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Atlantic Sharks Found in Rapid Decline
By ANDREW C. REVKIN
Shark populations in the northwest Atlantic Ocean have plunged by more than half since scientists began keeping careful track in 1986, with marquee species like the hammerhead and the great white falling more than 75 percent, researchers are reporting.
Such an abrupt decline in the ocean's dominant hunters could substantially alter marine food chains in ways that are impossible to predict and 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 described their findings today in the journal Science.
The Dalhousie researchers, led by Julia K. Baum, a doctoral candidate at the university, said similar declines had probably occurred elsewhere and that "pervasive overfishing of these species may initiate major ecological changes."
They said there was no evidence that the decline was the result of any natural cycle, partly because similar trends have been recognized in the Pacific and other waters under heavy fishing pressure.
Other biologists had reported declines 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 ax 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, 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," said Dr. Robert E. Hueter, the director of the center for shark research at the Mote Marine Laboratory in Sarasota, Fla. "We're playing with the future of our marine food resources."
Shark experts said that the decline in sharks was unlikely to affect the number of attacks on humans, which remain rare, and in any event are mainly the result of the rising number of people in the water.
In the area studied, which included coastal and deep waters from Newfoundland to northern Brazil, only mako sharks showed no substantial drop in numbers, the scientists said. The mako ranges widely offshore.
The researchers found the trends by using various statistical models to analyze catch records from American vessels pursuing tuna and swordfish with longlines — miles-long strands with hundreds of baited hooks.
Sharks are usually an unintended catch for such fleets — which changed gear a decade ago to allow sharp-toothed sharks to 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 European boats that fish for sharks because of the growing popularity of their meat and from recreational fishing, Ms. Baum said. Federal regulations restrict shark fishing by American boats.
Big declines in sharks were found in coast-hugging species like hammerheads and deep-ocean wanderers like the thresher, with its distinctive elongated sickle-shaped tail fin.
The number of threshers has dropped 80 percent since 1986, and even then the number was below what it had been in the 1950's, the study's authors said in interviews. The population of great white sharks declined 79 percent since 1986, they said. But hammerheads appear to have fared worse, the scientists said, with a population decline of 89 percent from 1986 to 2000.
Some researchers expressed skepticism about this particular finding, saying that 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, were very convincing and troubling.
Little is known about the way various shark species live, and so it remains unclear why some might be more affected by fishing than others, Ms. Baum said.
Sharks, along with rays and skates, evolved hundreds of millions of years ago along a very different path than most fishes. 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. The slow reproductive rate is likely to delay a recovery even if fishing pressure abates, the Dalhousie researchers said.
Dr. Hueter of the Mote Marine Laboratory 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."