An introduction to mark-recapture methods

When an absolute estimate of population size is required, these methods are the main alternative approach to those based on the count of animals within a fixed unit of the habitat. They offer the advantage that accuracy does not depend on an assessment of the amount of habitat; their disadvantage is that accuracy does depend on capturing a large proportion of the population.

Seber (1982,1986) reviews the statistical theory that underlies the mark-recapture approach. Originally developed to estimate population size, capture-recapture methods are now frequently used for the estimation of birth, death and emigration rates. Lebreton et al. (1992) reviews methodologies for the estimation of survival rates. Populations can be classified as either open or closed. A closed population remains unchanged over the period of study whereas an open population changes because of some combination of birth, death and emigration. Reviews of methods applicable to closed and open populations are those of White et al. (1982) and of Pollock et al. (1990) respectively. Descriptions of the application of the methodology to particular animal groups are given by, try Nichols et al. (1981) for birds, by Burnham et al. (1987) for fish and by, Eberhardt et al. (1979) for marine mammals.

A mark-recapture method was first used for ecological study in 1896 by Petersen to estimate plaice, Platichthys platessa, populations; later, Lincoln (1930) independently developed the method to estimate waterfowl populations. The principle used to calculate population size in these early studies, in which the population was assumed closed, still applies. If a sample from a population is marked, returned to the original population and then, after complete mixing, re-sampled, the number of marked individuals in the second sample would have the same ratio to the total numbers in the second sample as the total of marked individuals originally released would have to the total population. A basic prerequisite to the use of these methods is a technique for marking the animals so that they can be released unharmed and unaffected into the wild and recognised again on recapture.

Various assumptions underlie all methods of capture-recapture analysis. If the particular animal does not fulfil one or more conditions it might be possible to allow for this to some extent, but a method of analysis should not be applied without ensuring, as far as is practicable, that its inherent assumptions are satisfied. Parsimony, the use of the smallest possible number of parameters to model a situation, is essential, so though assumptions can be avoided by adding further parameters to a model, this inevitably leads to an increase in the variability of parameter estimates (Cormack ,1979; Begon, 1983).

1.	The marked animals are not affected (either in behaviour or life expectancy) by being marked, and the marks will not be lost.

The probability of capturing a marked animal is the same as that of capturing any member of the population; that is, the population is sampled randomly with respect to its mark status, age and sex. Termed 'equal catchability', this assumption has two aspects: firstly, that all individuals of the different age groups and of both sexes are sampled in the proportion in which they occur; secondly, that all the individuals are equally available for capture irrespective of their position in the habitat.

4.	Sampling must be at discrete time intervals and the actual time involved in taking the samples must be small in relation to the total time.

The violation of these assumptions may invalidate the capture-recapture method. Simply Tagging offers tests for equal catchability.