Shark Declines Threaten Shellfish Stocks, Study Says

Helen Scales
for National Geographic News
March 29, 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 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."

Wholesale Declines

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.

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