Geoscience Reference
In-Depth Information
Much of the development in mark-recapture methods has therefore
been driven by the goal of modeling changes of population param-
eters in terms of one or more covariates (explanatory variables).
Along with these basic concepts, methods of capture-recapture share a
number of theoretical assumptions:
1. All marks are permanent and are correctly recognized in the recap-
tures. This assumption applies only for the period of study.
2. There is no effect of the fact that an individual is caught, handled,
and marked on one or more occasions on its later probability of cap-
ture. In other words, individual catchability is not affected by being
caught.
3. The possibility that an individual dies or migrates is unaffected by
its handling, and any migration is permanent and therefore indistin-
guishable from mortality.
4. All the animals, marked or not, have the same probability of being
captured. This means that the sampling is effectively done at ran-
dom, regardless of age, sex, or the physical condition of individuals.
5. All individuals, marked and not, have the same probability of dying
or emigrating. That is, movements out of the population are not
affected by whether individuals are or are not marked.
7.3 Closed-Population Methods
7.3.1 The Petersen-Lincoln Method
As noted above, a population is said to be closed if there is no change in the
number of individuals in the population during the study. That is, there are
no deaths, recruitment, emigration, or immigration in the population.
Suppose that a random sample of
n
1
animals is taken from a population
consisting of
N
animals. These animals are then marked and released back
into the population. On a second capture occasion, a sample of
n
2
animals
is taken from the population, of which
m
2
are found to be marked. Then,
it might be expected that the proportion of marked animals in the second
sample will be approximately the same as the proportion in the population.
That is,
m
2
/
n
2
≈
n
1
/
N
, so that
N
≈
n
1
n
2
/
m
2
. It follows that an estimate of the
population size is given by
ˆ
Nnnm
12 2
.
/
( 7.1)
