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“Unprecedented changes” that have warmed the ocean off the west coast of North America may portend a dramatic decline in the biological productivity of coastal waters, explaining recent strandings of emaciated sea lion pups and a mass die-off that began last fall of small seabirds called Cassin’s auklets.

That’s the word from fishery experts and ecologists at the National Oceanic and Atmospheric Administration, who say populations of tiny organisms at the base of the marine food web already have diminished and could take a toll on everything from salmon to seals because of especially intense variability in regional weather patterns.

Scientists remain in “wait and see” mode, but, “Our guess is the primary productivity of zooplankton and phytoplankton will probably be reduced this year,” and perhaps even longer, said Toby Garfield, director of the Environmental Research Division at NOAA’s Southwest Fisheries Science Center in La Jolla.

A shift in atmospheric winds and the flow of unusually warm waters south from the Gulf of Alaska have raised ocean surface temperatures between 2 to 6 degrees along a band of Pacific Ocean from Alaska to Mexico, according to Nate Mantua, leader of the landscape ecology team at the science center’s Santa Cruz facility.

“Right now, the ocean is very warm, and we have lots of indicators pointing to low productivity and low availability of some of the more normal prey items for things like seabirds and marine mammals, including seals and sea lions,” he said.

In addition, an extended period of winds from the south and weak winds from the north has depressed the upwelling of cold, nutrient-rich water that would normally fertilize the surface waters and stimulate a more productive food web, he said.

But there are mixed signals in the wind and water, including some indication that north winds and ocean upwelling may be beginning from Cape Mendocino, in Humboldt County, north to southern Oregon, Mantua said.

Whether it continues, grows in strength and spreads southward is still in question, but it could help mitigate “food stress” to some degree, he said.

On the other hand, NOAA has recently declared development of a weak El Niño at the equator.

If it strengthens and spreads, “we’re potentially getting warm conditions from two directions,” Garfield said.

That could be good for fish species like sardines and anchovies, which tend to thrive in warm conditions, scientists said.

For cold water fish, like salmon, the reverse is true.

“The patterns that we’re seeing,” Garfield said, “are part of the natural variability that we expect to see. But in this particular instance, it’s been much stronger than in past instances.”

Mantua said strong upwellings and cold water conditions in 2012 and ’13 suggest the current trend reflects regional atmospheric conditions rather than long-term climate change, which is expected to become more dominant in years to come.

But the rapid warming that began last year and continues now could easily persist through next year, he said.

Scientists warned of implications for the marine food web as early as last fall.

When emaciated juvenile Cassin’s auklets began showing up dead along the California Coast in early November, wildlife biologists said it was likely because krill, their usual forage prey, had disappeared from the warm waters near their breeding colonies.

In December, focus shifted to very young, undernourished sea lion pups who began washing ashore, especially in the southern part of the state, well before they should have left their mothers’ sides. The pups are apparently unable to get the nourishment they need.

Insufficient food supplies are likely exacerbated by exponential growth in sea lion populations over the past few decades, Mantua said.

More than 1,800 stranded sea lions have since been recorded, though that number reflects only those that have been admitted to marine mammal rehabilitation facilities such as the Marine Mammal Center in Sausalito. That facility had 220 sea lion pups in its care Thursday, a spokeswoman said, and has cared for 568 so far this year.

Many pups have died before they could be admitted to care or have been euthanized upon arrival, said Justin Viezbicke, coordinator of the California Stranding Network.

Those that have been released back to the wild still face the same challenging ocean conditions that sent them ashore in the first place, as well, he said.

“We don’t believe we’ve peaked yet,” Viezbicke said.

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

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