5.2 Estimating a Fish Population

Comparing and Scaling

The Department of Natural Resources manages the fish population in the Bay and sets catch limits to protect certain species of fish. They place a tag on a certain number of fish. Then, they catch a sample of fish, and based on the number of tagged fish in the catch, they estimate the size of the total fish population. We are going to try to estimate the size of a fish population.

Go to the Estimating Population Gizmo at Explorelearning.com:

  1. Set the number of fish to tag to 30
  2. Set the number of fish to catch to 100

  1. Click on the “Catch and check” button.


Example

Number of fish caught / 100
Number of tagged fish in catch / 14
Percentage of tagged fish to caught fish / 14%
  1. Perform the capture-tag-recapture method (steps 1-3) a few more times, and report your results here:

Trial 1 / Trial 2 / Trial 3 / Trial 4 / Trial 5 / Trial 6 / Trial 7
Number of fish caught / 100 / 100 / 100 / 100 / 100 / 50 / 50
Number of tagged fish in catch
Percentage of tagged fish to caught fish
  1. Why is it important to take multiple samples of fish?
  1. Based on the results of your trials and using what you know about making comparisons with ratios, how can you estimate the number of fish in the whole population? (Remember that you know the number of tagged fish in the whole population and the number of tagged fish in the sample population.)

To calculate the estimated number of fish in the pond, this is the formula that is generally used:

Using this formula, show your work, in calculating the Estimate of the Fish Population with your data from before. Show your results in the table below.

Trial 1 / Trial 2 / Trial 3 / Trial 4 / Trial 5 / Trial 6 / Trial 7
Number of fish caught / 100 / 100 / 100 / 100 / 100 / 50 / 50
Number of tagged fish in catch
Estimate of Fish Population

Check your estimates by looking at the calculations made by the Gizmo. Click on the “Calculate” tab for each of your trials, after you are done calculating.


To check your results against the actual number of fish in the pond, go to the “Pond” tab and click on the box that says: “Show total fish in pond”

How close were your estimates to the actual number? Why might your estimates have been different from the actual number?

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