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How to Use the Tagging & Recapturing Technique to Estimate the Number of Fish in a Pond

Scientists use the mark-recapture technique to estimate the population size of many types of animals, including fish in a lake or pond. The trained researcher inserts a small tag under the skin of the fish before releasing it into the water. This may be done before releasing fish raised in a fish hatchery, or during a study where researchers trap fish for the purposes of tagging and checking for previously implanted tags. For example, California’s Department of Fish and Game tag captive-bred and released trout, a population in decline, before planting them in lakes and ponds around the state. The data from these programs can give you an estimate of the total number of fish in the pond or lake.

Things You'll Need

Mark-recapture data

Instructions

- 1
  Collect the data from a tagging and recapture expedition. You will need to know how many fish were originally tagged in a previous capture and tagging expedition, how many fish were recaptured in a second expedition, and how many of those fish recaptured had tags.
- 2
  Multiple the total number of fish recaptured in a second expedition, both with and without tags, by the total number of fish previously tagged in that lake. For example, if 200 fish were recaptured and 100 were tagged previously in the original capturing, you would get 20,000 (100 x 200).
- 3
  Divide that answer by the total number of fish recaptured with tags. Continuing with this example, if 50 of the 200 recaptured fish had tags the answer would be 400 (20,000 / 50). In this example, the pond has an estimated population of 400 fish.

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