Why Does the Capture Recapture Method Work?

Capture-recapture methods have a long history in the study of fish and wildlife populations. The simplest of these techniques is known as the two-sample model, which is used to estimate the unknown size of a population. The first sample provides individuals for marking and is returned to the general population, while the second sample provides recaptured individuals. Using the numbers of individuals caught in both samples plus the numbers caught in one sample, it is possible to estimate the number that were not caught in either sample, providing an estimate of the total population size.

This is represented by the equation R(recaptured)/C(captured in 2nd sample)=M(marked initially)/N(total number in population). In this equation, the numbers marked, captured, and recaptured are known, so the equation may be rearranged to give N=(MxC)/R.¹ There are many and more complex variations on this equation which allow for open and closed populations, as well as seasonal and field conditions and sampling requirements.

Application and Efficiency

There are three assumptions required for estimates using the above equation to be valid:

1. There is no change to the population during the investigation (the population is closed).

2. There is no loss of tags, so that individuals can be reliably matched from capture to recapture.

3. In each sample, each individual has the same chance of being captured.

It is important to bear these assumptions in mind as results are interpreted. Additionally, there are precautions and requisites that also have a bearing on the accuracy of results.

• Individuals that are tagged or marked have the same probability of survival as other members of the population. Thus, it is important to choose marking methods that do not harm the animal or decrease its chances of survival, such as a brightly-colored mark or tag that makes it easy for predators to spot the animal.
• The intervals between samples or sample groups must not be sufficiently long that births and deaths occur in significant numbers between the time of release and the time of recapture.
• Immigration and emigration to and from the population do not occur in significant numbers between the time of release and the time of recapture.
• Marked individuals continue to mix randomly with the population at large.
• Marked animals are neither easier nor more difficult to capture a second time. If marking an animal frightens it sufficiently and is a species that can successfully evades capture a second time, then recaptures will be underrepresented in the second sample.
• Marks or tags do not come off of the marked animals. Invertebrates can molt and shed marks, while fish and some mammals can lose tags. If this occurs, recaptures will be undercounted, resulting in higher, inaccurate estimates.
• Recapture rates are high enough to support an accurate estimate.¹

This model was originally developed by Z.E. Schnabel in the 1890s to estimate the size of fish populations.² It was later refined by Peterson and by Lincoln in the 1920s to estimate wildlife populations, thus, it is widely known as the Lincoln-Peterson Index. This model is based on the principle that the proportion of marked individuals in the second sample is equivalent to the proportion of marked individuals in the total population. For estimates of absolute numbers, mark-recapture methods have been considered very effective and reliable, hence their longevity.

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!