Geoscience Reference
In-Depth Information
0.651 ± 1.96 × 0.0675 or 0.519 to 0.784. It is also possible to estimate the
1965 survival rate. For this,
S
ˆ
2
= 0.631, with standard error 0.0647, and the
approximate 95% confidence interval is 0.504 to 0.758.
8.5 Estimation Using Radio-Tagged Individuals
Advances in technology in recent years have made it possible to monitor
the movement of even small animals by fitting them with radio-tags. This
allows the use of mark-recapture methods in which the animals fitted with
transmitters are thought of as marked but there is a substantial advantage
over more conventional methods of marking because it becomes possible to
know which marked animals are present in a study region without needing
to take into account survival and migration.
Essentially, the radio-tagged animals that are known to be in the study
region when a sample of animals is taken serve as the marked animals for
the estimation of population size. Formally, either the Petersen-Lincoln esti-
mator
N
ˆ
=
n
1
n
2
/
m
2
or, better still, the bias-corrected estimator
N
ˆ
*
= (
n
1
+ 1)(
n
2
+ 1)/(
m
2
+ 1) - 1,
(8.18)
is used where
n
1
is the number of marked (radio-tagged) animals available for
capture in the area,
n
2
is the number of animals captured in a sample from
the region, and
m
2
is the number of marked animals found in the sample of
size
n
2
. This is then a valid procedure providing that the probability of an
animal being included in the second sample of size
n
2
is the same irrespective
of whether or not it is radio-tagged. Furthermore, the standard error for the
estimator from Equation (8.18) can be approximated using the usual equation
·
ˆ
{
}
*
SE(
N
)
=
(
n
+
1)( )(
n
+
n mnmmm
−
)(
−
)/ ( )(
+
+
2)
.
(8.19)
1
2
1
2
2
2
2
2
If
k
independent surveys of the study area are made at different times to
find animals, then each one of these surveys can be treated as the second
sample for population estimation. The surveys will then provide
k
indepen-
dent estimates
ˆ
ˆ
ˆ
k
*
*
*
NN N
,
,
,
of the population size, with corresponding esti-
…
1
2
··
·
ˆ
ˆ
ˆ
*
*
*
mated standard errors
.
The appropriate treatment of the series of population size estimates will
depend on whether the population is open or closed. If the population is
open, then the series of estimates can be used to monitor the size changes.
This should give better results than the JS method because the number of
marked animals available for capture is known instead of being estimated.
On the other hand, if the population is closed, then all of the estimates should
SE(
NN N
), SE(
),
…
,SE(
)
1
2
k
