Mark-Recapture Population Calculator

The mark-recapture method (also called capture-recapture) is a technique used in ecology and wildlife biology to estimate the total size of a population. A portion of the population is captured, marked, and released. Later, a second sample is taken and the proportion of marked individuals in this second sample is used to estimate the total population size.

This calculator uses the Lincoln-Petersen estimator: N̂ = (M × n) / m, where M is the number marked and released in the first survey, n is the total captured in the second survey, and m is the number of marked individuals recaptured in the second survey. A Chapman correction can also be applied for small samples to reduce bias.

A population estimate is a statistically derived value representing the likely total size of a population, based on sampling data. It is not a guess — it is a calculated approximation with a measurable level of uncertainty expressed through confidence intervals. Managers use population estimates when a full census is impossible.

Animals can be marked in many ways depending on the species: physical tags (ear tags, fin tags), bands on birds, paint or dye, radio collars, PIT tags, or natural markings like fin shapes and coat patterns. The key requirement is that marks must be permanent, non-harmful, and identifiable upon recapture.

The Lincoln-Petersen method relies on several key assumptions: (1) the population is closed between surveys (no births, deaths, immigration, or emigration), (2) marks are not lost and remain visible, (3) all individuals have an equal probability of capture, and (4) marked individuals mix randomly with the unmarked population before the second survey.

The confidence interval gives a range within which the true population size is likely to fall. A 95% confidence interval means that if you repeated the sampling process many times, 95% of the resulting intervals would contain the true population size. Narrower intervals indicate more precise estimates.

If very few marked individuals are recaptured in the second survey, the population estimate will be very large and highly uncertain. This usually means either the population is very large, sampling effort is too low, or the assumptions of random mixing are not being met. A very small m value leads to wide confidence intervals and less reliable results.

Yes. Capture-recapture methods are widely used beyond wildlife biology. Epidemiologists use them to estimate the size of hidden populations such as people who inject drugs, homeless individuals, or unreported disease cases. The mathematics is identical — two independent survey samples are compared for overlap to estimate a total count.