By Alex Kirby
BBC News Online environment correspondent
The steep decline of the Pacific Ocean leatherback turtle has gone so far the species could be extinct within no more than a decade, conservationists fear.
Leatherbacks can grow to 2.75m (9ft) (Image: Matthew Godfrey)
A report by the US group Conservation International says leatherback numbers there have fallen by 97% in 22 years.
Five of the six other species of sea turtle are also at risk of extinction, though not necessarily as acutely.
Threats include fishing practices and the poaching of the turtles' eggs, but scientists say they can still be saved.
One in three dies
CI released its report on the plight of the Pacific leatherbacks at the 24th annual symposium on sea turtle conservation and biology, meeting in Costa Rica.
It says their numbers have fallen from about 115,000 breeding females to fewer than 3,000 since 1982.
The loggerheads predate the dinosaurs (Image: Alan F Rees/ARCHELON)
James Spotila, professor of environmental science at Drexel University, said: "The Pacific leatherbacks currently face an annual mortality rate of up to 30%.
"That rate is clearly unsustainable, and without dramatic intervention we can expect to see them disappear in as soon as a decade."
Of the other species, the Kemp's ridley and hawksbill turtles are also both classified by IUCN-The World Conservation Union as critically endangered, the designation given to the leatherbacks.
Green, olive ridley and loggerhead turtles are classed as endangered, and only northern Australia's flatbacks are not thought to face extinction.
Roderic Mast, vice-president of CI and president of the International Sea Turtle Society, said: "On land, the canary in the coal mine warns humans of impending environmental danger.
One hatchling in 1,000 is likely to reach adulthood (Image: Conservation International)
"Sea turtles act as our warning mechanism for the health of the ocean, and what they're telling us is quite alarming. Their plummeting numbers are symptomatic of the ocean as a whole."
One particular threat to turtles is the practice of longline fishing, which involves the use of lines up to 145km (90 miles) long baited with as many as 8,000 hooks each.
Scientists are urging a dual approach to saving the turtles:
One promising development, CI says, is the investment of several million dollars over the next three years to consolidate a marine protected area stretching from Ecuador to Costa Rica.
- better protection and management for their nesting beaches, and control of land-based lights: the turtles mistake them for the Moon, walk towards them, and become stranded on the beach
more marine protection and safer fishing techniques - less than 0.5% of the oceans are formally protected, and inexpensive changes to fishing gear and practices could mean radical cuts in the mortality rate.
The money is coming from four Latin American states, the United Nations Foundation, Unesco, and CI itself.