8096

• UCREFRP Catalog Number: 8096
• Author: American Fisheries Society.
• Title: Proceedings of the 19th Annual Meeting, Colorado - Wyoming Chapter, American Fisheries Society.
• USFW Year: 1984.
• USFW - Doc Type: March 7-8, 1984.
• Copyright Material: NO

largest impoundment in the ~rainage basins Beaver Creek Reservoir has a
storage capacity of 5.87 hm (4760 acre-feet). This reservoir and several
other very small storage reservoirs have only minor impacts on the discharge
of the South Fork of the Rio Grande.
Three sections of the river have been electroshocked the last week in
September from 1976 through 1983. Brown trout (Salmo trutta L.) are the
dominant species, comprising 95-99% of the wild trout populations. Occasion-
ally, brook trout (Salvelinus fontinalis) and cutthroat trout (Salmo clarki)
are collected in the sampling and stocked catchable-size rainbow trout (Salmo
gairdneri) are also taken at some of the stations in some years.
Initially the electroshocking surveys were only part of a region-wide
inventory survey by regional fishery biologists responsible for the south-
west region of Colorado. However, regional biologists and the authors were
intrigued by the vast differences in the levels of brown trout recruitment
between years in 1976 and 1977. For this reason the South Fork of the Rio
Grande River was incorporated into two different studies under the Fish
Research section of the Colorado Division of Wildlife (Nehring 1979; Nehring
and Anderson 1983).
The years 1976,. 1979, and 1980 were years of negligible recruitment of
young-of-the-year (YOY) brown trout. During 1977, 1978, and 1981 we observed
very high levels of YOY brown trout recruitment. In 1982 and 1983 brown trout
recruitment was moderate. Over the years we have investigated a number of
theories that might have explained the between year variations in recruitment.
In this paper we present the results of these investigations.
METHODS AND MATERIALS
A simple Petersen Mark and Recapture populat-ion estimate was completed
on the trout population 15 cm and larger in the South Fork of the Rio Grande
River in September each year from 1976 through 1983. In addition, constant
efforts were made each year to collect YOY brown trout as they were encounter-
ed in the electroshocking process. All trout were measured to the nearest cm
and a scale sample was taken from yearling and older brown trout each year
for age and growth analysis.
The South Fork of the Rio Grande was subjected to flow modeling twice
during the study, once in 1978 (Nehring 1979) and again in 1981-83 (Nehring
and Anderson 1983). Both times we used the theories, techniques,-and computer
models developed by the U. S. Fish and Wildlife Service's Instream Flow Group
in Fort Collins, Colorado (Bovee 1978; Bovee and Milhous 1978; Milhous, Wegner
and Waddle 1981), We used the incremental method for collecting the field
data. The field data were subjected to computer modeling with IF-G-4 compu-
ter model and interfaced with the PHABSIM (Physical Habitat Simulation System)
to generate weighted usable area (WUA) for the life stages of the brown trout.
This information was in turn interfaced with hydrologic data available through
the U.S.G.S. monitoring system (U. S. Geological Survey 1976-1983), to gener-
ate the stream discharge - WUA relationship over the study period (1976-1983).
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