Shark Declines Threaten Shellfish Stocks, Study Says
Dramatic declines of large North Atlantic sharks due to overfishing have upset the balance of entire marine ecosystems, a new study shows. Now scallops, clams, and oysters are paying the price.
Smaller sharks, skates, and rays that are normally eaten by the large sharks have become so abundant that they are ravaging shellfish stocks, the researchers say.
The shark declines, fed by growing worldwide demand for shark-fin soup, are indirectly causing some scallop fisheries to collapse entirely, the scientists add.
The study, which appears in this week’s issue of the journal Science, is the first ever demonstration of how wiping out top-level predators causes impacts that cascade down through the rest of the food web, the study authors say.
“Industrial fishing has left so few big sharks that they no longer perform their role as the top predators,” said study co-author Julia Baum of Dalhousie University in Nova Scotia.
“The predators of smaller species of sharks and rays have been virtually wiped out.”
A 2003 study by Baum and fellow Dalhousie biologist Ransom Myers used fisheries’ logbooks to track severe declines in large sharks since the 1980s.
In the new research, Myers, Baum, and three other marine biologists compiled additional fisheries’ records and independent research surveys going back to the 1970s to reveal that the original study underestimated the declines.
“This time we saw some species declining by 99 percent and more,” said co-author Charles Peterson, a biologist at the University of North Carolina (UNC).
What was most alarming was that all 11 major species of predatory shark—including sandbar, blacktip, tiger, hammerhead, and bull sharks—drastically declined, Peterson said.
“As a consequence we can explain why 12 of their 14 prey shark and rays species shot up in abundance in the same time frame,” he added.
One species to benefit from the shark declines is the cownose ray, which has increased 20-fold in the last 30 years to around 40 million individuals. But the shellfish they feed on have been suffering.
“Twenty years ago the abundance of rays was not sufficient to make a dent in the population of scallops,” said UNC’s Peterson, who has studied bay scallops in North Carolina since the 1980s.
“With the arrival of more rays, the scallop populations became decimated, and the [area’s] century-old fishery has closed,” he said.
“By excluding rays from small areas using underwater stockades, we showed that scallop populations do fine when they can escape the foraging rays.”
The new shark study “is an excellent illustration of the keystone importance of big sharks for marine ecosystems,” said Sarah Fowler, co-chair of the World Conservation Union’s (IUCN) Shark Specialist Group.
“Big sharks are likely to be an important and beneficial influence not only on scallop stocks, but also for other commercially important species in the food web that we haven’t yet considered,” she added.
For example, one worry is the impact cownose rays might have if they shift their diet to other species.
“Herds of rays may destroy seagrass beds as they go through looking for smaller buried mollusks,” Baum, of Dalhousie University, said.
“Our concerns are now that the cascade is going to continue one step further and threaten crucial nursery habitats for many marine species.”
Worrying New Plans
Eager to turn disaster into profit, U.S. entrepreneurs are now moving to establish cownose ray fisheries and promote new markets.
That’s exactly the same situation as the early 1980s, when other sharks were regarded as an underutilized resource, Baum said.
But just two decades later there’s a very real possibility of their extinction of the Northwest Atlantic.
“It would be a huge mistake to think that people can rush in and fish cownose rays without taking into account how vulnerable they are.”
Cownose rays take seven years to mature, and females give birth to only one pup a year.
Furthermore, IUCN’s Fowler added, “recovery of the predatory sharks will not happen if some of their most important prey—the rays—have also been depleted.”
“This study,” Baum noted, “underlines the need to consider ecosystems as a whole.”
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