Sharks range in size from the 0.1-meter ( 6-inch) long dwarf-dog shark to the 18-meter ( 60-foot) long whale shark---the world’s biggest fish. The whale shark, like two other large shark species---the barking shark and the megamouth shark---are harmless to people because they feed on plants and small aquatic animals.
Sharks have extremely sensitive sense organs (器官). Some sharks can detect (发现,察觉) the scent of decaying fish or blood even when it is diluted to only one part per million parts of seawater. They can probably hear underwater sounds that originate as far as 3 kilometers (2 miles) away and can tell the direction from which underwater sounds are coming.
Sharks are key predators (食肉动物) in the world’s oceans, helping control the numbers of many other ocean predators. Without sharks, the oceans would be overcrowded with dead and dying fish.
Every year, we catch and kill over 100 million sharks, mostly for food and for their fins. Dried shark fins are used to make shark fin soup, which sells for as much as $ 50 a bowl in fine Hong Kong restaurants. Other sharks are killed for sport and out of fear. Sharks are vulnerable (易遭攻击的) to overfishing because it takes most species 10 to 15 years to begin reproducing and they produce only a few offspring.
Influenced by movies and popular novels, most people see sharks as people-eating monsters. This is far from the truth. Every year, a few types of shark injure about 100 people world-wide and kill about 25. Most attacks are by great white sharks, which often feed on sea lions and other marine mammals. They sometimes mistake human swimmers for their normal prey, especially if they are wearing black wet suits.
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