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

M = number of fish marked in the first sampling
C = number of fish captured in the second sampling
am = number of marked fish which have moved into the study site between
mark and capture sampling
Q = estimated ratio of the total number of fish/number of marked fish
in the area where immigrants originated
R = number of fish in second sampling that were marked during
the first sampling.

(See Appendix B; Tabl e 4 for derivati on of equation 2). Estimates were made at
approximately two-month intervals for length groups approximating the year
classes 1976 through 1972 and 1971 and older (AppendixB; Table5+). Fish
populations in Logan control were estimated on 9 October 1975 and 5 February
1976. Confidence intervals (95%) were calculated for individual estimates
treating the ratio of recaptures to captures (R:C) as a binomial variable
(Davis 1964, Seber 1973) and obtaining intervals from the tables of Crow (1956)
and graphs of Clopper and Pearson (1934).

The first population estimate for Blacksmith bulldozed (11 August 1975)
was made using the "two-catch" method of Zippin (1958) (equation 3):

C2
N = 1
Cl- C2

(3)

where: N = estimated number of fish in the population
Cl = number of fish in first catch
C2 = number of fish in second catch

The estimate was made by grouping the fish by size class and calculating the
95% confidence intervals (Nt2 SE) with a normal distribution. The standard
error (SE) was calculated using (equation 4):

SE =

C1C2
2
(Cl - C2)

(el + C2)

(4 )

The two-catch estimate was used for the first sampling period because of
the possibility that the proposed alteration by bulldozer at this site would
occur before the recapture sampling could be made using the Petersen estimator.

Growth. Growth rates for the time intervals between population estimates
in a study site were calculated for each year class from mean weights. Growth
was assumed to be an exponential function and was calculated by following
Chapman (1968) (equation S):

InW2 - InWl
G = t.t

(5 )

where: G = instantaneous growth rate
wl,W2 = mean weights of an age group at times tl and t2, respectively

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