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Empty seas: Since 1970, ocean shark numbers have fallen 71 percent



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It seems like he’s explaining the Jedis of the ocean while marine biologist Stuart Sandin is asking about sharks. “They are wonderful predators, fast swimmers and they have amazing senses that can detect any disturbance from a great distance in the ocean, such as smells or small changes in water currents.”

Their ability to easily detect something about their world beyond the ordinary lets them locate prey in the open ocean’s vastness. Although, in the face of increased international fishing demand, it also leaves them increasingly weak, as global fishing fleets have doubled since 1950.

Sandin, who works at the Scripps Institution of Oceanography, said, “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.”

For decades, scientists have recognised that individual shark species are dwindling, but a recent analysis focused on 57 global databases highlights how significantly global stocks have declined over the past half century.

According to a report released Wednesday in the journal Nature, the abundance of oceanic sharks and rays fell more than 70 percent between 1970 and 2018 globally.

And 24 of the 31 shark and ray species are threatened with extinction, while three species are considered critically endangered, including oceanic whitetip sharks, scalloped hammerhead sharks and massive hammerhead sharks.

“For global shark populations, the last 50 years have been quite devastating,” said Nathan Pacoureau, a scientist at Canada’s Simon Fraser University and a co-author of the report.

Sharks are often deliberately caught by fishing vessels, but most frequently accidentally reeled as “bycatch,” during fishing for other species such as tuna and swordfish.

Sharks and rays are all fish composed of cartilage with skeletons, not bone. They usually take several years to achieve sexual maturity, in comparison to most other forms of fish, and they have fewer offspring.

“They reproduce more like mammals in terms of timing, and that makes them particularly vulnerable,” said Pacoureau. “Their populations are unable to fill up as quickly as many other types of fish.”

Since the 1950s, the number of fishing vessels trolling the open ocean has risen steeply as motor capacity increased the reach of ships. And while climate change and pollution also jeopardise shark survival, the biggest danger to all oceanic shark population is increased fishing demand.

“When you remove the ocean’s top predators, it affects every part of the marine food web,” said Stuart Pimm, a Duke University ecologist who was not interested in the research. “Sharks are like lions, tigers and bears in the world of the oceans, and they help to balance the rest of the ecosystem.”

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