WASHINGTON – Scientists reported a 90 percent decline in large predatory fish in the world’s oceans since a half century ago, a dire assessment that drew immediate skepticism from commercial fishermen.

Analyzing nearly 50 years of data, two marine scientists at Dalhousie University in Canada say in Thursday’s issue of Nature that commercial fishing killed off all but 10 percent of populations of large prized tuna, swordfish, marlin and other fish species.

Average weights of those remaining also have declined sharply, they say.

“Although it is now widely accepted that single populations can be fished to low levels, this is the first analysis to show general, pronounced declines of entire communities across widely varying ecosystems,” scientists Ransom A. Myers and Boris Worm report in Thursday’s issue of Nature magazine.

Nelson Beideman, who directs the Blue Water Fishermen’s Association in Barnegat Light, N.J., said the report seemed aimed at developing “Chicken Little-type scenarios” to please the Pew Charitable Trusts, which helped finance the study. Pew backs a number of environmental groups.

“Fishermen are not seeing the whole ocean down at 10 percent,” said Beideman, whose trade group represents Atlantic longline fishermen.

Myers, a marine biology professor, and Worm, a research fellow, also found giant commercial fishing operations generally take less than 15 years to kill off 80 percent of a new fishing ground’s abundance. But they said marine life can recover if smaller, fast-growing species are given a chance to fill in for the large, overfished predators.

Myers began work on the report a decade ago, collecting data only for commercial fish that could be canned. He covered Japanese longline fishing between 1952 and 1999. Longlines, the most widespread fishing gear used on open oceans, catch tuna, marlin and swordfish by floating for miles with baited hooks dangling vertically below.

No marine fish stocks were known to be overfished when large-scale fishing fleets began spreading globally just after World War II and the Japanese were catching 10 fish per 100 hooks.

Now, they are lucky to catch one per 100, Myers said. The report uses other research to verify the results and expand them to other species.

The trends outlined in the report echo a 1994 estimate by the U.N. Food and Agriculture Organization that almost 70 percent of marine fish stocks were overfished or fully exploited. A U.N.-sponsored world summit in South Africa called for restoration of global fisheries by 2015.

Myers and Worm hope their work helps guide those efforts.

David Burney, who directs the U.S. Tuna Foundation in San Diego, said the report raises legitimate concerns about overfishing but probably overstates the problem.

“It just highlights what we’ve been saying for the last five years – there’s way too much fishing capacity out on the open seas. It’s a combination of more, bigger boats and technology advances that are allowing you to find the fish so much easier than before,” he said.

Michael Sissenwine, head of fisheries science at the National Oceanic and Atmospheric Administration, agreed with the report that fishing can cause big reductions in populations quickly but cautioned against drawing larger conclusions.

“There’s nothing that assures us that the data they are using is representative of all populations in the world,” he said, adding that fishing typically reduces a species’ population by at least 50 percent.

Barbara Block, a Stanford University marine biologist and one of the world’s leading tuna researchers, said the report’s findings are solid.

“What the paper is doing is bringing to the public the reality of what’s happening in our seas,” she said. “We’re systematically removing the large carnivores from the seas.”

Block said “some of the most magnificent creatures on Earth” are being eliminated before researchers fully understand them.

“Do we want a world without white sharks and giant tunas?” she asked. “Do we want a world without mako sharks? Industrial large-scale fishing is making that choice for all of humankind.”

Daniel Pauly, a leading fisheries expert based in Canada, also praised the report for its unusually comprehensive data illustrating the shortcomings of fisheries management.

“We always regulate the closing of barns after the horses have already left,” he said. “What it means is that the high seas fisheries that are opened up in the deep seas, they are a completely law-free environment like the Old West.”