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

~t = time interval (tl - t2)

For the Logan and Blacksmith bulldozed sites mean weights were calculated
for the entire study site as were growth rates because of low numbers of fish
in these areas.

Population change rate. Changes in population numbers between successive
estimates result from either movement or mortality or a combination of these
changes. Population change rates were calculated using both components since
there was no way nor need to effectively distinguish between them. This rate
was assumed to be exponential and was calculated as follows (equation 4):

1 n Nl - 1 n N2
Y = ~t (6)

where: Y = instantaneous population change rate
Nl, N2 = population estimates for a year class at times tl and t2

ln = natural 10garithm
~t = time interval between estimates (t2 - tl)

Production. Biomass (standing crop) of a year class was calculated as
follows (equation 7):

B = "WN

(7)

where: w = mean weight for a year class
N = estimated number for a year class

Mean biomass of a year class (Bl) between collections was obtained by fo11owing
Chapman 1968 (equation 8):

Bo {e(G - Y} - l}
B. =
1 (G - Y)

(8)

where: Bo = biomass of a year class at the beginning of the time
interval between samplings
G,Y = instantaneous growth and population change rates,
respectively, for the period between samplings

Production {P} is calculated for a cohort (i) over a time interval (t) using
equation 9:

Pit = GitBit

( 9)

where: Git = instantaneous growth rate for cohort (i) during time
interval (t)
Bit = mean biomass of cohort (i) during time interval (t)

For dates when mean weights were available, but population estimates were
not (e.g. during spring floods), the production was calculated for the time

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