8141A

• UCREFRP Catalog Number: 8141A
• Author: Wydoski, R. S. and W. T. Helm.
• Title: Effects of Alterations to Low Gradient Reaches of Utah Streams.
• USFW Year: 1980.
• USFW - Doc Type: \
• Copyright Material: NO

this study since each fish was marked with a numbered tag and the section of
capture was always recorded. A limited amount of movement data for fish under
50 9 was obtained using freeze brands and dangler tags. Movement data were
obtained during all sampling for population estimates as well as sampling for
data on fecundity and growth. Several special collections were made outside
the boundaries of study sites to obtain movement data for the entire study
area (Appendix A; Table 2 ).

There were two objectives for obtaining information on fish movement: (l)
To check assumptions made in making population estimates, and (2) for a better
understanding of the ecology and life history of brown trout and whitefish.
Data were stratified by size group of fish and time between recaptures and then
analyzed to determine the number of fish moving, direction of movement (up-
stream or downstream), mean distance moved, and number remaining stationary.
The distance the fish traveled was determined to the nearest 100 m since move-
ment within sections could not be followed. Movement into adjacent sections
may reflect a change in locality of only a few meters across an imaginary
boundary; consequently, movement between adjacent sections was treated separately
in data analysis. These data were used to determine whether any immigration had
occurred between mark and recapture sampling for population estimates. Fish
movement was also analyzed by size group for different streamflows (including
a separate season for spawning) to determine the effects of changes in stream-
flow on fish movement.

Age Composition. Scale impressions were made in cellulose acetate and
examined on a scale projector at 80X. At least two independent readings were
made~of the scales from each fish. The number of annuli, fork length of fish,
and date and location of sampling were recorded and analyzed. The scale method
of age determination was validated by comparison with length frequencies of
the fish and other information on fish from the Logan River (Sigler 1951,
Bergersen 1973) and Blacksmith Fork River (Gosse 1977). The final age divisions
were drawn on length frequency histograms by date of collection.

Estimates. Whitefish and brown trout populations were estimated
modification of the Petersen population estimator

N = (M + l)(C + l) -1
R + 1

where: N = estimated number of fish in the population
'M = number of fish marked on the first sampling
C = number of fish captured on the second sampling
R = number of fish in the second sampling that were marked during
the first sampling.

(l)

When fish movement into a study site occurred between the mark and capture
dates, it was incorporated into the estimator (equation 2):

N* = (M + l){C - am(Q) + l} _ 1 (2)
R + 1

where: N* = revised estimate of the number of fish in the population

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