Zeeleeuwen Pier 39 San Francisco Bay (VS) plotseling verdwenen - Biologen staan voor raadsel....

S.F.'s vanishing sea lions baffles experts

30-12-2009 Kelly Zito, Chronicle Staff Writer, San Francisco Chronicle, USA


Since Oct. 23, when volunteers with Sausalito's Marine Mammal Center counted just over 1,700 of the pinnipeds sprawled out on the docks at Fisherman's Wharf, the number of sea lions has plummeted.

Sea lions have been a major tourist attraction at Pier 39 on Fisherman's Wharf since 1990. Now docks are deserted and no one knows where the animals went.
Photo: Brant Ward / The Chronicle

Tuesday afternoon, as holiday visitors and tourists swarmed the pier for magic shows, ferry rides and funnel cakes, the only empty spot was the network of docks reserved for the federally protected sea lions.

A half dozen of the slippery sea mammals swam around behind the breakwall, but only hauled out at adjacent docks busy with boat traffic where men with hoses quickly persuaded them to jump back into the frigid water.

"I've been coming here for 21 years and every time it's been packed with sea lions," said Arsalan Ziazie, 63, a writer from Los Angeles visiting his daughter in San Francisco. "It's very strange."

Biologists concur. Sea lion numbers can fluctuate due to changes in food supply and water temperatures. But so far, scientists haven't found any evidence that the mammals' prey is diminished overall or that the warming ocean waters of El Niño would decrease, rather than increase, local populations.

"It's hard to say why they've departed," said Joe Cordero, a biologist with the National Marine Fisheries Service. "As to when and if they come back, no one can say. It's puzzling."

A lone sea lion rests on a nearby pier. The sea lions have disappeared from Pier 39 in San Francisco, where there once were hundreds.
Photo: Brant Ward / The Chronicle

The animals have been a fixture on Pier 39 since 1990, when a big herring run lured the sea lions into San Francisco Bay. The Marine Mammal Center gets so many questions about the 1,000-pound creatures that the nonprofit staffs a small kiosk on Pier 39; the pier's insignia includes the silhouette of a sea lion.

Though scientists can't point to a cause for the sudden departure at Pier 39, 2009 has been an unusual year for sea lions, said Jeff Boehm, executive director of the Marine Mammal Center.

A spike in the number of sea lions born in 2008 in the Channel Islands taxed the animals' food supply, leading this spring to record rates of starvation and injury among yearlings. Months later, the number of adults at Pier 39 shot to all-time highs.

Then, the exodus. On Nov. 21, volunteers counted 927 sea lions at Pier 39. Seven days later, there were 20. While some observers wonder whether the animals now prefer Seal Rocks near the Cliff House, Boehm said it's highly unusual for the creatures to decamp to another spot so abruptly.
"Normally, they don't leave en masse - there should be a steady population of at least several hundred" at Pier 39, he said.

The unruly, smelly creatures' exit from San Francisco's piers may not be entirely unwelcome, however. This fall, as the sea lion population spilled over onto the Hyde Street commercial fishing pier and kayakers and swimmers reported more incidents of being bumped and bitten, port authorities began using a fireboat-type sprayer on the animals at Hyde Street and installed 200 feet of knee-high deterrent fencing topped with a slick roller.

The measures, which are permitted under federal law to prevent damage or to protect the public, appeared to work: Sea lions have largely vacated the area.

While scientists and wildlife rescuers will continue to monitor the Pier 39 sea lion population, visitors are likely to be left with one less travel photo.

Melissa Christensen, 18, came to Fisherman's Wharf from the East Bay with three friends on Tuesday as part of a birthday celebration.
Holding her camera and looking out at the empty docks, she said, "It's weird - the sea lions are one of those things you come to San Francisco to see."

E-mail Kelly Zito at kzito@sfchronicle.com.

(Bron: http://www.sfgate.com/)
(Bron foto's: San Francisco Chronicle)