英語(yǔ)語(yǔ)言學(xué)習(xí)：鯊魚

This is SCIENCE IN THE NEWS in VOA Special English. I'm Barbara Klein. And I'm Doug Johnson.

On our program this week, we tell about sharks. They are among the oldest animals on Earth. Sharks are famous for attacking other sea creatures and even people. Yet they also have been threatened by human activities.

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. Most sharks are about two meters long. The dogfish shark, however, is less than twenty centimeters in length. And, the biggest whale shark can grow to a length of twenty meters.

Sharks do not have bones. The skeleton of a shark is made of cartilage. Human noses and ears are also made of cartilage.

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.

Sharks grow slowly. Some kinds of sharks are not able to reproduce until they are twenty years old. Most 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.

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 mean 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.

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 published in nineteen seventy-four. It told about people in an American coastal town who sought protection from a great white shark that killed swimmers in the ocean. Thirty years ago last month, the film version of the book was released. "Jaws" became one of the most popular American movies in history. The movie was extremely frightening.

However, experts say not all sharks are like the one shown in "Jaws." Still, sharks attacked sixty-one people around the world last year. Twenty-seven of those attacks took place in North American waters. Twelve were in waters near the southeastern state of Florida.

The International Shark Attack File keeps records of all reported shark attacks. The list has been in existence since nineteen fifty-eight. The world attack totals last year were similar to those of the most recent years. Yet the number of attacks has risen during the past century. However, the number of deaths from shark attacks each year around the world remains very small. Experts say sharks killed only seven people last year.

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.

Many people disagree with that idea. That is because of media reports about shark attacks and resulting deaths. On June twenty-fifth, a shark attacked a fourteen-year-old girl as she swam near the coast of northwestern Florida. The girl was swimming with a friend in the Gulf of Mexico. They were more than ninety meters from the coast when a shark bit one of the girl's legs. A man surfing in nearby waters brought her back to land. But medical workers were unable to save the girl. She died of her wounds.

Two days later, there was another attack about one hundred kilometers east of where the girl died. A sixteen-year-old boy was attacked while standing and fishing in waters near an area calledCape San Blas. He survived the attack, but doctors were forced to remove one of his legs.

Shark experts are attempting to discover why many shark attacks take place within the same general area. They say warm weather conditions may influence both fish and shark activity. The warmer waters moving close to the coast carry many fish to that area. Experts say sharks may have followed the fish into the same area where many people were swimming.

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 metaljewelry. 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.

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 sharkfins. Experts say a fisherman can earn about fifty dollars a kilogram for shark fins. Collectors pay thousands of dollars for the jaws of a shark. Shark liver oil is a popular source of Vitamin A. Some people believe that a shark's cartilage and 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.

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 Earth.

Many scientists say the number of sharks worldwide has dropped by fifty percent over the past fifteen years. Among some kinds of sharks, the number may have decreased by more than seventy percent. 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.

Many nations have approved laws to protect the great white shark. These nations include Australia, South Africa, and the United States. Last October, the great white shark gained international protection for the first time. The recognition came at a meeting of the Convention on International Trade in Endangered Species of Wild Fauna and Flora, orCITES. Delegates at the meeting approved a plan to require a permit for selling the jaws, teeth and fins of great white sharks.