Shark Populations Dropped 71 Percent Since 1970
Scientists have known for decades that the numbers of some shark species are decreasing. But a new study shows just how severely worldwide populations have dropped in the past 50 years.
The numbers of oceanic sharks and rays fell more than 70 percent worldwide between 1970 and 2018. The information comes from a study that appeared recently in the publication Nature.
Stuart Sandin is a marine biologist who works at the Scripps Institution of Oceanography. He says that sharks are great hunters, fast swimmers and have extraordinary senses.
Sandin adds that they can identify any changes in the ocean from a great distance, such as smells or tiny changes in water flow.
Their ability to quickly sense anything outside the norm in their environment helps them find food in the open ocean. But it also makes them especially at risk in the face of increased international fishing pressure.
"You drop a fishing line in the open ocean, and often it's sharks that are there first — whether or not they're the primary target," said Sandin.
Twenty-four of the 31 species of sharks and rays are threatened with extinction. Three species — oceanic whitetip sharks, scalloped hammerhead sharks and great hammerhead sharks — are considered critically endangered.
Nathan Pacoureau is a biologist at Simon Fraser University in Canada and one of the writers of the study. He said, "The last 50 years have been pretty devastating for global shark populations."
Sometimes sharks are caught on purpose by fishing boats. But more often they are caught while fishing for other species such as tuna and swordfish.
Sharks and rays are both fish with skeletons made of cartilage, not bone. Unlike most other kinds of fish, they are not able to reproduce for several years, and also produce fewer young.
Pacoureau said sharks reproduce more like mammals than fish, so "their populations cannot replenish as quickly as many other kinds of fish."
The number of fishing boats in the open ocean has risen sharply since the 1950s. Climate change and pollution also place shark survival at risk. But increased fishing pressure is the greatest threat for every oceanic shark species.
Stuart Pimm is an ecologist at Duke University and was not involved in the study. He said the removal of top hunters like sharks from the ocean affects all sea life.
"Sharks are like the lions, tigers and bears of the ocean world, and they help keep the rest of the ecosystem in balance," Pimm said.
I'm Jonathan Evans.
Athletes need a good sense of balance.
They believed that the greater the diversity the more stable the ecosystem.
Worldwide sales reached 2.5 billion.
Pollution can harm marine life.
5.Climate change and pollution also place shark survival at risk.
Climate change 气候变化
Climate change is still very much a subject for debate.
We are all victims of climate change.
6.Climate change and pollution also place shark survival at risk.
at risk 处于危险中
He was putting himself at risk.
Hundreds of thousands of people are at risk.
斯图尔特·桑丁(Stuart Sandin)是斯克里普斯海洋研究所的海洋生物学家。他说，鲨鱼是优秀的猎手，游泳速度飞快，而且有着非凡的感官 。
内森·帕库鲁（Nathan Pacoureau）是加拿大西蒙弗雷泽大学的生物学家，也是这项研究的作者之一。他说：“过去50年对全球鲨鱼数量来说是毁灭性的 。”
自20世纪50年代以来，公海上的渔船数量急剧增加。气候变化和污染也使鲨鱼的生存面临风险 。但不断增加的捕捞压力是所有海洋鲨鱼物种最大的威胁 。
斯图尔特·皮姆(Stuart Pimm)是杜克大学的生态学家，他没有参与这项研究。他说，像鲨鱼这样的顶级猎手从海洋中消失会影响到所有海洋生物 。