Environmental Engineering Reference
In-Depth Information
2.4.2.2 Using marked individuals
There are two basic forms of mark-recapture that can be used to estimate survival
rates. First, individuals may be recaptured or resighted alive at discreet intervals.
Second, individuals may be recovered dead, often through harvest. It is also pos-
sible to use both types of information in combination. Williams et al . (2002) and
Amstrup et al . (2006) provide comprehensive details on the current state of the art
in these methods. The methods are implemented in widely available free software
(especially MARK; Section 2.7.1), and we refer the reader to these programs and
their associated literature for detailed advice on the their practical application (in
particular, the online introduction to MARK by Cooch and White 2006). Here we
outline the basic concepts, and briefly discuss the most important considerations
relevant to designing a field study.
Live recaptures
Survival analysis based on recapture or resighting is often referred to as the
Cormack-Jolly-Seber (CJS) model, after those who pioneered the approach. It
works by using information over several discrete observation occasions to separate
the probabilities of observation and survival. Imagine that 100 animals are caught
and marked, and that during a second capture session, some time later, 30 of them
are seen again. This indicates that the probability of being both alive and observed
is 0.3, which could mean anything between 100% survival rate with 30% chance
of being seen, and 30% survival rate with all of the survivors being seen. At this
stage we can say no more than this. Suppose that a third capture occasion produces
18 individuals that were marked on the first occasion, only nine of which were also
seen on the second occasion. This suggests that the probability of observing a
marked individual was 9/18
0.5. Given that the probability of being both alive
and observed is the product of the survival and observation probabilities, the sur-
vival rate can now be estimated as 0.3/0.5
0.6. The basic data requirement for
this type of survival analysis is therefore a set of individual capture or observation
histories over three or more occasions.
The method is closely related to the closed population mark-recapture method
of abundance estimation, and apart from population closure, the assumptions
of that method also apply here (see Section 2.3.4). In addition, mark-recapture
survival analysis assumes that:
The marked individuals are a random sample of the wider population;
Sample occasions occur more or less instantaneously relative to the gaps
between them.
Random sampling of marked individuals is necessary if the results are to be
assumed representative of the population as a whole. If the capture method is
biased towards certain subsections of the population, inference about the wider
population will be impossible. The most worrying example of this is when the
capture or marking procedures themselves create a sub-population by reducing
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