For many years, members of the Ojibwe [o-JIB-way] tribe in Minnesota have been catching a fish they like to eat called walleye. Today they are using mathematics and science to measure the changing population of walleye in their lakes. Will they be able to continue fishing for walleye, or will changes in the environment make their tradition no longer possible?

In this lesson, you will learn how the Ojibwe use population sampling to measure the abundance, or number, of different fish in their lakes. You will also learn how to interpret, or make sense of, the data that they find and make predictions based on those numbers.

Goals
Here are the big ideas you will learn about in this lesson:

• Population sampling is a method you can use to gain information about a large population.
• You can use data to make predictions about what conditions will be like in the future.

Strategies
You will be using many reading and writing strategies to complete this lesson. The key strategies that you will focus on are:

• Making inferences
• Determining important information
• Identifying and using text features
• Understanding problem/solution
• Sequencing events

• random
• representative
• sample

The Ojibwe need to keep track of the abundance of walleye fish to make sure that they keep showing up year after year. But counting the entire population of fish in a lake is an impossible task. How can they get the information they need?

In this video, members of the Ojibwe tribe take a survey, or poll, to find out if the number of walleye fish in the lake is growing or shrinking. They will catch a small group of fish, called a sample, and use this information to understand the entire population in the lake. This method is called population sampling.

First, they set out two fishing nets in different parts of the lake. Later on, they will pull up the nets and count the types of fish that they caught. They hope that the fish they catch are representative of all the fish in the lake.

Click "View" on the left to watch the video. After you watch, answer the question below.

In the video, the Ojibwe placed two nets in different parts of the lake. Why do you think they didn’t just use one big net and put it close to shore? Write two to three sentences to explain your answer.

The two nets in the lake will catch a random sample of fish. This means that some fish will get caught and others will not; which ones get caught is a matter of chance. Data from a random sample often lead to more accurate results than data from a sample that is not random, so this is a good way to estimate the health of the walleye population over time.

Here's one way to think about sampling. Imagine a jar filled with gumballs: there are a lot of red, some green, and only a few yellow ones. If you reach in and grab a big handful, you will likely end up with mostly red gumballs and a couple green ones (and maybe a yellow, if you are lucky!). The gumballs in your hand will probably look very much like the rest of the jar.

It's the same with fish in the lake. When the Ojibwe pull up the nets, they will find many fish trapped inside, including bluegill, pumpkinseed, and walleye—all types of fish that live in the lake. Hopefully, this catch is representative of the larger population of fish in the lake. Scientists can use this information to estimate the abundance of each kind of fish in the lake.

Click "View" on the left to watch the video and find out what the teens learned by sampling the fish in the lake. After watching, answer the question below.

How were the teens able to learn that the walleye population was healthy without catching all the fish in the lake? Explain what they did and how they decided that the walleye was healthy. Write your answer in two to three sentences.

Surveys that use sampling only gather data from a small part of the total population. If the sampling is truly random, then the data gathered is usually representative of the larger population. This means that if there are 10 times as many bluegill as walleye in the lake, then a representative sample will find 10 times as many bluegill as walleye in the nets.

But sampling is not a perfect way to gather representative data. Depending on the day, weather, and location of the nets, the types of fish caught may change—and those changes may affect the data analysis.

Look at the two pictures on the left. Each lake contains the same population of blue and red fish, but the two samples are slightly different. This shows that population sampling is good for estimating, but not always a good way to gather complete data.

Which sample is more representative of the whole population of red and blue fish in the lake? How do you know? And what does this example tell you about random samples? Explain your answer in three to four sentences.

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.

In this lesson, you have learned about how the Ojibwe people use sampling to help keep track of the fish populations in their lake. You will now read more about the importance of fishing and about the problems of overfishing.

Click "View" to begin the reading.

To prevent the overfishing of walleye, the Ojibwe want to set some limits on the size of walleye that are caught in a local lake. Describe how they could use a population survey to help them figure out what fish are okay to catch and what should be put back into the lake.

In this lesson, you have learned how to use sampling to estimate the abundance of fish in a lake.

Now show what you have learned so far by answering these three questions. Be sure to save all your answers before you go to the next screen.

You will now check your understanding of the vocabulary words for this lesson by placing them in sentences. Click "View" on the left to begin the activity and then continue reading below.

Before moving to the next screen, choose two words from the vocabulary list and write a new sentence for each word.

Vocabulary words: abundance, data, estimate, population, random, representative, sample

Knowing how to collect representative data is important. But so is knowing how to plan a survey! In this activity, you will put the steps of planning a survey in the correct order. Two different survey questions are featured.

Click "View" on the left to begin the activity. Your teacher will review the answers with you.

Then, before moving on to the final assignment, you may want to watch this lesson's videos again or reread the passage.

Lake Science

Video: 1m 26s

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Counting Fish

Video: 1m 50s

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Keeping a Steady Population

Document

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In this lesson, you have learned how sampling can help you estimate the size of a population in a hard-to-measure environment, like a lake. You will now write an essay that shares what you have learned.

Begin by reviewing your notes. Click "review my work" above and view or print out the screen that pops up.

Final Assignment Instructions

• Click Population Sampling: Fish—Final Assignment (PDF).
• Choose one of the topics in the PDF and write an essay.
• Organize your response so that it includes at least three supporting details that you have learned from the reading passage, video(s), and other materials in this lesson.
• Use at least three vocabulary words (or a form of the words) as part of your supporting details. For example, for the word jump, you could use jumps, jumped, or jumping.