The new research based on nearly 50 years of data offers a bleak outlook for some of the most commercially valuable trophy fish species and further debunks a notion that oceans are limitless blue frontiers teeming with boundless life.

"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. "Most scientists and managers may not be aware of the true magnitude of change in marine ecosystems."

Myers, a marine biology professor at Dalhousie University in Canada, and Dalhousie research fellow Worm found it generally takes less than 15 years for giant commercial fishing operations to kill 80 percent of a new fishing ground's abundance.

They also found marine life can recover from such commercial operations if smaller, fast-growing species are given a chance to fill in for the overfished predators, whose average weights also are declining sharply.

Myers began work on the report a decade ago, collecting data only for commercial fish that could be put into cans.

The data cover Japanese fishing between 1952 and 1999 for the most widespread type of fishing gear - longlines - used on the open oceans to catch tuna, marlin and swordfish. Longlines float for miles with baited hooks dangling vertically below, causing lots of other unintended catches.

Just after World II, as large-scale fishing fleets began spreading globally, no marine fish stocks were known to be overfished and the Japanese caught 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.

Michael Sissenwine, head of fisheries science at the National Oceanic and Atmospheric Administration, said he agrees 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.

"We shouldn't on the other hand conclude that a substantial reduction is a problem," he said. "The point is we shouldn't be thinking we can have fisheries and leave the ecosystem in a pristine state."

One recent success story is the rebuilding of North Atlantic swordfish populations to 94 percent of what they should be, up from 65 percent, because of stricter management since 1999, according to the International Commission for the Conservation of Atlantic Tunas.

Daniel Pauly, a leading fisheries expert based in Canada, said the report is significant, however, 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."

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 data serves as a guide for those efforts.

Barbara Block, a Stanford University marine biologist and one of the world's leading tuna researchers, likened the losses of big fish to the devastating population declines of great whales in the past century.

"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."

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