7116 ‎(2)‎

• UCREFRP Catalog Number: 7116
• Author: Baxter, G. and J. Simon
• Title: Editor
• USFW Year: Series
• USFW - Doc Type: 1970
• Copyright Material: YES

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Table 3. The population coefficient of variation Cp = SDIY for salmon and trout in streams of varying type. n = sample size.
I Section area varying, mean 100 m2. Cp alculated for density. Data from S. J. SaItveit.
2 Section area varying mean about 200 m2. Cp calculated for population size. Data from T. G. Heggberget.
3 Section area varying between 77 and 444 m2. Cp calculated for density. Data from E. Jutila.
4 Section length 100 m, area varying. Cp alculated per section. Data from T. Bohlin.
Spatial variation of some salmonid populations in streams.
Mean Time Age Atlantic salmon Brown trout
Stream width class
(m) n Cp% n Cp%
Laerdall -30 1980/9 0 13 113 14 83
I 15 110 13 116
1980/10 0 13 63 12 58
I 13 77 13 93
AIta 2 -70 1981/9 >0 15 82
1982/8 >0 16 85
1983/9 >0 16 72 r
SimojokP -40 1983 >0 14 101
Jorlanda4 3.6 1982/9 0 20 73
I 20 45
1983/9 0 20 65
I 20 29
Norum4 2.4 1982/9 0 20 96
I 20 60
1983/9 0 20 77
I 20 88

the target area (N = 43), it would take about
n = 26 sampling areas to obtain this precision. If
Class 3 is sufficient, n is reduced to about 12.
So, although the sample size calculated from
eq. (18) isjust a crude and preliminary magnitude,
it is clear that stream surveys even of moderate
precision levels will usually require a large number
of sampling areas. Therefore, choosing a sample
size n matching the aim of the study is one of the
most crucial steps in the planning procedure. We
have therefore used eq. (18) to construct Tab. 4,
illustrating how the sample size required depends
on C, Cp and N. From this table it is evident that
Class 1 might be hard to obtain unless the target
area is very small.
It is to be stressed that the sample size cal-
culated from eq. (18) should not be taken too
literally. We have ignored possible effects of stra-
tification (see below) etc. It might therefore be
valuable to make checks of the precision obtained
during the field work.

Stratification

We now have a rough idea of the sample size
required. The next problem is how the areas
should be distributed in the stream.
We often know enough of the relation between
the type of biotope and the fish abundance to state

Table 4. Sample size n required to reach a given precision
class for various combinations of N = total number of units
and Cp = population coefficients of variation = SDIY. C is
the precision requirement, expressed as the coefficient of
variation of the estimate. Sample size and precision classes.
independent data.

l)
!

N = 25 N = 50 N = 100 N = 00

'I
(

Class I Cp = 0.5 20 33 50 100
(C = 0.05) Cp = 1.0 24 44 80 400
Class 2 Cp = 0.5 13 17 20 25
(C= 0.1 Cp = 1.0 20 33 50 100
6 6 ~
Class 3 Cp = 0.5 5 6 l
(C = 0.2) Cp = 1.0 10 12 14 16