Dramatic declines of large North Atlantic sharks due to overfishing have upset the balance of entire marine ecosystems, a new study shows. Now scallops1), clams2) and oysters are paying the price.
Smaller sharks, skates3), and rays4) that are normally eaten by the large sharks have become so abundant that they are ravaging5) 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.
A 2003 study by Julie Baum and fellow Dalhousie biologist Ransom Myers used fisheries?logbooks6)to track severe declines in large sharks since the 1980s.
In the new research, Myers, Baum, and three other marine biologists compiled7) 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.
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-fold8) in the last 30 years to around 40 million individuals. But the shellfish they feed on have been suffering.
“Herds of rays may destroy seagrass beds as they go through looking for smaller buried mollusks9),” 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.”