---
title: "Estimating Population Size by Mark, Release and Recapture"
description: "A quadrat cannot count animals that run away, so mobile populations are estimated from the proportion of marked individuals that turn up in a second sample. The estimate is only as good as its assumpt"
canonical: https://lightmysky.com/learn/science/estimating-population-size-by-mark-release-and-recapture-mt_SrISE2trf9
source: https://lightmysky.com/learn/science/estimating-population-size-by-mark-release-and-recapture-mt_SrISE2trf9.md
retrieved: 2026-09-12
---

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# Estimating Population Size by Mark, Release and Recapture

A quadrat cannot count animals that run away, so mobile populations are estimated from the proportion of marked individuals that turn up in a second sample. The estimate is only as good as its assumptions, and each one of them can be broken by a careless mark.

Subject: Science · Area: Scientific Inquiry · Ages 15 to 16
Page: https://lightmysky.com/learn/science/estimating-population-size-by-mark-release-and-recapture-mt_SrISE2trf9

## Ready when they can

- Calculate a population estimate from the numbers marked, recaptured and marked-within-recaptured
- State the assumptions the method rests on, including mixing, survival and no loss of marks
- Explain how a mark that makes an animal easier for a predator to see biases the estimate, and in which direction

## Lesson: Counting animals that run away

A quadrat counts life that stays put: drop a square frame at random spots, count inside, and scale the average up. Mobile animals like fish or beetles need traps instead. Catch a sample, mark each animal harmlessly, release them, and come back later for a second catch.

**Example.** Say 20 fish were marked, and the second catch of 30 fish holds 6 marked ones. The marked fraction is 6 in 30, so the total is 20 times 30 over 6, which is 100. Total equals marked times second catch divided by marked inside the recatch.

The tidy maths only works if three promises hold. Marked and unmarked animals mix fully, so the fraction matches everywhere. Nothing changes between visits: no births, deaths, arrivals, or departures. Marks stay on and stay harmless, which is why studies keep gaps short and tags tiny.

**Tip.** Broken promises lean predictably. A mark that lures predators means fewer marked return, the bottom of the fraction shrinks, and the estimate inflates. Report about 100, never exactly 100: an estimate is not a census.

**Recap.** Mark, release, recatch, scale up the fraction, and trust the total only as far as its promises hold.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Sampling Ecosystems with Quadrats and Transects](https://lightmysky.com/learn/science/sampling-ecosystems-with-quadrats-and-transects-mt_jI8rAOYdmb)
- [Communities: Interdependence, Competition and Abiotic Factors](https://lightmysky.com/learn/science/communities-interdependence-competition-and-abiotic-factors-mt_su-CNKV8hK)