Capture and Recapture
Fish in a pond or animals in a population

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Is it possible to find how many fish there are in a pond without counting every fish?

Scientists use a method called catch and recapture to estimate animals' or a particular species' (kind or ainmal) population.

Purpose: To simulate counting animal populations

Materials: container or bowl; beans, cubes, or counters; pencil, erasable marker or stickers.


Put an unknown number of the similar objects (hundred or more) in the container. The container, or bowl will represent a pond or other environment. The beans or cubes will represent the animals in the pond or environement population or a particular species of animals. The pencil, marker, or stickers will be used to mark the animals that are captured, released, and maybe recaptured later.

Decide how many organisms to capture, tag and release (20, 25, 33).

Catch an animal, tag it, release it, and continue until the determined amount are tagged.

Then begin to catch the animals one at a time, record if it is marked or unmarked, release it, and continue to catch more animals until 20 or more have been caught. (be sure to draw the beans, cubes, or counters without looking, record if it is marked or not, put it back into the container, and draw another until 20, 25, or more have been drawn).

With the information you have, you should be able to estimate the total number of animals in the population...

Simulation Number of animals tagged Tagged animals caught Untagged animals caught Total animals caught Total Population

Does it matter if you catch one animal at a time and release it or of a whole group were collected at once (caught in a net or grab a handfull of beans)?

hint: Animals tagged / total population = Tagged caught / untagged caught


Dr. Robert Sweetland's Notes ©