On the high seas, far from any continent, sharks and rays were once abundant. Shortfin makos, the fastest sharks on the planet, chased after their prey at speeds of over 20 miles an hour. Scalloped hammerheads plied the waters, scanning the ocean expanse for food with their wide-set eyes and other specialized sensory organs.
These animals traveled widely across open waters so vast and inaccessible that many fishermen, and even some biologists, found it hard to believe that overfishing would ever endanger them.
“A decade ago,” recalls Nicholas Dulvy, co-chair of the International Union for Conservation of Nature’s Shark Specialist Group, “we would have extremely heated debates about listing an oceanic shark as threatened.”
Now, a sweeping analysis of current and historic population numbers have created a clearer—and sobering—picture. Dulvy and co-author Nathan Pacoureau, both at Simon Fraser University in Canada, found that populations of 18 shark and ray species have declined by 70 percent since 1970, according to the study, published in the journal Nature this week. At this rate, many of the species might disappear entirely in a decade or two, the authors caution. (Read about the most fascinating shark discoveries of the last decade.)
When the research team crunched the numbers for the oceanic whitetip shark, a species that was common back in 1970, we “just looked at them in stunned silence,” Dulvy says.
The oceanic whitetip “had declined by 98 percent in the last 60 years. That pattern was consistent across all three oceans.” The International Union for Conservation of Nature now lists the species as critically endangered.
Scalloped and great hammerhead sharks have met a similar fate. Though fisheries rarely target oceanic sharks, if they're caught, their meat, fins, gill plates, and liver oil is often sold.
This is troubling news both for the sharks and ocean health, as these top predators play a crucial role in the food web, in part by keeping smaller predators in check, experts say.
Diving into the data
For the study, Dulvy and Pacoureau collected all the data on those 18 species they could find across the globe, much of it buried in government reports or collecting dust on old hard drives.
A growing public awareness of shark conservation has prompted fisheries management agencies to start collecting shark data, providing the team with an influx of brand-new information.
The scientists ended up with 900 datasets spanning 1905 to 2018, each representing a species’ population changes over time within a particular region. With the help of international experts and computer modeling, the team extrapolated these data into best estimate of the global change in abundance. (Read more about sharks, lords of the sea.)
They also took into account the development of open-ocean fishing techniques. Long lines studded with hundreds of hooks or enormous purse seine nets often accidentally ensnare sharks, and their use has doubled in the last half century, while the number of oceanic sharks caught in them has approximately tripled.
“Combined with the increasing rarity of sharks, this means that the chance for an individual shark to get caught is now 18 times higher than it was in 1970,” says Dulvy.
Dulvy adds that uncertainty is inevitable in his analysis, and that the authors have likely underestimated some of the species’ declines, particularly in areas where overfishing has occurred for many decades.
Tropical fish hardest hit
The biggest decrease occurred in shark and ray populations in the tropics, where offshore fisheries have expanded in recent decades.
As larger sharks and rays become rare, fisheries are pivoting toward smaller species, says co-author Holly Kindsvater, a population biologist at Virginia Tech who has studied several species of devil rays, some populations of which may have fallen by 85 percent in the past 15 years.
Though people do eat their meat, their gill plates have recently become more popular in Chinese medicine. This shift shows how fishermen can pivot to other species when their original prey becomes scarce, she says. (Read how reef sharks are also in big trouble worldwide.)
“I don’t think there are many boats on the high seas exclusively targeting sharks and rays. But if you’ve started out fishing for tuna, and you’ve overfished them, you’ll start catching other things, and you’ll find a way to sell those, as well.”
Fishing for solutions
The impact of overfishing, accidental or otherwise, on sharks should motivate governments to impose more regulations, with the goal of making fisheries sustainable, Dulvy says. It's also important, he adds, to limit the international trade of threatened shark and ray species.
But there’s a long way to go. A proposed ban on fishing of endangered shortfin makos in the North Atlantic was recently blocked by the European Union and the United States, in part because the bulk of the catch is by Spain, says Dulvy.
“Sharks are kind of the last unregulated area,” he says, “which I think is why there is a bit of resistance to managing them.”
Such a ban has been shown to work for other species, says David Sims, a biologist at the U.K.’s University of Southampton who was not involved in this study. Sims has published research showing hopeful signs of recovery for great white sharks and porbeagles in the northwestern Atlantic, both of which are protected from fishing. (Learn about six sharks you’ve probably never heard of.)
Other solutions include creating marine reserves or setting aside no-fishing zones in shark-rich hot spots, Sims says.
Jessica Cramp, founder of the marine research and conservation organization Sharks Pacific and a National Geographic Explorer, agrees. She has helped establish several protected areas and a shark sanctuary in the Cook Islands to benefit migratory species, including sharks.
“These may offer refuge for species like the oceanic whitetip and silky shark,” she says, ”which this study has confirmed are in big trouble.”