Shark declines threaten shellfish stocks Published: 30 March, 2007
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 said.
According to National Geographic, the shark declines, fed by growing worldwide demand for shark-fin soup, are indirectly causing some scallop fisheries to collapse entirely, the scientists added.
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% 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 sharkincluding sandbar, blacktip, tiger, hammerhead, and bull sharksdrastically 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.
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 underutilised 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 preythe rayshave also been depleted.”
“This study,” Baum noted, “underlines the need to consider ecosystems as a whole.”
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