Study Finds North Atlantic Shark Populations in Decline

ANDREW C. REVKIN / NY Times 16jan02

Shark populations in the northwest Atlantic have plunged by more than half since scientists began closely tracking them in 1986, with marquee species like the hammerhead and the great white falling more than 75 percent, researchers say.

Such an abrupt decline in the ocean's dominant hunters is likely to alter marine food chains in ways that are impossible to predict and that might take decades to reverse, the researchers and other experts said.

The researchers, from Dalhousie University in Halifax, Nova Scotia, ascribed the drop to intensifying commercial and recreational fishing for sharks, which reproduce slowly compared with other oceanic fish. They describe their findings in Friday issue of the journal Science.

The Dalhousie researchers said similar drops had probably occurred elsewhere and said "pervasive overfishing of these species may initiate major ecological changes."

They said there was no evidence that the drop was due to any natural cycle, partly because similar trends have been recognized in the Pacific and other waters with heavy fishing pressure.

Other biologists had reported drops in shark populations in particular coastal areas, but several experts not involved in the new study said it provided the first detailed overview of an oceanwide decline with broad implications.

"This is a very important synthesis," said Dr. James F. Kitchell, a biologist at the University of Wisconsin, who specializes in the role of predators in ecosystems. "Like the axe and the plow, the hook and the net can create major changes in ecological structure and function. We've been fishing the top off the food web."

The impacts on other marine life, both shark prey and other predators, remain unknown, but could last for generations, other experts said.

"It's a giant experiment, and we're not just playing in the laboratory here, we're playing with the future of our marine food resources," said Dr. Robert E. Hueter, the director of the center for shark research at the Mote Marine Laboratory in Sarasota, Fla.

Shark experts said that the decline in sharks was unlikely to affect the number of attacks on humans, which remain rare and are mainly a result of rising numbers of people in the water.

In the studied area, which included coastal and deep waters from Newfoundland to northern Brazil, only mako sharks —- a widely ranging offshore species — showed no substantial drop in numbers, the scientists said.

The researchers detected the trends by using various statistical models to analyze catch records from American vessels pursuing tuna and swordfish with longlines — mileslong strands studded with hundreds of baited hooks.

Sharks are usually an unintended catch for such fleets — which changed gear a decade ago to let sharp-toothed sharks break loose — but the catch rate provides a barometer of their abundance, Ms. Baum said. The Dalhousie researchers said they accounted for the change in fishing gear in their analysis.

The main strain on shark populations comes from other kinds of longline vessels that target sharks and from recreational fishing, Ms. Baum said. European boats still actively seek sharks for a growing market for their meat.

Big drops were found both in coast-hugging species, like hammerheads, and deep-ocean wanderers, like the thresher, which has a distinctive, elongated, sickle-shaped tail fin. Thresher numbers have dropped 80 percent since 1986, and even then numbers were already below what they were in the 1950's, the authors said in interviews.

Hammerheads appear to have fared worst, the scientists said, exhibiting an 89 percent population drop from 1986 to 2000. Some researchers expressed skepticism about this particular finding, saying these sharks tend to concentrate near coasts in waters not well scoured by longline tuna and swordfish boats.

Federal fisheries officials said they had measured smaller declines in hammerheads and other coastal species and saw signs that some species, like blacktip sharks, were starting a slow recovery.

Over all, though, many experts said the new findings, particularly for deep-ocean sharks, looked both convincing and troubling.

Little is known about the lifestyles of various shark species, and so it remains unclear why some may be more affected by fishing than others, said Julia K. Baum, a doctoral candidate at Dalhousie, who was the lead author of the study.

Sharks, along with rays and skates, evolved hundreds of millions of years ago along a very different path than most fish. They have skeletons of cartilage, not bone, and take much longer to reach sexual maturity — 12 to 18 years for some species — and produce far fewer progeny than bony fishes like bass, sometimes just one or two live-born pups per female.

These habits suit their role as the ocean's dominant hunters, but also put them at greater risk when they are killed by people. The slow reproductive rate is likely to delay a recovery even if fishing pressure abates, the Dalhousie researchers said.

Dr. Hueter agreed. "Sharks are adapted to being the predators, not the prey," he said. "If we take them to the brink and decide we don't like what's happened, that'll be too bad, because it'll be impossible to bring them back quickly."