1. Have students place approximately 250 fish (beans) of one color into the pond.
2. Have one student in the group capture a small handful of the fish from the pond (about 80-90 fish). This sample is referred to as the number of fish captured designated as "C". Record number on data sheet.
3. Replace these captured fish with the same number of the other colored fish (if your fish are white then replace with brown fish and visa versa). Important! Return the captured fish into the main class containers of beans. Do not put these fish back into the pond because you want to keep the number of fish in the pond constant. These fish are the number of individuals originally marked designated as "M".
4. Add the marked fish into the pond. Make the fish swim by dispersing them through out the pond (i.e., mix with hand for 30-60 seconds).
5. Repeat step 2.
6. Have students count the number of captured fish and record on the data sheet. Record the number of marked fish which were recaptured designated as "R". Place the recaptured fish back into the pond.
7. Replace all of the unmarked fish in the sample with marked fish as in step 3.
8. Repeat steps 4 through 7 ten times. Twenty samples would be better statistically; however, the number of samples depends on the time available.
9. At the end of the ten samples, count the actual number of fish in the pond (both colors). Record on data sheet.
10. Using the data recorded on the data sheet, determine the three methods to estimate the population in the pond. Remember the Petersen Method can only be used for a single marking and recapturing period. You may want to have the students average the ten samples.
11. Using the population estimates for the three methods, calculate the percent error for each method using this formula: number of fish estimated / number of fish actually in the pond 100 = % error
12. Discuss the importance of consistent sampling techniques (capturing the same number of fish); sampling bias of methods and researcher (do not look at the beans when sampling, students tend to sample the marked fish); did the fish swim and disperse into the environment for the same amount of time; and how well did you mix the fish.
13. Compare the percent error and determine which sampling method was the most accurate.
14. Compare the data between student groups. Variations will occur with small sampling sizes versus large sampling sizes.
15. Discuss the importance of multiple samples compared to single samples, and how the number of recaptured fish in the sample determines the estimated size of the population.