Mark and recapture methods were developed to study animal populations for which it was not feasible to estimate population size just by counting all the individuals.

Basically, these methods consist of capturing part of the population by means of traps. Individuals are captured alive, marked individually and then released. This routine is repeated several times in successive days and note is taken of the individuals recaptured (i.e., captured more than once). Based in the total number of individuals captured and of individuals recaptured population size can be inferred, as well as individual survival and other demographic variables of the population.

This method is very easy to utilise, provided that live trapping and permanent marking of individuals can be carried out.

Data to be recorded are number of individuals captured at each sampling date, and the number of those which had been previously captured, i.e. those having already a mark. In addition, other basic data on the individuals can be recorded, such as sex, size or condition.

Population size estimates from those data are a bit complicated and there are several alternative algorithms. We will not provide these details here; interested readers should check the specialized literature. Two references can be useful as a starting point:

Krebs, J. R. 1989. Ecological methodology. HarperCollins, New York.

Sutherland, W. J. (ed.) 1996. Ecological census techniques: a handbook. Cambridge University Press, Cambridge.