Big Sharks Now Functionally Extinct, CBC, 03/30/07
Source: CBC News
The overfishing of large shark species off the eastern seaboard of the United States has upset the balance of marine life, a team of Canadian and U.S. scientists says.
"We're chopping off the top of the food chain," said Julia Baum of Dalhousie University in Halifax, co-author of a study outlining the huge decline in big sharks over 35 years.
Overfishing of several species of sharks, including hammerheads, is causing a dangerous ripple effect through the marine food chain, a new study says.
(Yves Lefebre - Fundacion Malpelo/Associated Press)
Fewer big sharks mean there are more of the fish they once ate, such as the cownose ray.
The population of the rays in Chesapeake Bay has grown by 20 times in 30 years, said Charles Peterson, a professor of marine sciences at the University of North Carolina at Chapel Hill and co-leader of a study published in Science on Friday.
As the rays increase — they may now number 40 million — they have slashed the populations of bay scallops, oysters and soft-shell and hard clams, Baum said. The century-old bay-scallop fishery is now just a memory.
So many sharks have been killed that they are "functionally extinct," which means they can no longer perform their role of controlling middle predators in the marine ecosystem, Baum told CBC News.
The population imbalance shouldn't be a surprise considering the drop in large sharks in recent decades. While they are fished to meet the growing demand for shark fins and meat, the single largest problem is the sharks taken accidentally as "bycatch" by fishermen seeking other species, she said.
As many as 73 million sharks are killed worldwide each year for their fins, and the number is rising rapidly.
The new study, funded by the Pew Institute for Ocean Science, updated a 2003 study by Baum and noted Dalhousie fisheries biologist Ransom A. Myers, who died recently.
The new study concluded that the original estimates of declines in big sharks had been too conservative.
Now the population of scalloped hammerhead and tiger sharks may have fallen by more than 97 per cent since 1970, while bull, dusky and smooth hammerhead sharks are down by more than 99 per cent.
As the shark populations fell, the fish they once ate boomed and the marine life at the bottom of the food chain — scallops, shrimp, clams — have been ravaged.
The rise of the cownose ray is even affecting the underwater vegetation.
"They're known in the Chesapeake Bay, in their great schools, to excavate and denude large areas of sea grass," Peterson said.
Increased protection for great sharks, including reduced fishing of all shark species and enforcing bans on shark finning, is needed to rebalance the system, Baum said.