Decline of Big Sharks Lets Small Predators Decimate Shellfish
Friday, March 30, 2007
A sharp decline in big sharks along the Eastern Seaboard has prompted a boom in other marine species that is devastating valuable commercial fisheries, researchers are reporting today in the journal Science.
The study -- by a team of Canadian and U.S. scientists -- found that intense fishing for sharks in the northwest Atlantic over the past 35 years has produced a cascade of unexpected effects. With fewer large predators in the sea, the number of rays, skates and small shark species has exploded, and these species are decimating such shellfish populations as North Carolina bay scallops and the Chesapeake Bay's American oysters.
As many as 73 million sharks are killed each year to supply fins for shark-fin soup, a Chinese delicacy.
Charles H. Peterson, a professor of marine sciences biology and ecology at the University of North Carolina at Chapel Hill, who helped write the paper, said he and his colleagues calculated that between 1970 and 2005, the number of scalloped hammerhead and tiger sharks may have declined by more than 97 percent along the East Coast and bull, dusky and smooth hammerhead sharks have dropped by more than 99 percent.
"This group of coastal sharks have very important roles in organizing the ecosystem," Peterson said in a telephone interview, adding that the team determined that the populations of 12 of the sharks' 14 prey species increased during the period. "We've just scratched the surface of the implications."
The cownose ray, which spends much of the summer in the Chesapeake and preys on several local species, has benefited significantly. The rays, whose wingspan can reach six feet, eat massive amounts of bay scallops, oysters and soft-shell and hard clams in the Chesapeake Bay, Delaware Bay and other estuaries in the area.
Peterson and a co-author, University of South Alabama marine sciences professor Sean P. Powers, determined from ocean-bottom sampling that migrating rays are eating nearly all adult scallops in North Carolina's sounds. Their disappearance prompted the state to shutter its century-old bay scallop fishery in 2004. The population of cownose rays has risen to as much as 40 million, and Peterson said scientists are worrying that the rays will target other species.
"What are these rays going to be eating now to fuel their energy for migration, and just to stay alive?" Peterson asked. "That's the fear."
In another part of the study, three marine biologists at Canada's Dalhousie University -- Ransom A. Myers, Julia K. Baum and Travis D. Shepherd -- used data from commercial fisheries and research surveys to document a sharp drop in 11 species of great sharks in the northwest Atlantic since the mid-1980s. Myers, a renowned fisheries scientist, died Tuesday of a brain tumor in Halifax, Nova Scotia.
Overfishing has also translated into fewer mature sharks along the Eastern Seaboard, according to a separate UNC survey. That study, undertaken every year since 1972, suggests that the average lengths of blacktip, bull, dusky, sandbar and tiger sharks have shrunk by between 17 and 47 percent.
Baum noted that just last month, the World Conservation Union's shark specialist group listed great hammerhead and scalloped hammerhead sharks as in danger of extinction, and dusky and sandbar sharks as vulnerable to extinction.
"We're at an extremely critical point for sharks worldwide right now," Baum said in an interview. "Their populations are plummeting, and at the same time the demand for shark fins in Asia is skyrocketing right now. If the world doesn't get serious about shark conservation right now, we really risk losing a lot of these species."