CAPTURE MARK RECAPTURE METHOD

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The Lincoln Index

• CAPTURE MARK RECAPTURE METHOD

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The Lincoln Index

• Used to estimate the size of an animal
  population.
• Capture animals
• Mark them with a tag
• Release them and wait a period of time
• Capture animals again

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Assumption

• The proportion of animals caught to the whole
  population is the same as the marked animals in
  the second sample.
• First Capture Marked animals, second
  capture
• Whole population second capture

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More assumptions

• The population of organisms must be closed, no
  immigration or emigration.
• The time between samples must be small compared
  to the life span of the organism
• The marked organisms must mix completely with
  the rest of the population during the time
  between sampling.

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Equation

• n1 m2
• N n2
• n1 number of animals first marked and released.
• n2 number of animals captured in the second
  sample
• m2 number of marked animals in the second
  sample
• Solve for N n1x n2 to find the total
  population
• m2

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Limitations

• Marking or capturing the animal might change its
  behavior
• The mark might be toxic or cause predators to see
  it
• Marks might cause the animal to be rejected by
  its own population
• Trapping might cause the animal to avoid the
  traps in the future or to choose to be trapped to
  get food!

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Investigation with beans

• Separate black and white beans
• Count 150 black beans into a cup
• Capture some black beans, n1
• Trade the captured black beans for white beans to
  mark them.
• Release the marked beans into the cup
• Capture a second sample, n2
• Count the number of marked beans in the second
  sample, m2
• Repeat for a second and third trial, change the
  size of the capture

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Data table Numbers of animals
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Data Processing Calculate N
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Data Analysis

• How does the size of the capture affect the
  accuracy of N?
• Does the size of the first capture affect the
  accuracy more than the size of the second capture
  or vice versa?

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Analysis of results

• The Lincoln Index is more reliable if many
  animals are captured and released.
• This is a problem because the animals do not
  usually like to be captured, nor are their
  actions random.

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Example

• pond turtles 1992 18
• 1993 34 (12)
• 1994 30 (18)

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Results1992 18 1993 34 (12) 1994
30 (18)

• 1992-93 N 18(34) 51 turtles
• 12
• ____________________________________
• 1993-94 N 34(30) 56.7 turtles
• 18
• ____________________________________
• So 51 56.7 107.7 53.9 turtle estimation
• 2