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The populations of three shark species in the Atlantic off the coast of North America have plunged by more than 75 per cent in the past 15 years, raising the threat of local extinction for some of the world's best known marine carnivores, according to a new Canadian study.

The research, conducted by a team of biologists at Dalhousie University in Halifax, has found a precipitous decline in the number of white sharks -- the species popularized in the Hollywood Jaws movies as a human-killing machine -- and thresher sharks and hammerheads, which had a drop of 89 per cent, the largest recorded in the study.

The findings are being published in today's issue of the journal Science, and show that all eight shark species for which catch records are being kept, with the exception of makos, have declined by more than 50 per cent in the past eight to 15 years.

The plunge suggests that sharks are at risk of becoming extinct off the eastern coast of Newfoundland and Nova Scotia, where they are common, along the U.S. eastern seaboard, and in the Gulf of Mexico and Caribbean Sea.

"There is a very real risk that we could lose these populations in the Northwest Atlantic if something isn't done now," said Julia Baum, a PhD biology student at Dalhousie and the main author of the Science paper.

And though humans may be fearful of sharks because of occasional attacks on swimmers, Ms. Baum said sharks have a "bad rap" and have far more to fear from their human predators because the main reason for the population decline is intense commercial fishing.

"We think of them as these really fierce animals. Yes, they are because they're predators, but as a group, as a population, they're really fragile," Ms. Baum said.

Sharks are of particular interest to biologists because they are the top predator in many oceanic food chains, and their extinction could cause a major disruption to the world's seas.

Top predators control the number of animals they feed on, and the disappearance of sharks could "initiate major ecological changes," the Science paper warned.

Until now, sharks have been supremely well adapted to their aquatic environment, with an astonishing ability to survive that is rivalled by few other species.

The fossil record shows that sharks have existed for at least the past 400 million years in temperate and tropical oceans, and have managed to live through several mass extinctions that wiped out most other species.

But sharks now appear to be in trouble around the world, though an exact status on the fish's plight is difficult to determine because few records are kept on catches in the open ocean.

The one exception is in the Northwest Atlantic, where good statistics are available because of record keeping on catches by the U.S. National Marine Fisheries Service.

These records were used by the Canadian researchers to develop their population-trend estimates.

Many of the sharks were taken as an inadvertent catch by the swordfish and tuna fisheries. The sharks were caught on so-called long lines, or fishing lines that can be kilometres in length with hundreds of baited hooks, set in the open ocean. These lines frequently catch sharks, giving an indication of their abundance. Other sharks were caught by the recreational and trophy-sports fishery.

According to Ms. Baum, sharks, as the top predator in their ecosystem, have difficulty maintaining population levels when they themselves become prey to aggressive human exploitation.

One reason for this vulnerability is that sharks have an unusual reproductive pattern, unlike most other fish. Salmon, for instance, lay hundreds of thousands of eggs. But sharks give birth to small numbers of live young -- sometimes as few as two pups -- that are fertilized internally in a reproductive pattern similar to that of whales.

Sharks are also long-lived, and have a late sexual maturity. This means that populations can be hurt through overfishing, and have little ability to bounce back quickly even when catches are relatively small.

The records used by the researchers indicated that about 1.3 million sharks were caught in commercial and recreational U.S. fisheries over a period starting in either 1986 or 1992 and ending in 2000.

"They're not used to having predators and we're really the top predator. They haven't evolved a strategy to deal with these levels of exploitation," Ms. Baum said.

The paper recommends extensive marine reserves to preserve sharks, the same approach used for other endangered marine creatures, such as sea turtles.

Shark populations take deep plunge A 15-year study of sharks in the Northwest Atlantic shows that some species have declined more than 75 per cent since 1988. In the case of the oceanic whitetip, the recent count was compared with one in 1992. Hammerhead Down 89% Study group (60,402) is primarily composed of scalloped hammerheads. Thresher Down 80% The area for this study group (23,071) encompasses the known distribution of Northwest Atlantic populations. Therefore, the observed numbers suggest that they have declined. White Down 79% The small number in this study group (6,087) resulted in less precise trend estimates than for other species. Oceanic whitetip Down 70% Study group (8,526) Tiger Down 65% Study group (16,030) Blue Down 60% Study group (1,044). Conflicting patterns between the areas of highest catches could indicate density-dependent habitat selection, with blue sharks moving into preferential habitat as the population declined. Mako Down 40% Study group (65,795) is mostly shortfin makos. SOURCES: DEPARTMENT OF BIOLOGY, DALHOUSIE UNIVERSITY/ATLANTIC FISHES OF CANADA

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