The week-long meeting that began Tuesday is aimed at bringing together marine biologists, environmental activists and fishing industry representatives to seek ways to ensure the survival of the only sea turtles without shells.

"The decline is an example of the greatest extinction of a population of animals that we have probably witnessed since humans have recorded this sort of thing," James Spotila, a researcher at Philadelphia's Drexel University told a telephone news conference.

"It is almost as rapid as the extinction of the bison in North America in the 1800s."

Currently there are some 30,000 leatherbacks swimming in the world's oceans, down from about 115,000 in the 1980s, the researchers said. The turtles can reach 9 feet (2.7 metres) in length and weigh 2,000 pounds (907 kg). The females come ashore once a year to lay their eggs.

PACIFIC NUMBERS DWINDLING FAST

But the situation of Pacific leatherbacks - which are genetically distinct from those in the Atlantic Ocean - is particularly perilous because their numbers have sunk to about 3,000 from 91,000 just 20 years ago.

This creates a downward spiral where fewer and fewer females are left to reproduce, leaving the leatherbacks swimming toward an uncertain future, scientists said.

For example, last year just 68 of the animals, which spend most of their time in the water, nested on Costa Rica's most popular leatherback beach, down from 1,600 only 15 years ago.

More worrying, along Mexico's coast - once described as the largest and most important leatherback nesting area in the world - only 50 females returned to lay eggs this past season, researchers said.

"The decline in the last five years is nothing short of catastrophic," said Sylvia Earle, an Explorer in Residence at National Geographic and leatherback expert. "The number has dropped at a precipitous rate."

Pacific leatherbacks are facing extinction mainly because of commercial fishing and from commercial development of their nesting areas, the researchers said.

The main ocean threat comes from longline fishing where a ship can send out thousands of baited hooks on hundreds of lines that total 60 miles (96 km) in length, snagging sea turtles, seals and other sea animals, in addition to the target catch of swordfish and tuna, the scientists said.

BEACH HOTELS A BIG THREAT

Problems on land stem from rapid development of hotels and resorts that encroach on the beaches where leatherbacks come to lay their eggs, they added. Egg poaching is also a danger.

But scientists also said there is hope for the sea turtles, so long as something is done soon. This makes it important to hold conferences like the one in Monterey where representatives from disparate groups can search for ways to address the problem, they added.

"Scientists have been talking to each other for a long time," Todd Steiner, director of Turtle Island Restoration Network, the nonprofit group that put on the conference. "But it really needs to get into the policy arena."

Some ideas for staving off Pacific leatherback extinction included better regulating fishing or exploring technology that could scare the turtles away from the hooks.

Other scientists said solutions were as simple as donating money to environmental groups looking to buy beaches to hold off development of hotels and resorts to protect the Pacific leatherbacks' nesting areas.

"If we could just set aside some of these beaches that historically are critical for the survival this species, we can make a difference," said Frank Paladino, a biologist at Purdue University. "It is not going to cost billions of dollars, it is only going to cost a few million dollars."