Predators: Sharks in Decline
IDEAS FOR THE INFORMAL SETTING
- To explore the impacts of overfishing, combine with Facing the Future’s Fishing for the Future activity.
- Display actual fishing equipment along with the simulations.
Students carry out group simulations of common fishing methods and assess why these methods and sharks’ reproductive biology are together contributing to a rapid decline in shark populations.
- Describe several methods by which sharks are captured.
- Discuss some of the advantages and disadvantages of each method.
bycatch, conservation, consumer, gill nets, longlines, species, trawl nets
2 sessions, 1 night for homework
- Bandanas or other strips of cloth
- 3 Nerf balls or other soft objects that can be thrown safely
- 2 12-foot ropes/clothesline
- 8 to 12 clothespins
- Notebook paper
- 8 or more 7-inch embroidery hoops
- 4 mesh bags or fabric with holes of different sizes (such as bags for onions, bait, or hosiery)
- Mixture of dry beans (1/4 pound each of lima, pinto, kidney, lentil, and black beans)
- 4 large (10-inch diameter) plastic containers
- Copies of handouts: Fishing Worksheets A and B (one per student)
- Educator Page: Answers To Fishing Worksheet B
NATIONAL SCIENCE EDUCATION STANDARDS
This activity supports the following National Academy of Sciences Science Education Standards (Grades 5-8):
- Unifying Concepts and Processes—Evidence, models, and explanation
- Unifying Concepts and Processes—Form and function
- Standard A: Science as Inquiry—Abilities necessary to do scientific inquiry
- Standard A: Science as Inquiry—Understandings about scientific inquiry
- Standard C: Life Science—Reproduction and heredity
- Standard C: Life Science—Diversity and adaptations of organisms
- Standard F: Science in Personal and Social Perspectives—Populations, resources, and environments
When shark-attack stories make the news day after day, people start to think that sharks are becoming more aggressive or that their populations are growing. However, sharks are not increasing in numbers or ferocity. In fact, sharks are suffering significant population declines. Scientists estimate that some species of coastal sharks have declined by between 50 and 75 percent in just the last 20 years.
One reason that shark populations have declined so rapidly is that many common fishing methods accidentally capture sharks in addition to the targeted fish. Another reason is that a growing market for meat, fins, and other shark products has made these fish a direct target of fishers who previously did not capture sharks, or at least did not keep them if they were caught.
But these practices might not take such a dramatic toll on sharks if it were not for some basic aspects of sharks’ reproductive biology. Sharks are slow growing, late-maturing animals that do not reproduce very quickly. And they are extremely susceptible to population declines if large numbers of them are killed. This activity contains a series of simulations that explore different fishing methods and how they intentionally or unintentionally lead to the capture of sharks. Then the activity highlights why some fishing methods are so disruptive to shark populations, particularly in light of sharks’ reproductive biology.
BEFORE YOU BEGIN
1. Before beginning any of the simulations, push desks to the sides of the room, leaving a large open space in the middle.
2. Gather the materials for the simulations.
3. Copies Charts A, B, C, and D from
Fishing Worksheet A
onto the board, and make a copy of
for each student.
WHAT TO DO
1. Discuss fishing. Ask students if they have any idea how people catch fish in the open ocean. Have a few students share what they know about the topic, then tell them that you are going to conduct a series of classroom exercises to show different fishing methods and their effectiveness in catching targeted species.
Write the following list on the board:
a. Hook and line
b. Gill nets and drift gill nets
2. Simulation A: Hook and Line
The hook and line fishing method is used by sport fishers as well as by some commercial fishers. In this simulation, some of your students are going to be fishing for yellowfin tuna using a hook and line. The other students are going to be the tuna, sharks, and other sea creatures. Ask three volunteers to be fishers. Have the fishers stand aside while you divide the remaining members of the class as follows:
a. Three to four pairs of students (with arms linked) = adult tuna
b. Three to four individual students = juvenile tuna
c. Three to four pairs of students (with arms linked) = adult sharks
d. Three to four individual students = juvenile sharks
e. Remaining students = other fish
Tie a bandana or strip of cloth around the arm of every tuna. You need not label the other students, but they should remember what identity they have been assigned.
Now present the rules of the game. The fishers will have one minute to “fish” for a tuna from the group. Since it would not be safe to throw a hook and line at their classmates, they will “fish” by throwing the Nerf ball or other soft object. To make things harder for the fishers, they have to be touching a desk with a part of their body when they throw the ball. None of the fish may run. Any fish the fishers hit is considered “caught,” but if it is not an adult tuna, the fishers should “throw” the fish back into the group and toss the ball again. Have the adult tuna that are caught stand next to the fishers who caught them. Whichever fisher has caught the most adult tuna when the minute is over wins the game.
To begin the game, group the fish in the middle of the room. Then tell the fishers to begin. As the fishers catch their fish, record the results on the board on Chart A. (Be sure to count every fish caught, even if the fish is thrown back.) You might want to do another round of fishing if time permits. To do this, “restock” the waters and select new fishers.
Afterward, have the students copy the results from the board onto
Fishing Worksheet A
and analyze the results. How many fish were caught that were not adult tuna? Tell the students that sharks are generally able to survive when they are caught using a hook and line and then thrown back. That being the case, what was the expected total shark mortality in these simulations? (Answers will vary, but it is unlikely that many would die.)
3. Simulation B: Gill Nets and Drift Gill Nets
Explain to the students that some commercial fishers use gill nets to catch fish in the open ocean. Gill nets allow the head and gill covers of a fish, but not its fins or other parts of its body, to fit through the net holes. The gill covers get caught in the net and prevent the fish from wriggling loose. So any fish that are larger at the gills than the holes in the net will get stuck. Once pulled onto the deck of a fishing boat, the fish will quickly die. You might point out that, in addition to being directly targeted by commercial fishers, a lot of sharks are accidentally caught in gill nets by fishers that are targeting tuna.
Some gill nets are fixed in one place and collect fish until they are hauled in. Others are allowed to float through the open water. The floating gill nets are called drift nets. Sometimes drift nets get lost and can float for years gathering fish, and other sea creatures.
To simulate gill net fishing, select one student to be the fisher. Have that person place the two ropes down on the floor to create three equal-sized “lanes.” Then have that person secretly designate one lane to be where the gill net will be. (Be sure the person tells you which lane he or she has selected before the other students start “swimming.”)
Meanwhile, divide the rest of the students as follows (you need not label them, but they should remember the identity they have been assigned):
a. One-fourth of the students = adult tuna
b. One-fourth of the students = juvenile tuna
c. One student = sea turtle
d. One student = dolphin
e. Two to four students = small fish
f. One-half of remaining students = adult sharks
g. Other one-half of remaining students = juvenile sharks
Now gather the students at one end of the classroom, and tell them they have to walk to the other end. When they reach the ropes, they should continue down one of the three lanes. Tell them that the fisher has placed a gill net across one of these lanes, but since fish cannot see gill nets, neither can the students. Tell them that they cannot change their lane once they have selected it.
The marine creatures should “swim” from one end of the room to the other, and they should stay in their lanes at the other end of the room. Then have the fisher announce which lane had the gill net, and have him or her count up the catch. All the small fish would have been able to swim through the netting in the gill net. The remaining creatures should be considered caught.
Run through the simulation again if time permits, recording both simulations on Chart B. Have the students copy the figures onto
Fishing Worksheet A
, Chart B and compare with the results logged on Chart A.
4. Simulation C: Longlines
Explain to the group that longlines are just what they sound like: long, thin cables or monofilament strands that stretch as far as 64 kilometers (40 miles) across the ocean. You can help your students understand by comparing the distance to a place about 64 km (40 miles away) from your classroom. Tell the students that on a longline, there is a float attached to the cable every hundred meters (few hundred feet) and a baited hook every meter (few feet). Longlines are often used to capture tuna and billfish such as swordfish. But they also unintentionally catch many sharks.
Choose two people to be longline fishers. Give them one rope, the clothespins, and 10 or more pieces of paper. Then have them go out into the hall and clip the paper on the rope in whatever distribution they want. Tell them that they will learn how to “fish” with their longline when they get back into the room. While the fishers are out of the room, divide the group as follows (again, you need not label them, but the students need to remember the identity they have been assigned):
a. One-fourth of students = adult tuna
b. One-fourth of students = juvenile tuna
c. Two students = sea turtles
d. One student = dolphin
e. One-half of remaining students = adult sharks
f. Other one-half of remaining students = juvenile sharks
Tell the fish to stand around the room in any configuration they want. The only thing they may not do is stand directly behind another fish. Tell the fish you have not yet decided which side of the room (front or back) the fishers will start from, so there is no point in bunching up at the back of the class.
Bring the two fishers in and have them stand at the front or back of the room with their rope stretched out across the classroom. Explain that the papers on their longline are meant to represent their baited hooks. They should hold the rope so that the papers will pass over the heads of some fish and brush against others. Then have them walk slowly down the length of the classroom, being sure not to shift their longline just to hit a particular fish. The fish may not duck or shift their bodies to avoid one of the “hooks.” Every time a fish is brushed by a piece of paper, that student should remove the paper. (In real life, once a hook has caught a fish, no other fish can be caught on it.) Then the fish that are caught should go to the front of the room and identify themselves. Repeat the simulation if time permits. Discuss the outcome of the fishing, record it on Chart C (with students copying the figures to
Fishing Worksheet A
, Chart C), and compare the results with those recorded on Charts A and B.
5. Simulation D: Trawling
Explain that trawl nets are large, heavy nets that are dropped to the ocean floor, and then dragged just above or along the ocean bottom to catch shrimp and other fish. The nets are then hauled to the surface and emptied onto the deck of a fishing boat. Fishers sort through the catch, throwing back what they do not want. While trawl nets make it relatively inexpensive to catch lots of fish, they also catch many unwanted animals, which often do not survive. Dragging the heavy trawl nets along the ocean bottom also damages sensitive seafloor habitats.
Organize the class into four teams. Give each team at least two embroidery hoops with different sized mesh and a container filled with beans.
In this simulation, your students will play the part of shrimp fishers. Tell the students to assume that the different beans are different species of fish. Ask each team to choose one variety of bean to represent shrimp and another to represent sharks. Remember that shrimp are quite small relative to most other marine animals, including mammals, fish, and other crustaceans.
The team members should take turns selecting a net and dragging the net through the beans. After each turn, have that person count the results of his or her catch. How many shrimp did that person catch? How much of the catch was bycatch? Tell the students to record their results on Chart D and
. Encourage different team members to try out different size nets, which may mean trading nets with other teams until they have used all four net sizes.
6. Discuss simulations. Ask the students if they have any questions about the simulations. In each simulation: Were they surprised by how many sharks and other fish were caught, even though they were not the targeted species? Explain that this unwanted catch is called bycatch. Some students may express dismay that fishers are responsible for killing so many marine mammals and fish that they do not use. You might explain that people are working to minimize this bycatch, but that it is difficult and expensive to change common and ingrained practices.
Fishing Worksheet B
for homework. Use the worksheets as a means of assessing each student’s understanding of the concepts (see ASSESSEMSNTS). Then return the sheets to the students and set aside a class period to review and discuss the answers. (Answers to
Fishing Worksheet B
are provided at the end of the activity.)
8. Discuss status of sharks. Tell your students that because of current fishing practices, many kinds of sharks are experiencing huge population declines. In fact, scientists estimate that humans kill at least 100 million sharks every year.
What are some ways that people could try to reduce this number? (Answer: Set limits on shark catches, set limits on the size of sharks that fishers may catch, reduce consumer demand for shark fins, or change fishing methods.)
Why might these changes be difficult to implement? (Answer: It is hard to rally public concern for sharks; many sharks move from one country’s waters to another’s, so fishing limits set by one or two countries will not guarantee that sharks are protected; current fishing methods are profitable to the commercial fishing industry, so any changes are likely to be resisted.)
9. Research shark conservation. As a wrap-up to the activity, have your students research current efforts in shark conservation. They should search the Web, contact environmental organizations, check the newspaper for articles, and so on. Allow students to share their findings with the rest of the class.
Fishing Worksheet B
to assess students’ understanding.
Needs Improvement—Provides incomplete or insufficient answers.
Satisfactory—Adequately answers each question.
Excellent—Provides thoughtful responses to each question.
- Have your students look into the reproductive biology of several shark species. Do sharks reproduce in the same way that other species of fish do? Or are sharks’ reproductive habits closer to those of large mammals? Explain.
Activity adapted from Oceans of Life—An Educator’s Guide to Exploring Marine Biodiversity, a resource of World Wildlife Fund’s Windows on the Wild biodiversity education program. For more information on WOW please visit
Note to Teachers: This lesson and others relating to National Geographic’s Strange Days on Planet Earth can be found online at