Overfishing is causing a sharp decline in shark populations in the northwest Atlantic, Canadian biologists have found. Conservationists warn the toothy predators need immediate protection, or they could disappear in a few years.

Sharks were once viewed as little more than an irritant that got caught in nets and took hooks meant for catching tuna and swordfish.

Now scientists view sharks as a valuable indicator of the health of the ocean ecosytem.

Disappearing shark

But when six researchers at Dalhousie University in Halifax looked at 14 years of mostly American fishing data, they found a number of shark species have declined to the point of no return for some.

They used mathematical models to track shark populations, allowing for under-reporting by fishing captains. The study appears in Friday's issue of the journal Science.

Between an area stretching from the coast of Nova Scotia to New England, the team found declines of:

• 89 per cent for hammerhead sharks
• 65 per cent for tiger sharks
• 60 per cent for blue sharks

Great white sharkCourtesy: Stanford University

The great white shark of Jaws fame has been reduced by 79 per cent.

Shark numbers were also way down at the northern tip of Brazil, in the Gulf of Mexico and the Caribbean.

Sharks need immediate protection: biologists

Julia Baum, a marine biologist at Dalhousie, said sharks are seen as vicious predators who don't get much pity. But their decline in the Atlantic signals troubles for the entire ocean system.

"The ocean ecology is like a big machine," said Baum. "Each part works with the other parts. When you take one part out you don't know what effect it's going to have."

• FROM CBC ST. JOHN'S:Shark populations in enormous decline: study

Overfishing is mostly to blame. Some sharks suffocate on hooks because they are pulled out of oxygen-rich water.

Other times, fishermen cut off the sharks' fins, which are valued by the Chinese for a soup.The sharks are released in the sea where they die.

The sharks' problems are compounded because they don't reproduce until they are almost 10 years old. Sharks carry their babies for a year, so heavy fishing disrupts their sensitive breeding cycle.

"They're slow growing and have very few young and so they can't bounce back quickly," said Dr. Jeff Marliave of the Vancouver Aquarium. "This means that the whole balance of life in the ocean gets changed."

• FROM JAN. 3, 2001:Satellites reveal sharks stray from shore

David Conover is a fisheries expert at the Marine Sciences Research Center at the State University of New York.

He said the study shows a "particularly clear and compelling example" of how bycatch in commercial fishing has contributed to the unrecorded decline of a group of species.

The study suggests shark reserves should be set up off the coast, and fishing for tuna and swordfish should be reduced to prevent sharks from being accidentally caught.