Capture-Recapture Method

Capture – recapture

Learning objectives

• How does ‘Capture-Recapture technique’ work to estimate the total population?
• Predict the total population using the formula given and reasoning
• List some factors that can affect the accuracy and precision of a population estimate

A quick recap

1. [pic 1]
2.  = [pic 2][pic 3]
3. of the class is going on the field trip. If 24 students go on the trip, how many students are in the class?[pic 4]

Questions to be answered

How can we estimate the total population of fish in a lake?

How can we estimate the total population of squirrels in a park?

How can we estimate the total number of black taxis in London?

How can we estimate the total population of kangaroos in a forest?

Phew

This is a familiar problem for ecologists and it’s solved using a method called Capture-Recapture, devised by Danish marine biologist Carl Georg Johannes Petersen in 1896 to study European plaice in the Baltic Sea and later proposed by Lincoln (1930) to estimate numbers of ducks

Have a look please : https://www.youtube.com/watch?v=tyX79mPm2xY 

[pic 5]

The principle

The PROPORTION of marked individuals in the second sample should be the same as the proportion of marked individuals from the first sample in the total population

[pic 6]

Cases:

1. If many of the animals you capture the second time are tagged, then you have probably captured a large portion of the total population

[pic 7]

1. If few of the animals in the second capture are tagged, then the total population is probably much larger than the size of your capture

[pic 8]

Extension:

[pic 9]

[pic 10]

Assumptions :

• There is no death, immigration or emigration (ie the population is closed)
• The sampling methods used are identical
• The marking has not affected the survival rate of the animals 
• Animals do not lose their marks
• Marking does not affect the likelihood of recapture
• Sufficient time is left between marking and recapture for all marked individuals to be randomly dispersed throughout the population

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