icta1urids. Data for individual gamefish was collected as described <br />previously. Gape size of individual gamefish collected in nighttime sampling (excluding channel <br />catfish) was measured by opening the mouth to its fullest extent without force and measuring <br />width and height of the gape in mm. Gape areas for each gamefish were estimated as the product <br />of these two measurements. On July 9 backwaters at RM 154.8 and 135.2 were sampled. On <br />July 23-24, backwaters at RM 153.8 and 147 were sampled. Larval fish drift nets were set in the <br />main channel near these backwaters and fished concurrently during these nighttime samples to <br />document species composition and relative abundance of drifting larval fishes as part of the <br />potential prey base in backwater habitats. <br /> <br />ISMP seine data from 1986-1991 were used for analysis of species composition and catch <br />rate for gamefish species and Colorado pikeminnow. Catch rates were estimated for each year <br />by dividing total number of each fish species captured by the total area seined, and adjusting to <br />100m2 seine area. Correlations between number of seine samples made, seine sample area, <br />number of backwaters sampled, backwater area, river miles sampled, and annual catch rates of <br />gamefIsh species and Colorado pikeminnow were estimated using Pearson's correlation coefficient. <br />Length-weight regressions were used for channel catfish to assess general growth and condition <br />per year. The estimated slopes of the regression lines for Colorado River channel catfish were <br />qualitatively compared between years sampled and with similar data for the Yampa River. A <br />slope of 3.0 indicates increase in fISh weight equal to increase in fish length. A slope > 3.0 was <br />considered to indicate robust growth. For analyses of prey ingested by gamefish, percent <br />frequency of occurrence of fish species ingested was used as a measure of predator choice among <br />alternative prey selections. This was considered to be more important in evaluating interaction <br />between predator and prey rather than characterizing prey items in the diet by weight or volume, <br />which provides a better measure of the nutritional importance of food items <br /> <br />RESULTS <br /> <br />Seine samples--abundance <br />Seine sampling resulted in 11-20 backwater habitats being sampled, and 2245 seine <br />collections made annually (Table 1). Additional seine samples for the purpose of augmenting <br />sample size of gamefish species was found to have negligible results and was discontinued in <br />1987. Under ISMP protocol, the 3o-mile reach of river sampled allowed for 12 backwaters to be <br />sampled over six 5-mile segments. In 1986 and 1989 only 11 backwaters were available. In the <br />entire 55-mile study reach, 22 backwaters could be sampled over 11 five-mile segments, but the <br />
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