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Scientists up and down the West Coast are monitoring what appears to be a large-scale die-off of young Cassin’s auklets, small seabirds whose breeding grounds include a colony in the Farallon Islands west of San Francisco.

Emaciated, white-bellied birds have been washing ashore in Sonoma County and along a broad swath of California coastline since early November after a period of ocean warming in the Farallones region and disappearance of the tiny krill that provide their main source of food, researchers say.

Scientists are still collecting data, but the largest concentration of dead birds appears to be in northern Oregon, according to monitors in the Pacific Northwest. Birds have been washing up in Washington, as well.

Scientists say anyone who finds a dead bird should leave it alone so that monitors surveying the beaches can collect accurate records on the die-off.

Just what’s behind the phenomenon is far from clear, those involved in the research say.

One factor may in fact be the species’ recent breeding success, which means a particularly large number of inexperienced fledglings were introduced last summer to the harsh challenges of life at sea, they said.

But there’s concern, at least locally, about the drastic shift in ocean temperature and feeding conditions — from those that facilitated several very productive breeding seasons to those that prompted nesting pairs in the Farallones to abandon their second round of eggs in July — and the potential for linkage to climate change.

Jaime Jahncke, director of California Current at Point Blue Conservation Sciences, which has monitored the Cassin’s auklets in the Farallon National Wildlife Refuge for more than four decades, said mean sea surface temperatures recorded in July and August were the second-highest in 45 years, and rose substantially in September.

But “you can have ocean warming for different reasons,” he said.

An anomalous warming in 2005 and 2006 — though it was winter — resulted in a large die-off of birds, as well as a season in which the birds did not come to the Farallones to breed, Jahncke said. There was also a high mortality event in 1997 and ’98.

Scientists, so far, think cyclical weather patterns are most likely to blame this time, as well.

“We’re not seeing a widespread eco-disaster here,” said Julia K. Parrish, Aquatic and Fishery Sciences professor at the University of Washington and founder of its Coastal Observation and Seabird Survey Team, which monitors seabirds from Mendocino to the Arctic Circle. “We’re seeing a spike of (deaths in) one species that’s giving us clues, and the clues don’t suggest that the bottom is dropping out of the ecosystem.”

The Farallones region has the largest breeding colony of Cassin’s auklets in California, representing more than an estimated 28,000 breeding pairs this year, according to Point Blue Conservation Science.

Once fledged, they forage for krill far offshore, and are considered an excellent indicator species of ocean conditions and the end of the food chain, said Russell Bradley, Farallon Program Leader for Point Blue.

After many years of declining populations, the Farallones colony rebounded in recent years. Since 2008, many of the breeding pairs have been successfully producing young in early summer and, very unexpectedly, have produced second eggs that then resulted in young.

“It’s unbelievably rare in seabirds,” Bradley said. “It just doesn‘t happen.”

This year, 98 percent of the pairs bred successfully on the first go-round, he and Jahncke said. But most of those that attempted a second time abandoned their eggs during incubation, left the area and did not revisit the islands later, as would often be the case. In the absence of krill, blue and most humpback whales also left the area early, Bradley said.

In early November, beach monitors and others began spotting dying and, in most cases, dead Cassin’s auklets, both on the shoreline and in the water.

Members of the Gulf of the Farallones Marine Sanctuary Beach Watch team recently saw more than 100 at Salmon Creek Beach, said program manager Jan Roletto, sanctuary research coordinator. Scientists say thousands, at least, have died, though the longevity of adult Cassin’s auklets, generally, means there should not be a huge impact on the overall population.

In California, the dead and dying birds have been concentrated in the Point Reyes/Sonoma Coast area, south to San Luis Obispo, said Laird Henkel, supervisor of the Marine Wildlife Veterinary Care & Research Center, Office of Spill Prevention and Response for the California Department of Fish and Wildlife.

No evidence of disease or toxic exposure is so far evident, though necropsies on some recovered birds are being performed, said Doug Cordell, a spokesman for the U.S. Fish and Wildlife Service.

“This is believed to be a starvation event,” he said.

Parrish, who said the incidence rate in northern Oregon appears to be the highest overall, said she believes most of the juvenile birds were bred in British Columbia, by far the largest breeding grounds for the species and an area with what she described as enormous reproductive success this year.

What’s still unknown is whether so many young were fledged that it’s magnifying the always high rate of juvenile mortality; whether the young birds are coming farther into shore than usual to seek food, and thus are more noticeable this year than when they die at sea; whether the arrival of storms in the Pacific Northwest might have played a role in their deaths or their arrival onshore; or whether it’s just a question of food availability, Parrish said.

But if it’s all because of a warmer ocean, she asked, why are only Cassin’s auklets affected?

“Nobody has a lock,” she said, “on, ‘Is there something larger scale that’s happening in the environment that’s causing them to die at a higher rate?’ We don’t even know if they’re dying at a higher rate or just coming in closer to shore.”

The picture may be clearer in the Farallones refuge, however, where decades of sustained monitoring and data collection “is all indicating we are seeing a much higher die-off rate,” said Mary Jane Schramm, spokeswoman for the Gulf of the Farallones National Marine Sanctuary.

After what she called “an extraordinarily productive” period through June, “it was like somebody hit a switch.”

EDITOR’S NOTE: This story has been updated to reflect certainty in the Gulf of the Farallones region about the high die-off rate among young Cassin’s auklets.

You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.