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.
Two sessions, one night for homework
This activity supports the following National Academy of Sciences Science Education standards.
This activity supports the following National Council for the Social Studies standards.
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 aren't 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 shark meat, shark fins, and other shark products has made sharks a direct target of fishers who previously didn't capture sharks, or at least didn't keep the sharks if they were caught.
But these practices might not take such a dramatic toll on sharks if it weren't for some basic aspects of sharks' reproductive biology. Sharks are slow-growing, late-maturing animals that don't 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.
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:
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've 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 wouldn't be safe to throw a hook and line at their classmates, they'll “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's 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's unlikely that many would die.)
Explain to the students that some commercial fishers use gill nets to catch fish in the open ocean. Gill nets allow a fish to fit its head and gill covers, but not its fins or other parts of its body, 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're hauled in. Others are allowed to float through the open water. (These floating gill nets are called drift nets.) Sometimes drift nets get lost; they can float for years gathering fish and other sea creatures in them.
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've been assigned):
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.
Explain to the group that longlines are just what they sound like: long, thin cables or monofilament strands that stretch as far as 40 miles across the ocean. (Help your students understand this distance by comparing the distance to a place about 40 miles away from your classroom.) Tell the students that on a longline, there is a float attached to the cable every few hundred feet and a baited hook every 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'll 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've been assigned):
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 haven't yet decided which side of the room (front or back) the fishers will start from, so there's 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 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.
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 don't want. While trawl nets make it relatively inexpensive to catch lots of fish, they also catch many unwanted animals, which often don't 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 size 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 Worksheet A. Encourage different team members to try out different size nets, which may mean trading nets with other teams until they've used all four net sizes.
Use “Fishing Worksheet B” to assess students' understanding.
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 Diversity, a resource of World Wildlife Fund's Windows on the Wild biodiversity education program. For more information on WOW please visit www.worldwildlife.org/windows.
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