Capture-Recapture 09/08/2019 Learning Objectives: Able to use the capture-mark-recapture method for estimating population size

Capture-Recapture How can we estimate the number of the following? Fish in a lake? Deer in a park? Horses in the New Forest? Sweets in a jar? Black taxis in London?! 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

https://www.youtube.com/watch?v=tyX79mPm2xY

Capture-rECAPTURE The method is: Take a sample of the population Mark each item Put the items back into the population and ensure they are thoroughly mixed Take a second sample and count how many of your sample are marked The proportion of marked items in your new sample should be the same as the proportion of marked items from the population in your first sample

Capture-Recapture Example: 10 fish are caught in a lake, marked and released back into the lake. A week later, 20 fish are caught and 4 are found to be marked. Estimate the number of fish in the lake. 10 𝑛 = 4 20 𝑛= 10×20 4 𝑛=50 There are approximately 50 fish in the lake

Capture-Recapture Cameron wants to know how many snails are in his garden. He collects 5 of them and marks them and puts them back in the garden. A few days later he collects 10 snails and finds that 2 of them are marked. Estimate the number of snails in Cameron’s garden. 5 𝑛 = 2 10 𝑛= 5×10 2 𝑛=25 State two assumptions that must be made in order for this estimate to be as accurate as possible.

Capture-Recapture 09/08/2019 Answer the questions from the worksheet in your classwork book to crack the code and reveal YET ANOTHER hilarious joke!