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Sharks: The necessary killer?
29 MARCH 2007

Big shark, toothy shark: Scallop's best friend?
Marine ecologists have long been caterwauling about the disappearance of top predators: Tuna, swordfish, and shark are all becoming as rare as hen's teeth, or fish feet. And now we read a new study linking the decline of 11 top-predator shark photo of hammerhead sharkspecies to declining numbers of shellfish, and the shuttering of a century-old scallop fishery in North Carolina.

How do sharks benefit scallops? Indirectly. Big sharks eat intermediate predators which, in turn, have a taste for coquilles St.-Jacques and other shellfish specialties.

Hammerhead shark photo by Suneko, at Wikimedia.

The prey fish, including small sharks, skates and rays, are called elasmobranchs because they (like the larger sharks), have a collagen-based skeleton. The prey are also called "meso-predators," which is jargon for: "They eat me, but I eat you. Don't mess with Mr. In-Between."

Could this three-part relationship explain the problem with sharks and scallops?

Graph shows decline of 87% for sandbar sharks; 93% for blacktip sharks, 97% for tiger sharks; 98% for scalloped hammerheads, 99% or more for bull, dusky and smooth hammerhead sharks
Seven great shark species declined by 87 percent to 99 percent in the Atlantic over the past 30 years or so. As an ecological force, these top predators have largely been exterminated. Data from Myers et al, see below.

Hard to prove
Doing ocean ecology is expensive and confusing, and it requires sorting through tsunamis of data, if you are lucky enough to have data. To test the relationship between sharks, meso-predators, and scallops and related organisms, Ransom Myers of Dalhousie University and Charles Peterson, of the Institute of Marine Sciences at the University of North Carolina, and colleagues focused on great sharks that eat elasmobranchs, and on those elasmobranchs eaten by the great sharks.

Then they combed through fishing and fish-survey records from the U.S. Atlantic Coast, and found severe declines in the sharks that eat elasmobranchs, along with population surges among those elasmobranchs eaten by the great sharks. "All 11 of the great sharks declined dramatically over 35 years, and 12 of the 14 prey species increased," said Peterson.

The big sharks are killed by deliberate fishing, the cutting of shark fins for use in Oriental medicine, and as "bycatch" during hunts for other fish.

One meso-predator liberated by the shark's disappearing act is the cownose ray, an unlovable creature with a toxic barb on its tail and a big-time appetite for bivalve shellfish like the bay scallop and soft-shell (steamer) clam. Fishermen in North Carolina's salt-water sounds had already linked the explosion of cownose rays with the decline in scallops, says Peterson, and one purpose of the new study was to test the association.

Close-up of a large, wet, gray flat fish
On the U.S. Atlantic Coast, the cownose ray has a taste for bay scallop. The ray's population is soaring, but it is pesca non grata among fishing folk. You can see how it got that name... Copyright Joe Brown, NOAA

The study strongly points toward shark declines as the causative factor in bay scallop declines, says Peterson. And the increase in mid-level predators apparently now inhibits recovery of depressed populations of clams, scallops and oysters.

Association does not prove causation, so to test how rays affect mollusks, the researchers built small pens in coastal North Carolina, setting the poles too close for the rays to enter. While unprotected scallops quickly disappeared from the nearby water, populations inside the pens were nearly stable (the small declines were blamed on voluntary scallop departures). The pens provided experimental evidence to support the hypothetical relationship between sharks, meso-predators and scallops.

Brown shell in the shape of a fan.For a century, fishing people in North Carolina caught bay scallops. About three years ago, the scallop became scarce, and the fishery collapsed. What has this to do with broader issues of marine ecology? Copyright: Sean Powers

Simple. Occam loved simple.
Further support for the hypothesis came from a principle called "Occam's razor," which states that all things being equal, the simplest explanation is preferable. "This is the most parsimonious explanation," says Peterson. "The loss of predation by the great sharks is responsible for the increase in meso-predators, and then for the decrease in the prey, the bay scallop and for suppressed recovery of other bivalves."

Other marine researchers have found that changing the top predator causes cascading impacts down the food chain. The present study advances this concept by "focusing on groups of species with a similar ecological function," Peterson says.

"This is an interesting example of the trophic cascade ideas that developed from studies of lake ecosystems here in Wisconsin," says James Kitchell, professor of limnology at University of Wisconsin-Madison (a "trophic cascade" is the kind of chain reaction under discussion, where a change at one level of the food chain affects organisms at another level). School of sharks with hammer-shaped heads swim through the ocean"We would expect that this kind of evidence might first appear in the North Atlantic because of its history of exceptionally high fishery exploitation rates. Clearly, this result endorses the growing call for an ecosystem approach in the management of marine systems."

Populations of the toothy hammerhead shark have crashed over 30 years. Sure, they can eat a sub sandwich sideways, but how has this decline affected smaller, more common animals? Photo: USGS

Studies in the North Atlantic, Kitchell adds, showed that after intense cod fishing, the shrimp that the cod ate had a population explosion. "New fisheries developed for those and actually produced a more valuable catch than that for cod when it was abundant." Fishery management decisions, he says, involve tradeoffs. "Reducing exploitation of large sharks might effect recovery of scallop fisheries in the United States," but much of the shark killing occurs in international waters, and hence is difficult to control.

Peterson says it's not too late to promote shark preservation, and that better enforcement and stricter regulation could benefit top predators that play such a critical role in the ocean. "Recovery of the great sharks requires patience and time, but they are still there," and there are even signs that hammerhead and tiger sharks may be starting to rebound.

"The top predator plays a role in maintaining the structure, and the delivery of goods and services from the sea," says Peterson. "One predator affects organisms on the food chain below it, which in turn has a strong effect on its prey."

— David Tenenbaum


Bibliography
"Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean," by R.A. Myers, J.K. Baum and T.D. Shepherd, Science, 30 March 2007.

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