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# Capture - Recapture

Capture - Recapture. Capture-recapture is a method commonly used in ecology to estimate an animal population’s size. How does it work?. A portion of the population is captured, marked, and released.

## Capture - Recapture

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1. Capture - Recapture Capture-recapture is a method commonly used in ecology to estimate an animal population’s size.

2. How does it work? A portion of the population is captured, marked, and released. Later, another portion is captured and the number of marked individuals within the sample is counted. The number of marked individuals within the second sample should be proportional to the number of marked individuals in the whole population. An estimate of the total population size can be obtained by using a proportion.

3. Example • You have released 100 tagged fish into the pond. You must assume these tagged fish distribute themselves randomly and evenly throughout the pond. • When you pull 100 fish out of the pond you find that 12 of them are tagged. • There are several ways you can look at this. Since you know there are 100 tagged fish in the pond, how many groups of 100 fish will you have to remove to recapture all 100 of the tagged fish? • At 12 tagged fish per group you will have to haul in groups. Since there are 100 fish per group this translates to a total of about 833 fish in the pond. • You would need to pull 9 groups.

4. Another way to solve the problem is using a proportion! • For every 100 fish removed from the pond, 12 of them are tagged. The ratio of 100 fish to 12 tagged fish should reflect the composition of the total pond. • Again, you know that there are 100 total tagged fish in the pond. • Call the total fish population F. • You can then set up and solve the proportion to find that there are approximately 833 fish in the pond. • = Or • You must keep like quantities in the same position. OR…

5. Discuss with your group: How could you make a more accurate estimate using the capture-recapture method?

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