Conduct a Fish Survey in Your Class
In this activity, students will simulate a fish survey to determine the population of bluegills in a hypothetical lake using tagging and a capture/recapture technique.
Objective: Students will learn how to estimate the size of a fish population in a lake.
Materials: Each pair of students will need: 1 spoon and paper cup; small white beans (about 100 cup); colored markers; calculators; student worksheet.
- Divide the class into pairs and distribute a cup half filled with white beans (about 100) to each pair. Explain that the cup represents a lake and the beans a bluegill population.
- Without disturbing (touching) the bluegills in any way, have students guess the number of bluegills in their lakes and record their guesses on the student worksheet at the end of this activity. Discuss the difficulty of the task and the potential sources of error.
- Explain that your job as fish biologists is to gain information about the number of bluegills in the lake. You're going to net and tag some bluegills. Have the students pour out about 30 beans to represent the portion of the bluegills netted and tagged.
- Using a marker to color their fish, have students count and tag each bluegill that was captured (poured out) and then record the number on the worksheet. Point out that besides tagging the fish, biologists may also collect information such as age, sex, and size of each fish tagged.
- Have students release their bluegills by pouring them back into the lake. Simulate their daily movements and interaction with the rest of the fish in the lake by shaking the cups (holding a hand over the cup so the beans don't go flying.
- Now tell students to try to determine the total bluegill population of their lakes by taking a sample with a "net." Give each pair of students a spoon and have them scoop a sample of bluegill from their lake and record the total number of bluegills trapped and the number of these that were tagged.
- Explain that the total population of bluegills in the lake can be estimated mathematically using the following formula:
Total fish tagged X Total fish in trapnet
population = -----------------------------------------------
Tagged fish in trapnet
- Have each pair of students substitute their data in the formula, and using a calculator, perform a population calculation and record it on their worksheet.
- Have students return their bluegills to the lake, mix the population as before and then take a second sample. Record the new data on their worksheet and perform a second population calculation.
- Ask students to compare their two population calculations with each other and with the initial estimate. Discuss possible reasons for any differences. Then, have them determine the accuracy of their estimate and calculations by returning their second sample to the lake and actually counting their fish (beans). Stress that this is a luxury fisheries biologists never have! Ask students the following questions
- Which was closest to the actual population, your original estimate or one of the calculations? (You may want to have students average the two calculations.) Why?
- What conditions, not present in this activity, in an actual lake might affect the bluegill population?
To see the student worksheet, click HERE.
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