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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covering drastic differences, the same precision
may be sufficient. In the planning of surveys it is
therefore important to state the precision require-
ment as clearly as possible and try to design the
work for the precision level chosen. One way to
classify the precision is the following'. C is the
coefficient of variation as defined above.

Class 1. With this precision, a population change
of a factor of '" 1.2 in time or space (e.g.
83-100-120) is discovered in '" 95 cases of 100.
This corresponds, roughly, to C = 0.05 for inde-
pendent estimates. This precision may be required
if the estimate is to be divided or multiplied with
other estimates, or when good precision is called
for.

Class 2. With this precision, a population change
of a factor '" 1.5 in time or space (e.g.
67 -100-150) is discovered in '" 95 cases out of
100. This corresponds, roughly, to C = 0.10 for
independent estimates. This level is suitable in
many cases, e.g. when classifying fish density in,
say, 4-5 quality classes from 'very bad' to 'very
good'.

Class 3. With this precision, a population change
of a factor", 2 in time or space (e.g. 50-100-
200) is discovered in '" 95 cases out of 100, corre-
sponding to C = 0.20 for independent estimates.
This precision may be sufficient when classifying
fish densities as 'bad', 'intermediate' and 'good',
or when the object is to give alarm if a population
is reduced to less than half of its original size.
These precision classes will be referred to in the
folJowing text, especially in the section concerning
the sampling design.

Mark-recapture methods

We now turn to one of the methods of assessing
the population size in a closed area, the mark-
recapture methods. These are based on the recap-

I Based on normality of the estimates and not to small
samples.

ture of a known number of marked or tagged
individuals. In the more sophisticated versions,
requiring individual tagging or at least batch
marking and repeated marking-recapture (see
Seber, 1973; pp. 59-292), it might be possible to
estimate migration and mortality in addition to
population size. As electrofishing usually is used
to assess the number of small fish (for which
individual tagging is often difficult) in closed
sections of streams, the simplest mark-recapture
estimator called the Petersen method may be
used. If m individuals are caught, marked (e.g. by
fin clipping) and released, and if c individuals are
caught on a second occasion, of which r are found
to be marked, an estimate y of the populations
size y is

y = mc/r

(1)

An estimate of the sampling variance V(y) of Y
IS

V(y) = y2(y - m) (y - c)
mc(y - 1)

(2)

and of the Standard Error SE(y)

SE(y) = JV(y)

(2')

Approximate 95 % confidence limits are

^
^ + 2SE(^)
y - y

(3)

The catch probability, or catch ability, p, in the
second catch is estimated as

p = rim

(4)

Example 1. m = 66 trout yearlings were marked
and released into a closed section of a stream.
Later c = 54 trout were captured, of which r = 32
were marked. Then

y=1I1.4
P = 0.485

^ ^
SE(y) = 9.07
95 % confidence limits 93.3 - 129.5
^
C = SE(y)/y = 0.081 or 8.1 %

I .

I

1

y

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