Environmental Engineering Reference
In-Depth Information
2.3.4.3 Permanence of marks
When individuals are identified by external tags or marks, it is possible that these
may be lost. This is most likely to be a problem in studies focussing on survival
rates, in which longer intervals between captures are used, but may still be a prob-
lem when estimating the abundance of more delicate species that cannot easily be
permanently marked. Where natural markers such as genetic or coat patterns are
used, misidentified individuals can have a similar effect, which is to underestimate
recapture rate, and therefore overestimate abundance.
Fortunately, if the rate of tag loss or misidentification is known, it can be
corrected for at the analysis stage. If there is any suspicion that there might be
significant loss of marks during the survey, it is therefore important to measure
the frequency with which it occurs. This can be done by double-marking at least a
sample of the individuals caught, and recording the proportion of recaptures
that retain only one of the marks. Various models are available for estimating
rates of tag loss from patterns of double-tag retention (Bradshaw
et al
. 2000;
Rivalan
et al
. 2005).
2.3.4.4 Data requirements for mark-recapture
While simple mark-recapture methods can be applied using just a single recapture
occasion without identification of individuals, in practice this approach is almost
certain to yield strongly biased abundance estimates because of violations of the
assumptions. In order to guard against this, more complex modelling is required,
which requires more extensive data. This should usually be derived from several
capture occasions, with some means of recognising individuals in order to yield
complete capture histories for all individuals. In cases where individual-specific
Box 2.4
Closed population mark-recapture in action: tagged crayfish and
paint-balled elk.
Mark-recapture methods are most often applied to small or medium-sized
animals that can easily be caught in large numbers and individually marked. Jones
et al
. (2005) provide a good example of this, applied to a sustainability assessment
of crayfish
Astacoides granulimanus
harvesting from forest streams in Madagascar.
Streams were searched in 100 m sections, and all crayfish larger than 22 mm were
marked with an individually recognisable visible implant elastomer tag. A total
of 74 sections were covered, across a range of harvesting intensities, and each was
visited five times on consecutive days. This was a sufficient number of visits to
provide the information necessary to estimate capture probability, with gaps
between visits long enough to allow the population to mix, but short enough to
justify the assumption of a closed population (i.e. no births, deaths or migration).
In total, 26,096 crayfish were captured 44,286 times. In the analysis, a range
