And this is Sarah Long with Science in the News, a VOA Special English program about recent developments in science. Today, we tell about sharks, the large fish with sharp teeth that live in the oceans of the world.
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Scientists say sharks have lived in the world's oceans for millions of years. Today, sharks live the same way they did more than two-hundred-million years ago, before dinosaurs existed on the Earth. Scientists say there are more than three-hundred-fifty different kinds of sharks. For example, the dogfish shark is less than twenty centimeters in length. The huge whale shark more than fifteen meters long. Most sharks are about two meters long.
Sharks do not have bones. The skeleton of a shark is made of cartilage. Human noses and ears are made of cartilage.
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A shark has an extremely good sense of smell. It can find small amounts of substances in the water, such as blood, body liquids and chemicals produced by animals. Sharks also sense electrical and magnetic power linked to nerves and muscles of living animals. These powerful senses help them find their food. Sharks eat fish, other sharks, and plants that live in the ocean. Some sharks will eat just about anything. Many unusual things have been found in the stomachs of some tiger sharks. They include shoes, dogs, a cow's foot and metal protective clothing.
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Sharks grow slowly. Many kinds of sharks are not able to reproduce until they are twenty-five years old. Some reproduce only every two years. And they give birth to fewer than ten young sharks.
About forty percent of the different kinds of sharks lay eggs. The others give birth to live young. Some sharks carry their young inside their bodies, with a cord connecting the fetus to the mother, like humans do.
Scientists are beginning to understand the importance of sharks to humans. Medical researchers want to learn more about the shark's body defense system against disease. They know that sharks recover quickly from injuries. Sharks appear never to suffer infections, cancer or heart diseases. Many people believe that shark cartilage can help prevent cancer. Scientists have questioned this idea. Yet they still study the shark in hopes of finding a way to fight human disease.
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Most sharks live in warm waters, but some can be found in very cold areas. Most sharks live in the oceans. However, the bull shark leaves ocean waters to enter freshwater rivers and lakes. They have been found in the Zambezi River in Africa, the Mississippi River in the United States and Lake Nicaragua in southwestern Nicaragua.
Sharks are important for the health of the world's oceans. They eat injured and diseased fish. Their hunting activities means that the numbers of other fish in the ocean do not become too great. This protects the plants and other forms of life that exist in the oceans.
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People have long feared sharks because of their sharp teeth, aggressive actions and fame as fierce hunters. Jaws was the name of a popular book and movie a number of years ago. They told about people of an eastern American coastal town who sought protection from a great white shark that killed swimmers in the ocean.
Experts say not all sharks are like the one described in Jaws. Still, fifty-two people have been attacked by sharks around the world so far this year. Forty-one of those attacks took place in the United States. Twenty-nine were in waters near the southern state of Florida.
About eighty people were attacked by sharks around the world last year. Experts say that in the Nineteen-Nineties the average number of shark attacks worldwide each year was fifty-four. They say about fifty to seventy-five shark attacks take place around the world each year, resulting in five to ten deaths.
Shark experts say bees, snakes and elephants kill more people each year than sharks do. They say there is no great need to protect people from sharks.
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Many people disagree with that idea, especially this year in the United States. That is because of the increased media reports about shark attacks and resulting deaths.
In July, a shark attacked an eight-year-old boy in waters near Saint Petersburg, Florida. The shark bit off his arm. Doctors re-attached it and saved his life. But he lost almost all his blood and is still in poor condition.
Earlier this month, sharks killed two people along the east coast of the United States. A ten-year-old boy died after being bitten by a shark in waters off the coast of Virginia Beach, Virginia. Just a few days later, a shark killed a man and seriously injured his girlfriend. They were swimming off the coast of North Carolina, south of Virginia Beach.
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Shark experts are trying to discover why so many shark attacks have taken place within the same general area this year. They say the warm weather conditions may have influenced both fish and shark activity. The warmer waters moving closer to the shore carried many fish to that area. They say sharks may have followed the fish into the same area where many people were swimming.
The experts believe that bull sharks were responsible for all these attacks. Bull sharks are also suspected of killing a man last year in waters near Saint Petersburg, Florida. Experts say bull sharks are especially dangerous because they will target people as food. They can live in both clean and dirty water. They will swim into rivers and other areas that put them closer to people.
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Experts say most sharks bite people by mistake. For unknown reasons, they think that a person is a large sea animal, like a seal or sea lion. That is why people should not go swimming in the ocean at the times of the day when the sun goes down or comes up. Those are the times when sharks are looking for food. Experts also say that people should not wear bright colors or shiny metal jewelry. These may cause sharks to attack.
The experts say shark attacks only seem to be increasing because more people are swimming in the oceans than ever before. They say the number of sharks in the world has decreased in recent years. Scientists say people are killing sharks faster than the sharks can reproduce.
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People hunt sharks for sport, food, medicine and their skin. Experts say the international market for some kinds of sharks has increased greatly because many parts of a shark are valuable.
For example, shark meat is good to eat. In Asia, people enjoy a special kind of soup made from shark fins. Experts say shark fins can earn a fisherman about fifty dollars a kilogram. Collectors pay thousands of dollars for the jaws of a shark. Shark liver oil is a popular supplier of Vitamin A. Some people believe that shark cartilage and shark liver can improve people's health. The skin of a shark can be used like leather. People also kill sharks because of fear.
Many sharks are killed by mistake. Each year, thousands die in traps set out to catch other kinds of fish.
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If too many sharks in one area are killed, that group of sharks may never return to normal population levels. Such hunting activities also have made some kinds of sharks in danger of disappearing from the Earth.
For example, the number of dusky sharks and sandbar sharks off the eastern United States decreased by more than eighty percent. This happened between Nineteen-Eighty-Five and Nineteen-Ninety-Five. The sand tiger shark and the great white shark are threatened around the world.
The United States government restricts the number of sharks that can legally be killed near its coasts. The government also requires that fishermen take the whole shark body instead of just the fins. This stops fishermen from cutting off the fins and throwing the rest of the fish back into the water. Many countries also have approved laws protecting the endangered great white shark. These nations include South Africa, Australia and the United States.
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This SCIENCE IN THE NEWS program was written by Nancy Steinbach. It was produced by Cynthia Kirk. This is Bob Doughty.
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And this is Sarah Long. Join us again next week for more news about science in Special English on the Voice of America.
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