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Sea turtles are back, noshing on jellyfish

David Perlman, Chronicle Science Editor

Monday, September 29, 2008

Endangered leatherback sea turtles, unseen off the central California coast only two years ago, have returned and are once again gobbling their favorite food: huge jellyfish that are swarming by the zillions from Monterey Bay to Point Arena.

The leatherbacks were spotted during a monthlong survey cruise aboard a government research vessel and repeated aircraft observations. Researchers said they were seen diving for meals close to shore and snacking now and then in deeper waters much farther out.

“We’re getting a better understanding of the leatherbacks and their coastal habitat here after several years when the population was much lower than usual – and after we observed none at all in 2006,” said Scott Benson, chief scientist for the National Oceanic and Atmospheric Administration’s leatherback survey mission based in Monterey County at Moss Landing.

Benson led teams of specialists aboard the NOAA research ship David Starr Jordan that carried sonar gear to scan for the jellyfish while crew members tagged the leatherbacks with temporary tracking devices – simple devices attached to their shells with suction cups – to record their enigmatic diving and feeding behavior.

The rare and little-known leatherbacks have been around during 100 million years of evolution, and their migration patterns are amazing: They nest and lay their eggs in the sandy beaches of Indonesia and the Solomon Islands, then swim 7,000 miles across the Pacific to their feeding grounds along the California coast. But in the past 25 years, more than 90 percent of the leatherback population has vanished, Benson said.

The abrupt decrease is largely because of egg-hunters raiding their nests, commercial long-line fisheries whose hooks can ensnare the turtles as “bycatch,” and most recently the erosion of many nesting beaches because of small rises in the sea level caused by global warming, said Michael Milne of the Sea Turtle Restoration Project, an environmental group based in Marin County.

The huge abundance of jellyfish (Chrysaora fuscescens), commonly known as sea nettles, is apparently caused by increased upwelling of nutrients like krill and plankton from just above the sea floor this year, Benson said.

Spotters aboard the NOAA Twin Otter aircraft found six leatherbacks “surrounded by miles of jellyfish” – along with humpback whales and large ocean sunfish – off the San Mateo County coast and in the midst of regular cargo shipping lanes leading to and from the Golden Gate. In one case, a leatherback was observed swimming among the jellyfish only 5 miles west of Benson’s home in Moss Landing, he said.

Another leatherback that was equipped with a more permanent satellite tag a year ago had returned to the same area this year, apparently after spending the winter a few hundred miles south of Hawaii along what Benson called “Jelly Lane.”

During one segment of the cruise off Pescadero, the mission’s trawling team of jellyfish specialists encountered huge hauls of the creatures, including one weighing 24 pounds with a bell 21 inches across and tentacles “taller than any of our scientists,” the team reported.

Hungry as they are, leatherbacks don’t eat the jellyfish’s transparent globular bells – it’s the viciously stinging tentacles they love, and Benson and his colleagues found themselves “covered with stinging jellyfish slime” whenever they hauled any of the turtles aboard, he said.

Although the survey found most of the leatherbacks feeding amidst the jellyfish no more than 30 miles from shore, the ship did venture as far out as 150 miles, and even there, Benson said, the turtles were feeding amid an abundance of jellyfish.

Eddies of cold and warm water there attracted the jellyfish, Benson said, “and it made the area a fast food stopover for the turtles – a good place for a quick snack on the way in toward the coast after that 7,000-mile swim.”

E-mail David Perlman at dperlman@sfchronicle.com.

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This article appeared on page B – 1 of the San Francisco Chronicle