Leatherback turtles, whose numbers in the Pacific have plunged in recent years, will all but disappear within a decade unless commercial fishing practices are changed, according to a new analysis of the giant animals' nesting activity.

The turtles, which nest on ocean beaches and then spend almost all of their lives in the open sea, have been dying in large numbers -- at least 1,500 females every year, by some estimates -- caught in the long lines and nets used by commercial fishermen. This level of mortality, combined with the natural high mortality of turtle hatchlings, means the population is unsustainable, according to the analysis, a computer model devised by scientists at Drexel University in Philadelphia who have studied the turtles' nesting activity at a major site in Costa Rica since 1988.

''The rate of adult mortality is too high,'' said Richard D. Reina, a marine biologist at Drexel and a co-author of the study, which is being reported today in the journal Nature. ''They just can't recover.''
From a high of 1,367 females that nested at Playa Grande, Costa Rica, in 1988, the model forecasts that fewer than 50 will nest by 2004, a number that means the turtles will be effectively wiped out in the Pacific Ocean.

Efforts to protect the turtles where they nest will only delay reaching that number by about five years, Dr. Reina said. ''Our model always predicted extinction,'' he said.

Cynthia Lagueux, a zoologist with the Wildlife Conservation Society who is working on a turtle project in Nicaragua, said scientists generally agreed that turtle populations were declining, particularly in the Pacific. ''But there's lots of disagreement or discussion on where the regression line will hit zero,'' she said.

John A. Musick, a sea turtle specialist with the Virginia Institute of Marine Science, said: ''There's no doubt that something has to be done for those Pacific leatherbacks. Extirpation of those breeding populations is probably inevitable.''

Leatherbacks, which can weigh more than 800 pounds and are listed as endangered by the World Conservation Union, return to the same beaches to lay their eggs on average every three years. At Playa Grande, the scientists tagged females to determine whether and with what frequency they returned.

The scientists found that not only was the overall number of nesting turtles declining sharply, but also that the number of returning females fell far below expectations.

The only explanation, the researchers said, is that the turtles are dying in the years they migrate throughout the ocean. And although the fishing industry's reports on the unintended victims hauled in with the fish being sought tended to be poor, particularly in Asia, the few reports available indicated that leatherbacks had been killed in substantial numbers. ''All the evidence showed that this was the cause,'' Dr. Reina said.

Last fall, a federal judge in Hawaii ordered the closing of fishing grounds to the state's commercial long-lines fleet until an assessment of the impact on turtles could be prepared.

Paul Dalzell, a pelagics biologist with the Western Pacific Regional Fishery Management Council, which has jurisdiction over fisheries in federal waters, acknowledged that the Hawaiian fleet had been ''slow off the mark'' in coming up with measures to protect leatherbacks.

But he said the estimate of turtle kills by the Hawaiian fleet was only 10 a year. The Hawaiian fleet is small, however, representing only about 3 percent of the commercial boats throughout the Asian Pacific. Boats using gill nets off South America are a large part of the problem, he said.

Dr. Reina said several modifications of commercial fishing practices were needed.

Exclusion zones, where fishing would be banned at certain times of the year, could be established in known turtle migration corridors, he said.And more independent observers were needed on boats to ensure that live turtles hauled on board were returned to the sea still alive, he added, contending that it is now more convenient and cost effective for the crew to kill or permanently disable the turtle to get the hook out.

''It really comes down to time and effort,'' Dr. Reina said. ''For a commercial fishing boat, time is money. Hooks are money. Nets are money. Turtles are not money.''