Mark Recapture Method

Mark-Recapture method

For organisms that move around, such as mammals, birds, or fish, a technique called the mark-recapture method is often used to determine population size. This method involves capturing a sample of animals and marking them in some way—for instance, using tags, bands, paint, or other body markings, as shown below. Then, the marked animals are released back into the environment and allowed to mix with the rest of the population.

Later, a new sample is collected. This new sample will include some individuals that are marked—recaptures—and some individuals that are unmarked. Using the ratio of marked to unmarked individuals, scientists can estimate how many individuals are in the total population.

Example: using the mark-recapture method

Let’s say we want to find the size of a deer population. Suppose that we capture 80 deer, tag them, and release them back into the forest. After some time has passed—allowing the marked deer to thoroughly mix with the rest of the population—we come back and capture another 100 deer. Out of these deer, we find that 20 are already marked.

If 20 out of 100 deer are marked, this would suggest that marked deer—which we know are 80 in number—make up 20% of the population. Using this information, we can formulate the following relationship:

This approach isn’t always perfect. Some animals from the first catch may learn to avoid capture in the second round, inflating population estimates. Alternatively, the same animals may preferentially be retrapped—especially if a food reward is offered—resulting in an underestimate of population size. Also, some species may be harmed by the marking technique, reducing their survival. The approach also assumes that animals don’t die, get born, leave, or enter the population during the period of the study.

Alternative approaches to determine population size include electronic tracking of animals tagged with radio transmitters and use of data from commercial fishing and trapping operations.