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A recent explosion of plankton off the Sonoma Coast has turned lethal for abalone and other shellfish.

White meat from the prized sea snails has been washing up on area beaches for more than a week, sending wafts of rotting flesh over bluffs, and dismaying those who consider themselves stewards of the ocean.

"It's like going up to an old growth forest and then coming back and it's been clear cut," said Matt Mattison, an abalone diver from Monte Rio, who was stunned by the extent of the die-off near Fort Ross on Monday. "In 28 years of diving up here, I have never seen anything like this."

State scientists say the destruction appears to be the result of a plankton bloom that began toward the end of August and is now dissipating.

Such red tides are common occurrences, named for the rusty, murky tinge they give the water. The blooms feed on periodic upwellings of nutrient-rich cold water from the ocean's depths.

Generally, the blooms pass without major harm to abalone. But the most recent red tide stuck during a stretch of unusually calm ocean conditions, which kept the plankton densely packed, said Ian Taniguchi, senior biologist with the California Department of Fish and Game

Scientists are awaiting lab results to see if toxins from the concentrated plankton poisoned the abalone as well as the urchins, chitins, sea stars and other animals that have been washing up in unusual numbers.

Alternatively, the abundance of plankton may have consumed the oxygen around the animals, suffocating them, Taniguchi said. The calm ocean conditions likely added to the problem by minimizing the churn that oxygenates the water, he said.

Either way, this has been the deadliest red tide for state abalone in at least three decades, Taniguchi said, though no precise tally is available. Deaths have been observed from Bodega Bay to Point Arena with most reports from between Salt Point and Fort Ross, he said.

Local divers called the carnage unprecedented with dead abalone easily seen onshore and in the water over the holiday weekend.

"I wasn't ready for what I saw," said Bill Mashek, a 30-year abalone diver who said he was taken aback after diving near Fort Ross Sunday.

"There was hundreds and hundreds of out-of-the shell abalone out on the rocks. I have just never seen anything like that before."

Abalone are a prized North Coast delicacy with their harvesting prohibited south of the Golden Gate. Mendocino and Sonoma counties account for more than 95 percent of the state abalone catch.

In 2009, nearly 51,400 abalone were harvested at Fort Ross and nearby Reef Campground, the most of any site in the state, according to the most recent data on Fish and Game's web site.

The fishery has remained open despite the die-off. It's too early to say what the death toll may mean for future restrictions, Taniguchi said. Already divers are limited to three abalone a day — and no more than 24 in a year.

Such limits help protect populations against catastrophes, Taniguchi said. Still he said extra protections may be necessary.

Next week Fish and Game officials will take previously scheduled samples of the abalone population at Salt Point State Park. They may also perform samples of the populations in Fort Ross and Timber Cove that were scheduled for next year, Taniguchi said.

"We need to assess the impact and determine if there is a need to adjust the management of the fishery," he said.

Mattison said he and other local divers who are active on NorCal Underwater Hunters, an online forum, are not waiting for additional mandatory limits. They're launching a grass-roots effort to encourage people to end their harvest for the rest of the year.

"Let's give it a break. Let it recoup," he said. "I personally will not be harvesting any more abalone off the Sonoma Coast for the rest of the year."