The issue of stocking a lake can be volatile. Anglers, local residents, and business owners may believe that a certain lake should be stocked with lake trout. After all, it may improve fishing in the lake, which can have a positive effect on the local economy. Like many issues faced by natural resources personnel, the issue of stocking is not that clear-cut. Before stocking a lake, natural resources personnel must determine if lake trout can succeed in a given lake. Like all animals, lake trout depend on specific environmental conditions for their survival. In this lesson, you will decide whether the temperatures and oxygen concentrations of Ice Lake provide some of the basic physical conditions required to support lake trout.

Knowledge Base
Imagine the Ice Lake Association asked you if lake trout would survive if they were stocked in Ice Lake. You plan to investigate this question by looking up summer and winter temperature and oxygen levels in a typical Northeastern Minnesota lake that does support lake trout.

Before you begin, consider what you know about the water quality needs of lake trout.

1. What factors need to be considered before stocking a lake?
2. What social and economic issues are related to stocking?
3. What can happen if a lake is "incorrectly" stocked?

Experimental Design
A natural resources technician has provided you with the following information (see Table 1) for a typical good lake where lake trout flourish.

4. Construct a graph of the concentration of oxygen at each depth for each season and describe the pattern you see. Now add the temperature data to the graphs you have constructed.
5. Do you observe declining oxygen levels with increasing temperature in any of your graphs?

Table 1

The technician reminds you that lake trout seek out water temperatures of about 9–13° C in the summer. If summer temperatures exceed approximately 20° C or if oxygen concentrations fall below about 8 mg/l conditions are not ideal for the survival of adult lake trout.

You know Mackinaw Lake supports lake trout. Now you need to collect data from Ice Lake to see if it can support lake trout. You will organize and analyze your data in the same way you have just reviewed the data from Mackinaw Lake. You need to begin by collecting data from one sampling period in each of the seasons: spring, summer, and fall.

Data Collection
Collect the temperature and DO data for Ice Lake on three different dates. Record the data in Excel or another spreadsheet program.

Data Management and Analysis

6. Create graphs for each season showing the temperature and dissolved oxygen profiles.

Interpretation of Results

7. Based on the data you have collected, would lake trout succeed in Ice Lake? Explain how you have arrived at your conclusion.
8. If you found habitat limitations for lake trout because of low oxygen levels or temperatures that are too high, would you expect these conditions to persist through times of the year for which you do not have data? Why or why not?
9. Explain why you would or would not recommend the Ice Lake Association should attempt to stock lake trout?

Reporting Results
Turn your answers and your graphs in to your teacher. Check to see if other groups got similar results. Your group may be asked to make a presentation of your findings to the class.