Advantages and Disadvantages of the Mark and Recapture Method

When wildlife biologists require absolute estimates of a population size, they rely on alternative approaches to those based on a count of animals within a fixed unit of a habitat. One such approach is the mark recapture method (also referred to as “capture-mark-recapture” or “mark-release-recapture”). Originally developed to estimate population size, mark recapture is now frequently used for the estimation of birth, death and emigration rates within populations.

The mark recapture method involves capturing a number of animals, marking them, releasing them back into the population, and then determining the ratio (proportion of marked to unmarked animals) of the population when marked and unmarked animals are captured at a later date. It has been used to estimate the populations of all manner of animals, from insects to the largest mammals.

The mark recapture method for estimating population size is summarized by the following equation:

                         N1 x N2

P = ------------------------

                             M

N1 = number of individuals marked initially

N2 = number of individuals in second or recapture sample

M = number of marked individuals in second or recapture sample

P = population estimate¹

In using the mark recapture method, populations can be classified as either open or closed. A closed population remains unchanged over the period of study, whereas an open population may change because of some combination of birth, death and emigration.

Use and Method Explained

The mark recapture method has a long history in wildlife research and management. Using this technique, individuals of a population are captured and marked with paint or a tag. The marked individuals are then released back into the wild and later recaptured. Information on the number of marked recaptures and unmarked captures can be used to estimate population size.

The mark recapture method is widely used in ecological studies to estimate survival rates within populations. For certain species, it is more commonly used as a tool for estimating population size. Most mark recapture data sets involve multiple sampling events, with each sample subsequent to the first providing information on recaptures as well as on individuals caught for the first time.^2,3

There are four assumptions common to most mark recapture scenarios:

• The subject (marked) animals are not affected (e.g., behavior or life expectancy) by being marked
• The marks (e.g., paint, tags) will not be lost
• The marked animals will become completely reintegrated into the population
• In subsequent captures, the probability of capturing a marked animal is the same as that of capturing any (unmarked) member of the population
• Sampling is carried out at discrete time intervals and the actual time involved in taking the samples is short in relation to the total study time¹

Advantages and Disadvantages of the Mark and Recapture Method

What are some advantages and disadvantages of the mark and recapture method? The mark and recapture method is often used to estimate population size from repeated sampling of the uniquely marked animals, but capturing and marking animals carries both advantages and disadvantages. Some of the advantages of mark and recapture include:

• Accuracy depends on capturing a fairly large proportion of the population.¹
• Many study populations are not geographically closed because only a part of a population’s range is included in the study area.
• Animals can move into and out of some study areas, and the number of animals present at a given time may be less than the population estimated.³

Due to these limiting factors, some have offered that the methodology can represent inaccurate estimation of population size in many circumstances. It has been argued that individuals individually captured may be more likely to be captured the second time than individuals that were not initially captured.³

Interested in learning more about safe capture? The San Diego Zoo now offers courses in safe capture techniques and best practices. Learn reliable, safe, and effective techniques for the species you work with and the scenarios you encounter!