[ti:Shark Populations Dropped 71 Percent Since 1970] [by:www.51voa.com] [00:00.00]更多听力请访问51VOA.COM [00:00.04]Scientists have known for decades [00:02.80]that the numbers of some shark species are decreasing. [00:07.92]But a new study shows just how severely [00:11.28]worldwide populations have dropped in the past 50 years. [00:17.96]The numbers of oceanic sharks and rays [00:21.48]fell more than 70 percent worldwide [00:24.84]between 1970 and 2018. [00:30.08]The information comes from a study [00:32.64]that appeared recently in the publication Nature. [00:37.80]Stuart Sandin is a marine biologist [00:40.84]who works at the Scripps Institution of Oceanography. [00:46.64]He says that sharks are great hunters, [00:50.16]fast swimmers and have extraordinary senses. [00:56.20]Sandin adds that they can identify any changes in the ocean [01:01.12]from a great distance, such as smells or tiny changes in water flow. [01:09.16]Their ability to quickly sense anything outside the norm [01:13.84]in their environment helps them find food in the open ocean. [01:20.20]But it also makes them especially at risk [01:23.76]in the face of increased international fishing pressure. [01:28.64]“You drop a fishing line in the open ocean, [01:31.64]and often it’s sharks that are there first [01:35.28]— whether or not they’re the primary target,” said Sandin. [01:40.76]Twenty-four of the 31 species of sharks and rays are threatened with extinction. [01:48.56]Three species — oceanic whitetip sharks, [01:52.76]scalloped hammerhead sharks and great hammerhead sharks [01:57.92]— are considered critically endangered. [02:02.36]Nathan Pacoureau is a biologist [02:04.84]at Simon Fraser University in Canada [02:08.24]and one of the writers of the study. [02:11.84]He said, “The last 50 years have been pretty devastating [02:16.60]for global shark populations.” [02:20.56]Sometimes sharks are caught on purpose by fishing boats. [02:25.60]But more often they are caught while fishing for other species [02:30.44]such as tuna and swordfish. [02:34.84]Sharks and rays are both fish with skeletons made of cartilage, not bone. [02:42.72]Unlike most other kinds of fish, they are not able to reproduce [02:47.60]for several years, and also produce fewer young. [02:53.08]Pacoureau said sharks reproduce more like mammals than fish, [02:58.08]so “their populations cannot replenish [03:01.08]as quickly as many other kinds of fish.” [03:06.20]The number of fishing boats in the open ocean has risen sharply since the 1950s. [03:13.96]Climate change and pollution also place shark survival at risk. [03:20.76]But increased fishing pressure is the greatest threat [03:24.56]for every oceanic shark species. [03:28.72]Stuart Pimm is an ecologist at Duke University [03:32.64]and was not involved in the study. [03:36.40]He said the removal of top hunters like sharks [03:40.16]from the ocean affects all sea life. [03:44.92]“Sharks are like the lions, tigers and bears of the ocean world, [03:50.32]and they help keep the rest of the ecosystem in balance,” Pimm said. [03:56.92]I’m Jonathan Evans. 更多听力请访问51VOA.COM