Estimating
an animal population
Capture/Recapture
This method of sampling is most useful when dealing with an animal population
that moves around. In this method individual animals are marked or tagged
and then released back into the environment. After leaving time for dispersal
the population is then resampled and the number of tagged animals in the
sample gives an estimate of the entire population.
The following equation is used to estimate the population
Population = M x
n
m
Where M = number of captured tagged and released organisms
n = number samples the
second time (tagged and untagged)
m = the average number of tagged animals
recaptured
In this field study we are going to estimate the number of a species
of leatherjacket. This is a good species to catch because they survive
netting well. Other species can be killed by the process of netting so
they would have to be sampled in another way.
Leatherjacket
The leatherjackets
were sampled using the netting method shown below.
The netting was done three times and each time 10 Leatherjacket were
caught and tagged. Therefore the total of tagged fish was 30. After a week
the area was resampled and out of 30 leatherjackets caught 5 were tagged.
Applying the formula above:
Population = M x
n
m
Where M = number of captured tagged and released organisms =
30
n = number sampled
the second time (tagged and untagged) = 30
m = the average number of tagged
animals recaptured = 30/5 = 6
Total population =
