If you look at data that has been collected over the course of a few years, you can begin to see patterns.

To figure out if the walleye population is healthy, the Ojibwe can compare the results of this year’s survey to the results from previous years. If they catch about the same number of fish as in previous years, then they might determine that the population is staying the same. If the numbers go up from year to year, then the population may be increasing (getting larger); if the numbers go down, then the population may be decreasing (getting smaller).

The table on the left shows the number of walleye caught in different sampling surveys over a period of 16 years. From this data, a scientist could conclude that the walleye population increased, then decreased, and is now increasing again, since three walleye were recorded in the most recent survey.