Annually, the researchers said 308,000 of the marine mammals die unintentionally in fishermen's hauls.
There are more than 80 species collectively known as cetaceans, or fishlike sea mammals. They range from porpoises weighing 100 pounds to the blue whale, the world's largest creature at more than 120 tons. Many species are near extinction because of centuries of overhunting.
The new study, conducted by American and Scottish biologists, suggests that accidental captures, known as "bycatch" in the fishing industry, may be the biggest immediate threat to these animals' survival - even more than ship collisions and pollution.
The report was released by World Wildlife Fund, a Washington-based advocacy group, as governments gather in Berlin for the 55th annual International Whaling Commission meeting that begins Monday.
Fishing industry officials did not respond to interview requests.
"This level of bycatch is no doubt significantly depleting and disrupting many populations of whales, dolphins and porpoises," said the study's lead researcher, Andy Read of Duke University Marine Laboratory in Beaufort, N.C. "Several species will be lost in the next few decades if nothing is done."
Marine researchers who did not contribute to the study said the new mortality estimate is "very distressing."
"There is need to harvest seafood," said Michael Moore of the Woods Hole Oceanographic Institute on Massachusetts' Cape Cod. "We should be able to feed the planet without driving non-food species to extinction. But I'm not sure we can."
Since the 19th century, commercial whaling turned millions of whales into lubricants, cosmetics, margarine and meal. The IWC banned most whaling in the 1980s. Norway ignores the ban, while Japan takes nearly 700 whales a year under a controversial IWC research exemption. Some native cultures are allowed to conduct strictly limited hunts.
Cetaceans still patrol the same waters, now described by biologists as an undersea forest of lobster pots, baited long lines and huge nets.
To prepare the bycatch estimate, Read and scientists at the University of St. Andrews in Scotland analyzed cetacean deaths in 125 marine mammal populations in 1990-99. Most of the deaths occurred in U.S. waters.
Commercial fishing advocates point out that cetacean deaths decreased 40 percent in the United States in the last decade as new federal laws were enacted and equipment improved.
Other measures, including underwater acoustic alarms, were employed in the late 1990s with the help of fishermen.
But, scientists say, some fishermen rush to cut away lines and nets cinching the bodies of dead whales before investigators can trace them through the equipment.
The decrease also reflects the collapse of once-productive fisheries, as well as drastic declines in cetacean populations overall.
To reach the worldwide estimates, the researchers resorted to multiplying the U.S. statistics. They acknowledged their results were "very crude," but said mortality figures in more remote countries were not available.
The true numbers may be even higher, Read said.
Cetacean survival depends on species' behavior, too.
In the North Atlantic, humpback whales are increasing 7 percent annually, but the northern right whale remains critically endangered, numbering just 350.
The disparity may be due in part to the way a humpback safely eats, by taking one big gulp of food. The right whale, however, swims with its mouth wide open through curtains of plankton. Lines snag in the creatures' filtering combs called baleen, becoming what researchers call "whale floss."
"The whale starts to struggle and spin and the line wraps around its flippers and tail," Moore said.
Most northern right whales bear entanglement scars.
In 1991, researchers tried to rescue a right whale tangled in rope at a cost of more than $250,000 but failed to save it.
Last August, spotters identified a 10-year old female ensnared in fishing line off southeastern Canada. The same whale was spotted off Cape Cod in March, still entangled.
"They lose weight and slowly go downhill," Moore said. "We don't see them regularly enough to know them when they are dead."
By Joseph B. Verrengia