WASHINGTON – North Atlantic shark populations have declined by more than 50 percent in the past 15 years, with some species approaching the point of no return due to relentless fishing pressure and scant international efforts to protect the toothy ocean predator, researchers say.

A study in Friday’s issue of the journal Science found that long-line fishermen harvesting tuna and swordfish from the Atlantic and adjacent waters are killing huge numbers of hammerhead, great white, tiger and thresher sharks.

This is consistent with other studies suggesting a decline in shark numbers in all of the world’s oceans.

A team of researchers at Dalhousie University in Halifax, Nova Scotia, analyzed the logbooks of long-line fishing fleets from 1986 to 2000 and found a sharp drop in the number of sharks killed while harvesting tuna and swordfish.

“We estimate that all recorded shark species, with the exception of makos, have declined by more than 50 percent in the past eight to 15 years,” the researchers reported.

“This is a worldwide phenomenon,” said Ransom A. Myers, a professor of biology at Dalhousie and a co-author of the study. “There are only a few areas in the world where we have good data, but wherever we go, they show the same thing – the shark is in serious decline.”

The study found that hammerheads declined by 89 percent in the Atlantic, while tiger sharks were reduced by 65 percent, blue sharks by 60 percent and threshers by about 80 percent.

The trend for great white sharks, the famed predator in the movie “Jaws,” is a decline of about 79 percent. The study found that in at least two fishing areas, no great white has been recorded since the early 1990s.

Myers said the shark is particularly threatened by intense fishing pressure because the animal cannot quickly replace its lost numbers.

“They are like humans,” he said. “They take a long time to mature and have relatively few babies. The bigger sharks have only about four pups a year. That makes them more vulnerable than other fish species.”

Although sharks in some parts of the world are targets of fishermen, in the North Atlantic they fall victim to fleets seeking other types of catch.

“The hammerheads concentrate in exactly the same places where the fleets fish for tuna and swordfish, so they are hit because they are at the wrong place at the wrong time,” said Myers.

The sharks routinely feed on the herring and squid commonly used for bait by the long-liner fishermen, he said, so catching sharks is just a routine part of fishing for the other species.

Myers said sharks could be protected by changing commercial fishing patterns. For instance, some of the sharks migrate along set paths at specific times of the year. Prohibiting fishing during those periods in the migration areas could reduce the bycatch of sharks, he said.

Also, establishing refuges where all fishing was forbidden would give sharks, along with other fish, a safe haven where they could feed and reproduce safely. This eventually would mean bigger catches for the fishing fleets, said Myers, and protection for the sharks.

“The shark is declining now; there is no reason to believe they would not recover if we fished in a responsible manner,” he said.

The United States now forbids harvesting of shark fins for shark-fin soup, a favorite in Asia, but long-line fleets from Spain and Japan continue to harvest, said Myers.

At a recent meeting of the Convention of International Trade in Endangered Species of the United Nations, Japan, a major shark-consuming nation with a vast fishing fleet, led a successful effort to defeat proposals that called for protection of two shark species.

Fisheries expert David O. Conover, a professor at the Marine Sciences Research Center at the State University of New York, Stony Brook, said the Dalhousie study shows “a particularly clear and compelling example where a group of species that are bycatch in commercial fishing are suffering a decline that has gone unrecorded.”

Conover said the shark is at the top of the food chain in the ocean and if those species were to be wiped out by overfishing, it could disrupt the entire ecosystem of the oceans.

“We know from many examples that once you start eliminating the predators at the top, it has a ripple effect throughout the food web,” said Conover.

“It is difficult to predict what would happen, but we know there are major effects in altering the food web.”