7116 ‎(2)‎

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

26

Sampling design and stock assessment in streams

Introduction

So far we have been dealing with the question of
how to estimate the fish population in a closed
area in a stream or in the litoral zone of a lake. We
have also found methods well suited for this
purpose, at least for some species.
It is evident, however, that even if an exact
estimate were obtained from, say, pool 'A', this
figure is in most cases quite uninteresting. It
becomes useful first when we from this figure can
say something about the fish population in the
stream. It is also evident that if the population is
heavily varying from site to site, then we have to
study several such sites to make conclusions
about the whole population. So first we have to
define the population about which statements are
to be made, and then decide the size and number
of sites to be investigated, and finally find out how
to select these sites and how to calculate popula-
tion parameters from the result.
Too little attention has been paid to these
questions (Bohlin, 1981; Hankin, 1984), though
great savings may be the result if the field work is
well organized (or great losses if it is not).

The target area

The first crucial point in the planning of a
sampling programme is to define the target area
(sampling universe). The target area is the total
stream area (or in some cases stream length) that
one wishes to make statements about and in
which it is possible to carry out quantitative elec-
trofishing. In most applications the aim is to make
statements of the population, or to compare e.g.
'disturbed biotopes' with 'natural biotopes'. In the
first case the target area is the total fish-producing
area in a specific stream (minus the fish-producing
areas in which electrofishing is impossible). In the
latter case there are two target areas, of which the
first one is the total area of 'disturbed biotopes' in
the stream and the second the total area of
'natural biotopes'. In large streams it is important

to note that statements about the fish population
are limited to the population fraction in areas
(biotopes) where electrofishing is possible, so that
the target area may comprise only a minor
fraction of the total stream.
Even small streams may be to large for a total
survey, and one has to select the sampling areas
and determine the number and size of these. On
the other hand, if the target area is very small a
total survey may be practical, and no subsampling
is needed.

A

o RIFFLE BIOTOPE
EJ POOL BIOTOPE

34
35

36

. 10
'. 11

r

I

<)
f
r

..,
(

Fig. 8. A stream divided into N sampling areas using (a)
units of equal length, (b) units of a size varying according to
biotope variation. A sample of units may be obtained using
simple random sampling in (a) and (b) or by proportional
probability sampling (b). The target area is the whole stream
except for the deepest part of the pool in the middle of the
stream. See also Fig. 9.

~

(