7116 ‎(2)‎

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

34

is to estimate relative population changes. Bohlin
(1981) suggested the following method, applied to
SRS with no auxiliary variable:

^ ^ ^
R = X/Y

(27)

^ ^
where X and Y is from eo. (21). The approxi-
^ -" ^
mate sampling variance VCR) of R is

VCR) = V(Y)X2 + V(X)y2 - 2rJV(Y)V(X)' XY
X4
(28)

^ ^ ^ ^
Here, V(Y) and VeX) is from eq. (22) and r is
the coefficient of correlation between x and y,
calculated in the usual way. Eq. (28) is approxi-
mate even for large samples. Note that a large
positive correlation will increase the precision
substantially.

On the choice of sampling methods with special
reference to large streams

Let us first recall the information required to
apply the three main methods proposed:
SRS - no auxiliary variable: N
SRS - auxiliary variable used: N, total area M,
section size mj for the n sections in the
sample
PPS - Section size mj for all N sections.
For large streams (N large) the PPS methods
will thus be impractical, so the options are the
SRS methods. The applications of these methods
to large streams, however, is not self-evident. In
many cases, electrofishing is possible only in
some areas, mainly along banks and in other
shallow areas where the current is not excessive.
In order to obtain the M value, the whole stream
has to be visited and measured with respect to
area and depth relations. Although this can be
simplified using e.g. aerial photography it may still
be a formidable task. If so, this leaves us to the
least precise of the methods, the SRS without
auxiliary variables. A practical compromise may
be the following in the case of large streams:

Fig. 9. A suitable sampling design in a large stream may be
to divide the stream into areas of approximately equal length
using a map, in this case N = 12 sections. A sample of n
sections (here n = 3) is drawn by simple random sampling.
The areas mj in which e1ectrofishing is possible (strippled) is
measured. For some estimates the total strippled area M
must be known (see text).

U sing a reliable map, the stream is divided into
N sections of approximately equal length (see
Fig. 9) from which are drawn a random sample of
n sections after stratification. As the 'functional'
width of the latter may vary greatly depending on
depth etc., the area mj of the random sample are
also measured. The population density within the
stratum or target area is then estimated from eq.
(23'). The sampling error of this estimate, how-
ever, cannot be estimated unless M is known (eq.
24'), so if an estimate of the sampling error is
important M has to be measured. In many cases,
however, it may be more useful to estimate popu-
lation change rather than absolute density. As
stated previously, the SRS without auxiliary varia-
ble may work well in this case provided a fair
correlation between x and y, so M may not have

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