Sharks Dwindle; Shellfish Suffer

A school of cownose rays. Credit: Steve Otwell

Thursday, March 29th, 2007--

The sea is running out of sharks and that's bad news for shellfish. An annual shark survey conducted off of North Carolina shows the sandbar shark population dropped 87 percent since 1972; tiger sharks dropped 97 percent; while scalloped hammerhead, bull, dusky and smooth hammerhead shark populations all dropped by 98 percent or more. These declines, reported in a paper in the journal Science this week, amount to the functional elimination of large sharks from the food web, at least off the coast of North Carolina, where the study took place.

“The decline of the sharks is not a huge shock,” says Charles Peterson, a researcher at the University of North Carolina Institute of Marine Sciences and co-author on the study. Previous studies have shown such dramatic declines: the news is that the sharks are taking the bivalves with them.

As sharks have disappeared, shark bait—skates, rays and small sharks (called mesopredatory elasmobranchs)—have gotten a new lease on life. The mesopredator populations have surged, which means more and more shellfish are getting eaten. As the sharks disappeared, so have the scallops and over a dozen other species of bivalves, according to the study.

This study is the first to document a “trophic cascade”—in which changes in top predator populations indirectly affect lower level organisms—in a complex food web. Cascade effects had previously been shown only in simple ecosystems that have only one top predator, Peterson says, explaining: “The reason we do see one in this more complex system is the almost uniform over-fishing that occurs at the top trophic level because of industrial very un-selective fishing.”

Incidentally, Peterson had been studying one particular elasmobranch, the cownose ray, on a tip from fishermen. Fishermen told him that they were seeing increased numbers of cownoses and were worried about the impact on the scallop fishery. “We had funding to test the hypothesis, which really arose from the fishermen, that cownose rays were diminishing the catch of bay scallops,” Peterson says. The cownose ray, which Peterson calls the “Canada goose of the marine world”, migrates through the waters off of North Carolina each year in schools of hundreds of thousands.

The fishermen’s concern was valid. “By accident, we were doing this work at the very time when the ray predation was so intense that it drove the fishery to extinction,” says Peterson. “We have had three straight years of a closed bay scallop fishery.”

To test whether the scallop disappearance was related to the rays, Peterson and his colleagues created pens, called stockades, to keep cownose rays out of some scallop grounds. “We did this over the entire highway of waterways that the rays move through in the sounds of North Carolina,” says Peterson, adding: “The scallops outside the stockades completely disappeared. Any population that had more than about one or two scallops per square meter was eliminated, whereas inside the stockades, we retained almost sixty percent.”

But, don’t blame the cownose rays entirely for the fishery crash. The elasmobranch upsurge is linked to fishing, too. Shark declines have been attributed to bycatch, the accidental catch of sharks during other fishing operations, and targeted fishing: in Asia, shark fins can go for as much as 22 dollars a pound: “It’s a high value product, so it still attracts a great deal of fishing pressure worldwide.”

Plus, cownose rays are just one species—of twelve—shown to be increasing. There are other hungry elasmobranchs that haven’t been accounted for. Peterson says: “The interesting thing is that nobody has studied the other eleven mesopredators that have increased, so the question arises: what are they feeding on and what kind of analogous impacts might they be having?”

What did you think of the story? Send us some feedback.

--Flora Lichtman

Sources

Charles Peterson
Institute Of Marine Sciences University Of North Carolina Chapel Hill, NC

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