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  Teaching Guide

Activity 2: Grades 5-8
Salmon Counting

Much of what is known about salmon and tuna populations is based upon population sampling. The assumption that a random sample is representative of the population's overall concentration is key to this strategy. When the observed numbers are adjusted to reflect the extent of the animal's range, an estimate of the total population is made. Uncertainties arising from movement into and out of the sample region and the extent of heterogeneous spread of individuals compromises the accuracy of sampling estimates.


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This activity page will offer:

  • Introduction to population sampling
  • Application of sampling technique
  • Opportunity to relate sample size to estimate accuracy

Population Sampling
In this activity, you'll infer numbers of a virtual population illustrated within a rectangular sampling grid. You'll observe the accuracy of the technique in relation to the sample size upon which the estimates is based.


  • Pencil and paper

Grid with salmon
Click here for a larger version of this grid


  1. Examine the counting grid above. It represents an area that measures four meters by six meters. What is the total area of this full-size grid?
  2. Make a guess before you do any counting. Estimate the number of fish found within these 24 square meters?
  3. Use a pencil and number the squares one through 24.
  4. Devise a method for picking six random numbers from that same range of twenty-four numbers .
  5. Examine the six squares identified by the selection of your six random numbers.
  6. Tally the total number of fish found in these six squares. Remember, you'll need to devise a plan to deal with fish that lie on a dividing line.
  7. Once you've tallied the number of your fish in your sample, multiply this number by four. The number you arrive at is the estimate for the number of fish in the24 square meter grid. Record this as your first estimate.
  8. Pick three random squares. Tally the total number of fish in these squares. Multiply your count by eight. Record this value as your second estimate.
  9. Randomly select one number. Count the number of fish in that square. Multiple this number by 24 to arrive at an estimate of the number of fish in the entire area.
  10. Repeat step 9 two more times.
  11. Count the actual fish that are in the entire grid. Compare and contrast this number to the estimates made based upon 6 samples, 3 samples, and the 1 sample calculations.


  1. Why was it important to devise a way of picking random numbers.
  2. Why did you need to develop a method for counting fish that were positioned on a grid line?
  3. In step 7, why was the number of counted fish multiplied by four?
  4. How did the number of samples on which the estimate was based affect the accuracy of the estimate?

More Math
Based upon your count and observed concentration, how many fish can be found in a square kilometer with the same population density?

Creative Journal
Based upon the natural history of the salmon, create a fictional journal that describes life as a salmon. Begin the story with the salmon's hatching within in a remote lake in Washington. Be sure to include entries that describe the hazardous journey both down and up river.

Build A Model
Research the structure of turbines and generators that are used in hydroelectric plants. Then, use classroom materials to construct a simple model of these devices. Include a model turbine with blades that rotate in response to the flow of water. If applicable, attach a small magnet to the turbine shaft. Place a coil of wire around the magnet and monitor the inducted current using a digital multimeter.

Web Connection

The Life History of the Pacific Salmon
Hyperlinked photographs explore the life cycles of different types of Pacific Salmon

Sampling in Research

An introduction to sampling and sampling errors.

Identifiying Populations

A paper written by scientists on the subject of salmon population structure.


Academic Advisors for this Guide:
Corrine Lowen, Science Department, Wayland Public Schools, Wayland, MA
Suzanne Panico, Science Teacher Mentor, Cambridge Public Schools, Cambridge, MA
Anne E. Jones, Science Department, Wayland Middle School, Wayland, MA
Gary Pinkall, Middle School Science Teacher, Great Bend Public Schools, Great Bend, KS



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