Wizened reptiles, some weighing more than half a ton, have swum across the Pacific Ocean just to dine in Monterey Bay.

As leatherback turtles gulp down jellyfish, local researchers take advantage of this chance to learn more about the critically endangered reptiles.

When a seasonal phenomenon known as the Davidson Counter-Current is in effect, upwelling winds calm and the California Current relaxes offshore. The normally frigid waters spanning from central California to Washington warm up, and jellyfish– aka leatherback food– arrive en masse, luring a population of lumbering reptiles 7,000 miles from Indonesia for the goopy fall harvest.

Last year’s conditions didn’t favor the jellies, so most leatherbacks didn’t make the trip. But this year, at least 20 have been sighted along the coast from Point Reyes to Monterey, according to Moss Landing Marine Laboratories scientist Jim Harvey. The number is encouraging considering the recent decline of the Pacific leatherback population, he says.

Scientists with MLML and the National Marine Fisheries Service have formed a team to investigate how the gigantic turtles behave. In the past they’ve attached time-depth recorders and tracking devices to the leatherbacks. This year, they’re trying out an underwater video camera.

“It turns out they are not eating the whole jellies,” says MLML Director Kenneth Coale. “They are going right for the guts and gonads, which are higher in caloric content.”

Presented with a smorgasbord of jellies, leatherbacks seem to prefer the more elegant sea nettles, which have roughly two to three times more energy than the locally dominant moon jellies.

While scientists analyze jelly guts in Moss Landing, researchers at Stanford University’s Hopkins Marine Station in Pacific Grove are studying the only other surviving Pacific leatherback population, which nests off Costa Rica’s west coast and forages near the Galapagos Islands.

Only about 100 adult females in the Costa Rican group are left– a 95 percent decline in 20 years, says George Shillinger, a Stanford doctoral student who tracks the population’s movement.

Leatherbacks have been around for 100 million years, outliving even the dinosaurs. But their numbers have crashed in recent decades due to egg poaching, long-line and gillnet fishing, and plastic litter in the sea, which they mistake for jellyfish. They also suffer from habitat loss– especially along Costa Rican shores, where foreign investors are building in the critical nesting area of Playa Grande. “It really puts the developers against turtles, sadly,” Shillinger says.

In California, too, the leatherback population has crashed over the course of one human generation.

By combining real-time environmental data with tracking data, scientists can forecast where the leatherbacks will go next, Shillinger says. Working with fishermen and governments, they can create “turtle caves”: areas temporarily closed or restricted to fishing while the leatherbacks migrate through.

The approach could be palatable to fishermen because it doesn’t close areas permanently and could prevent boycotts of turtle-killing fisheries.

Shillinger and his colleagues presented their proposal at the International Union for the Conservation of Nature’s meeting in Barcelona in early October. The 8,000-member IUNC overwhelmingly approved a resolution in support of protected turtle swimways.

“It sets the stage to do more for these animals,” Shillinger says. “The theory is that [the swimways] can have multiple- species benefits: Use the turtles as a flagship, and everything that shares the ocean with them benefits.”

To see the leatherbacks migrate across the Pacific, visit GreatTurtleRace.com.