Every winter in Minnesota, the numerous lakes in the state freeze over. As soon as the ice is safe, enthusiastic anglers arrive with their fishing rods, ice augers, and buckets of bait to try their luck with the walleyes, perch, crappies, and elusive muskies below. Fish populations are closely monitored and regulations adjusted to ensure a sustainable fish harvest. But how do you measure the population of fish in a lake when the fish are swimming randomly below the surface and can’t be seen?
To estimate fish populations, scientists use a method called mark and recapture, which is illustrated by the Estimating Population Size Gizmo. In the Gizmo, students choose a number of fish to tag and a number of fish to catch. For example, suppose a lake contains a total of 20 tagged fish. If students go out and catch 10 fish and only 1 is tagged, then it is reasonable to guess that the tagged fish represent about one in ten fish in the pond, so the total fish population is around 200. Students will learn how to use ratios to estimate fish populations and see how sample size affects the accuracy of their estimate.