What is the Mark-Recapture Technique?

The Mark-Recapture technique is a method used to estimate the size of populations, and the population dynamics of wild animals when it is impractical (or impossible) to physically examine each individual animal. Sometimes called Capture-Mark-Recapture (CMR), it is a survey method in which the count statistic is the total number of animals caught, and the associated detection probability is the probability of capture.¹

In the operative sense, Mark-Recapture 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. This is expressed using the following equation, known as the Lincoln Index:

N = (M*C) / R, where

N = the estimated number of individuals in a population

M = the number of individuals initially captured and marked

C = the total number captured the second time (with and without a mark)

R= the number of individuals recaptured (with a mark)¹

Mark-Recapture techniques have a long history in wildlife research and management of animals such as small invertebrates and fish because they can be used to estimate population size as well as population survival and growth. Individuals of a population are typically captured using a sampling method 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.

Mark–recapture analysis is widely used in ecological studies to estimate survival rates within populations. For certain species (e.g., marine mammals), 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 several assumptions that are made when estimating the size of population via Mark-Recapture, some of which by nature have a tendency to skew results:

• The population must be closed (there is no migration of individuals into or out of the population).
• All members of the have an equal chance of capture (e.g., they mix randomly, which is often not the case based on sexual behaviors, age, etc.).
• Identifying materials (e.g., tags, paint) are not lost between samplings.
• The identifying material does not harm the animal (e.g., compromise its health or make it more vulnerable to disease, predators).
• The identifying materials do not affect chances of recapture.²

Limitations to Mark-Recapture

Many Mark-Recapture study populations are not geographically closed because only a part of a population’s range is included in the study area. Animals typically move into and out of the study area, and the number of animals present at a given time may be less than the population estimated.

Due to some of these limiting factors (and the assumptions described earlier), 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. Some have argued that there should also be considerations (e.g., alternative equations) allowing for inequality of probability of capture due to such factors as behavior, while others hold that even births and deaths within a closed population tend to significantly skew results.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!