--- title: "Estimating Population Size by Mark, Release and Recapture" description: "Estimate a mobile animal population from the marked proportion in a second catch. Calculate the estimate, check its sampling assumptions, and explain how lost or harmful marks can bias the result." 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Estimating Population Size by Mark, Release and Recapture Estimate a mobile animal population from the marked proportion in a second catch. Calculate the estimate, check its sampling assumptions, and explain how lost or harmful marks can bias the result. 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 number marked in the first catch, the total in the second catch and the number marked in the second catch - State the assumptions about population closure, mixing, capture chances, survival and retained 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 Quadrats sample organisms that stay still or move slowly. Transects help investigate change along a habitat gradient. For suitable mobile animals, mark and recapture offers another way to estimate a population. Researchers capture a sample, mark and release it, allow mixing, then capture another sample and count its marked animals. Handling and marking need appropriate permission and animal care. **Example.** Researchers mark and release 16 fish. A later catch contains 24 fish, including 4 marked fish. The marked share is 4/24, or 1/6. If this represents the population, the 16 marked fish are about one sixth of the total: 6 × 16 = 96 fish. Use first marked count × second catch size ÷ marked count in the second catch. A zero marked count cannot be used as the divisor. *(drawing: Four of the 24 recaught fish were marked. Shade one sixth to represent that share, not individual fish.)* The two-catch method assumes no births, deaths, arrivals or departures between catches. Marks must remain readable and must not change survival or capture chances. Marked and unmarked animals need comparable chances of capture. Mixing should let the second catch represent the population's marked share; small groups do not all need to have the same fraction. Allow time for mixing without a long gap that permits major population change. With catch sizes fixed, fewer recognised marks in the second catch make the estimate larger; more make it smaller. Lost marks or extra predation on marked animals can therefore cause upward bias. A catch from a marked-heavy release area before mixing can cause downward bias. These are expected directions under the stated conditions, not a guarantee for every sample. Report an estimate rather than an exact census total. Chance affects which animals enter the samples even when the assumptions hold. Explain the method, uncertainty and suspected bias. Repeated, well-designed sampling helps assess uncertainty, but repeating a biased method does not remove its bias. **Recap.** Estimate the total from a second catch's marked share. Check mixing, capture chances, population change and marks, and report uncertainty. *Lesson, questions, worked solutions and flashcards written for this dataset. Population-demography sources informed the facts and teaching design.* ## 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) ## Optional connections - [Communities: Interdependence, Competition and Abiotic Factors](https://lightmysky.com/learn/science/communities-interdependence-competition-and-abiotic-factors-mt_su-CNKV8hK)