Experts say one of the world's most feared, but misunderstood, creatures is under assault by fishermen. A new study of all types of sharks finds that there are fewer of them than there used to be, and that is harming oceans' ecosystems.

A study in this week's journal Science is one of the first to look at the shark population. Researchers surveyed sharks in the northwest Atlantic, a vast area that encompasses Newfoundland to the north and northern Brazil to the south, and stretches from the Gulf of Mexico to a distant point east off the coast of South of America.

Lead author Julia Baum says fisherman using long lines with up to a thousand hooks, designed to catch swordfish and tuna, are also snaring sharks. And Ms. Baum says this has resulted in huge shark declines.

"For hammerhead sharks, an 89 percent decline is what we are estimating for the past 15 years. That is a phenomenal decline," she said. "Similarly, for the thresher shark, we are estimating an 80 percent decline."

The study also found a 79 percent drop for white sharks and a 65 percent decline in tiger sharks since 1986.

Ms. Baum and colleagues at Dalhousie University in Halifax, Canada used data from logbooks kept by fishermen, and they plugged it into a model to arrive at the estimates.

Ms. Baum says the use of hooked long lines is common all over the world, so it is safe to assume that shark populations are in steep decline globally. While many sharks are caught by accident in long lines, she says shark fins are prized throughout Asia to make soup, so there is a deliberate effort to catch them as well.

Many people might ask why it is important to worry about fewer sharks, because they are predators and will attack humans.

"The number of attacks throughout the world is fairly minimal," said Malcolm Smale, marine biologist with the Port Elizabeth Museum in South Africa.

"For example, in South Africa, we only have about three to five attacks per year on average. Less than one per year is fatal. And this is a much lower number than any other cause of injury or fatality, for example, road accidents, people getting shot in burglaries, people drowning, people struck by lightning and so on," he added.

But sharks are the oceans' top predator. As such, Mr. Smale says they eat smaller, weaker fish and those that may be unfit for human consumption.

"Now, one might think that is not such a bad thing. But in reality, what we would call pest species - those with very rapid cycles, in other words, they breed within a year or two - are likely to become more common," he said. "And on the one hand, that might be thought to be a good thing if they are edible, but in many cases these species might be pest species, in other words, not suitable for human consumption. And so this is detrimental for both humans and the food web."

Experts say unlike the fish they eat, sharks have long life cycles, and it could take years to replenish their numbers, if that is possible. In the meantime, Canadian researchers suggest preserving the shark population by discouraging over-fishing and establishing marine reserves.