Overfishing of sharks makes scallops vanish: study

WASHINGTON Thu Mar 29, 2007 6:58pm EDT

1 of 4. An undated image of a White Shark. With most of the great predatory sharks gone from northwest Atlantic waters, the rays and skates the sharks normally feed on had a population explosion, researchers reported on Thursday.

Credit: Reuters/NOAA/Handout

WASHINGTON (Reuters) - Overfishing of big sharks in the Atlantic has cut stocks by 99 percent, dooming North Carolina's bay scallop fishery and threatening other species including shrimp and crabs, researchers reported on Thursday.

With most of the great predatory sharks -- bull, great white, dusky and hammerhead -- gone from northwest Atlantic waters, the rays and skates the sharks normally feed on had a population explosion, the scientists said in the journal Science.

"With fewer sharks around, the species they prey upon -- like cownose rays -- have increased in numbers, and in turn, hordes of cownose rays dining on bay scallops have wiped the scallops out," said study co-author Julia Baum of Dalhousie University in Halifax, Nova Scotia.

Bull, dusky and hammerhead sharks have declined by more than 99 percent between 1970 and 2005, Baum said in a statement.

This coincided with a rise in Asian demand for shark fins for medicinal uses and for food. Shark fins currently sell for about $22 a pound, Peterson said, citing a local fisherman.

Now that the ravenous rays and skates have feasted on bay scallops, they are likely to look for food in protected areas along the coast where other fish and shellfish shelter in their early months of life, said co-author Charles "Pete" Peterson of the University of North Carolina at Chapel Hill.

"We know that once they eat the things that are the most easy, evident and obvious to get, which are those on the surface of the bottom like a scallop and an oyster, they turn to digging in the bottom to get buried shellfish," Peterson said by telephone.

RAIDING THE SEAGRASS NURSERY

Rays and skates are good diggers and can excavate seagrass beds along the Atlantic coast, Peterson said.

Seagrass beds are normally used as nurseries for young fish and shellfish like shrimp and crabs because they protect against predation by what Peterson called "wimpier predators" such as crabs; they are not build to stand up to raids by bigger species like rays.

"It's a nice hiding place and a productive habitat," Peterson said of the seagrass beds. "It tends to be warmer and less violent physically. ... You probably don't have nannies, but outside of that, you've got everything you'd want in a nursery in the seagrass bed."

Many Atlantic coast and Gulf of Mexico fisheries are dominated by animals and plants that depend on estuaries, the parts of rivers that connect to the sea.

If rays and skates prey on these shellfish and some of the young grouper and snapper fish that begin their lives in the seagrass, these species could also be threatened, Peterson said.

The overfishing of sharks may be a consequence of a previous overfishing of cod, Peterson said.

When fishing agencies looked for an unexploited resource to replace cod as a mainstay, they settled on shark about 25 years ago.

Sharks, in huge demand in Asia, are also frequently caught inadvertently by nets meant to snare swordfish, he said.

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