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Capture Recapture Calculator

Counting every fish in a lake is impossible, so scientists tag a sample, put them back, then catch a second sample and see how many carry tags. The fraction tagged in that second catch should match the fraction tagged in the whole population, and that single idea gives the estimate.

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    Worked Example

    40 fish are tagged and released. Later 50 fish are caught and 8 have tags. Estimate the population.

    1. In the second catch 8 out of 50 were tagged, a fraction of 0.16
    2. That same fraction of the lake should be tagged, and 40 were tagged in total
    3. So the population is 40 ÷ 0.16 = 250

    Answer: About 250 fish

    Try These Yourself

    20 tagged, 40 caught later, 5 tagged. Estimate the population.Show answerHide answer

    Answer: 160

    20 × 40 ÷ 5.

    Why must the tagged ones mix back in?Show answerHide answer

    Answer: So the second catch is fair

    Otherwise the sample is not representative.

    60 tagged, 30 recaught, 6 tagged. Population?Show answerHide answer

    Answer: 300

    60 × 30 ÷ 6.