How to Determine the Size of an Animal Population
Determining stocking numbers for fish or whether your turtle or frog population is declining year after year can be important. Whether it is estimating the number of fish or frogs in a pond in a person's yard or a large lake, the exercise known as mark-recapture is relatively easy to do.
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Instructions
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Capture as many animals within an area as possible. If necessary, constrain your search by time. For example, use a net to capture fish for one hour, or walk along the shore capturing frogs for 20 minutes.
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Give each animal a unique mark. This can be done by clipping toes, tails, or fins, or banding.
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Count the number of animals collected.
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Release the animals back into the area where you collected them.
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Wait a day or more for the marked animals to redistribute themselves among the non-marked animals.
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Collect animals again, using the same methodology and time constraint that you used before.
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Count the marked and unmarked animals.
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Use the following equation to estimate the population size:
n1 x n2 / m = N
n1 is the number of animals captured during the first visit
n2 is the number of animals captured during the second visit
m is the number of animals on the first visit that also were collected on the second visit -- the marked animals
N is the population estimateFor example, say you capture 20 frogs on your first visit and 10 on your second visit. The second visit you have five frogs that are marked and 15 that are not. Here is the equation and estimation:
20 x 10 / 5 = 40
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Tips & Warnings
Research exactly how to mark the animal you are interested in. There are different techniques for different animals.
Use caution when working around water and make sure to use water safety measures when collecting animals.
References
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