The grooveless hammerhead shark (pictured here is a trenchless hammerhead shark swimming off the coast of the Bahamas) is a critically endangered species.
摄影：BRIAN SKERRY，NAT GEO IMAGE COLLECTION
Written by: TIM Vernimmen
On the high seas, far from the mainland, sharks and rays were once abundant. The grey mackerel shark is the fastest shark on Earth, chasing prey at a speed of 32 kilometers per hour. The Hammerhead Shark travels freely through the water, using its wide field of vision and other special sensory organs to find food in the ocean. The range of these animals is extremely wide, and many of the seas are inaccessible to humans, so many fishermen and even some biologists have a hard time believing that overfishing threatens their survival. "Ten years ago," recalls Nicholas Dulvy, co-chair of the IUCN shark expert group, "we had a heated debate about whether to list a shark as endangered. Now, researchers have conducted a comprehensive analysis of current and past shark population data for clearer, more thought-provoking, comprehensive information. According to a new study published this week in the journal Nature, Dulvy and study collaborator Nathan Pacoureau of Simon Fraser University in Canada found that the number of sharks and rays in 18 species of sharks and rays has decreased by 70 percent since 1970. The study authors warn that at this rate, many shark and ray species could disappear completely within 10 to 20 years. When the team counted the number of pelagic whitetip sharks, a species that was common in 1970, we were "dumbfounded and stunned," Dulvy said. Ocean-going whitetip sharks "have decreased by 98% in the last 60 years." This trend of reduction is consistent across all three oceans. The International Union for Conservation of Nature now lists the species as critically endangered. The Luxe hammerhead shark and the grooveless hammerhead shark face a similar fate. Although fishing companies rarely target sharks, their meat, fins, gills and cod liver oil are often sold if they are caught. Experts say this is troubling news for both sharks and ocean health, as these top predators play a vital role in the food chain and play a role in controlling the number of small predators.
On iturup Island in the Russian Far East, two fishing boats are fishing in purse seines. Sharks are often caught in this fishing technique.
Photograph by Sergei KRASNOUKHOV, GETTY IMAGES
Dig deeper into the data
Over the course of the study, Dulvy and Pacifico collected all the data on 18 shark species that could be found globally, most of which were hidden in government reports or stored on old hard drives that had been stored for a long time.
As public awareness of shark conservation increases, fisheries management agencies begin to collect shark-related data, thus providing a wealth of new information for the research team.
The scientists ended up with 900 datasets from 1905 to 2018, each representing how the number of shark species in a particular sea area changed over time. With the help of international experts and computer simulations, the research team used this data to make extrapolations to better estimate changes in sharks around the world.
They also considered developments in distant-water fishing techniques. Long lines or huge purse seine nets with hundreds of fishing hooks often catch sharks by accident, and the use of such fishing gear has doubled over the past half-century, while the number of sharks caught using them has roughly tripled.
"Coupled with the fact that sharks are becoming more rare, this means that a shark is now 18 times more likely to be caught than it was in 1970," Dulvy said.
Dulvy added that, in his own analysis, uncertainty is inevitable, and the study authors may have underestimated the decline of some shark species, especially in waters where overfishing has been taking place for decades.
Tropical sharks have been hit hard
Shark and ray populations have declined the most in tropical waters. In recent decades, offshore fisheries in the tropics have expanded.
As larger sharks and rays become scarcer, fisheries begin to shift to smaller fish species, said study collaborator Holly Kindsvater, a population biologist at Virginia Tech who studied several species of manta rays, some of which may have lost 85 percent of their populations over the past 15 years.
While it is true that people eat manta rays, their gill plates have recently become increasingly popular in the TCM community. The shift, she said, suggests that fishermen can switch to other species when initial fishing objects become scarce.
"I don't think there are many boats on the high seas that specialize in sharks and rays. However, if you start out catching tuna, overfishing you'll catch other fish and find ways to sell them. ”
solution
Whether accidental or intentional, the impact of overfishing on sharks should prompt governments to implement more regulations to achieve the goal of sustainable fisheries development, Dulvy said. It is also important to limit international trade in endangered sharks and rays, he added.
Still, there's a long way to go. Recently, the European Union and the United States banned fishing for endangered mackerel sharks in the North Atlantic, in part because Spain catches large numbers of gray mackerel sharks, Dulvy said.
"In a way, sharks are the last unregulated fish," he said, "and I think that's why there's some resistance to managing them." ”
Such bans do show that such bans do help protect other species, says David Sims, a biologist at the University of Southampton in the United Kingdom, who was not involved in the study. A study published by Sims shows signs of recovery in the northwest Atlantic great white shark and porch shark, both of which are protected from fishing.
Sims said other solutions include establishing marine protected areas or no-fishing zones in shark-infested hotspots.
Jessica Cramp, founder of sharks pacific, a marine research and conservation organization, agrees. She has helped establish several protected areas and one shark sanctuary in the Cook Islands to protect migratory species, including sharks.
"These protected areas may provide shelter for species such as the pelagic whitetip shark and the silk shark," she said, "and this study has confirmed that both face a huge threat." ”
(Translator: Stray Dog)