The plight of the turtle

Science World, May 9, 2003 by Mona Chiang

VITAL STATS: Leatherback (Dermochelys coriacea)

Average weight: 499 kilograms (1,100 pounds)

Average length: 1.8 meters (6 feet)

Life span: About 50 years

Diet: Mainly jellyfish

Habitat: Leatherbacks are highly migratory and swim the world's oceans. They've been spotted as far north as Nova Scotia, Canada, and as far south as Chile.

Main nesting beaches: Mexico, South America, the Caribbean

Status: Endangered

Population: An estimated 30,000--down from 115,000 in 1980

Threats: Egg harvesting, disturbance of nesting grounds, drowning in fishing nets, choking on plastic bags mistaken for jellyfish

It's as ancient as the dinosaurs and the most giant reptile on Earth. But the leatherback turtle also ranks as the most endangered of all seven sea turtle species that exist today. Some scientists predict that without urgent conservation, this living prehistoric relic may go extinct in a decade. What's so amazing about the leatherback turtle and why is it worth saving? Marine biologist Scott Eckert of Wider Caribbean Sea Turtle Conservation Network talked to Science World.

Q: Why is the leatherback the most endangered sea turtle?

A: We divide leatherbacks into three or four large groups based on where they nest or live. While every population is a little different, the Pacific population dropped well in excess of 90 percent in the last 15 years. Now, fewer than 5,000 females nest in the region.

Q: Why this big die-off?

A: The common theme has been egg harvesting and the killing of adults on beaches, combined with fishing activities. For example, swordfish gill-netting was introduced off South America in the early 1980s: The fishing industry learned to use smaller vessels that go into the high seas, stringing 100-mile-long nets. While the nets catch a lot of swordfish, they also kill any marine life that swims into them. Fishermen in Chile and Peru were catching 3,000 leatherbacks per year. Before gill-netting, some 80,000 leatherbacks nested in Mexico and Costa Rica each year. Now they're down to about 600.

Q: Why are turtle eggs so valuable?

A: Turtle eggs have traditionally provided a protein source for people all over the tropical world. Leatherbacks produce anywhere from 65 to 80 eggs per clutch (nest) and lay about 11 clutches per season. A beach with 1,000 nesting females is a huge opportunity to harvest food. This reached epidemic proportions in the 1970s and '80s, when truckloads of eggs were sold on international markets.

Q: That's a huge problem!

A: These turtles have a life span that exceeds 50 years, and only 2 to 3 percent of adults die naturally each year. They don't become reproductively mature till they're about 20. By taking away the eggs, the leatherback is still going to be around for a while. But when all the adults die, the population will go extinct.

Q: The leatherback spends 99 percent of its life swimming. What propels it?

A: You don't find any other sea turtle designed quite as nicely for hydrodynamic efficiency (ease in moving through water). The leatherback has the classic Olympic swimmer's shape--big broad shoulders, long front limbs with giant paddles, and huge pectoral muscles. And those ridges down its back help the leatherback travel straight so it doesn't need to do a lot of sculling (rowing arms like oars) sideways. On top of that, the ridges have little bumps, which break surface tension (clinging force on the surface of a liquid): If the body is too smooth, water tends to suck at it, making it harder to swim. The bumps break this tension, allowing the turtle to glide through water more efficiently.

Q: What fuels this marathon swimming?

A: A large diet of jellyfish, which is almost pure protein. Somehow the leatherback can convert that protein into fat and oil very quickly. Even if the jellies aren't out, the turtle will have enough fat reserve for the energy to swim to the next foraging area. Also, leatherbacks have characteristics that other sea turtles lack.

Q: Like what?

A: Think about how far it is to swim 10,000 km (6,214 mi) a year or 45 to 60 km (28 to 37 mi) per day. These are just horizontal distances.. Their vertical distances are equally mind-boggling: This turtle can dive up to 4,000 ft (1,219 m). That's partly because the leatherback can keep warm: Besides heavy fat insulation, it also has the perfect shape for heat retention. In a cross-section of the turtle you'll see an almost perfect cylinder, which means maximum mass for minimal surface area. Also, constant swimming generates heat. These turtles have been spotted swimming by icebergs. No other sea turtle is as fearless of expanse.

Q: Unlike other sea turtles, the leatherback's carapace (shell) is leathery and flexible. Does that make it more vulnerable to predators?

A: They're certainly vulnerable to killer whales, which occasionally feed on them. But killer whales can't dive as deeply. The hard shell is great for turtles like the green or hawksbill, which live near reef areas. They can use the shell to hunker down and hide beneath a rock or use it as a shield against predators. But leatherbacks can't hide in the open ocean. And to be armored enough to protect it from a shark or whale would entail a shell too heavy to swim in.