North Coast mussel die-off an alarming sign of warming world’s threat to marine life

BODEGA BAY - As a scientist, Jackie Sones trains her focus on observable data - what can be documented, quantified and compared.

But it's taken some effort recently to keep her emotions at bay as she worked among tens of thousands of empty, gaping mussel shells, appraising the scope of a rare mass die-off along the rocky shoreline of the North Coast.

In yet another sign of the toll exacted by rising temperatures on the ocean environment, a period of extreme heat last month appears to have killed off a large portion of the mussel bed in Bodega Bay.

“It was just really hard to be surrounded by all these dead animals,” Sones, research coordinator for the UC Davis Bodega Marine Reserve, said after a weekend survey of the mussels. “You're doing this as a scientist, to document the situation. But as a person, that was a challenging thing to be doing.”

Up to 70% of the specimens died in the hardest hit, most exposed areas of the mussel bed along the mile or so that Sones has been able to survey so far.

Sones has heard from researchers and citizen scientists about similar episodes ranging from Dillon Beach in Marin County to an area of the Mendocino Coast near Westport, suggesting a widespread problem.

The alarm is not just over the enormous number of mussels that basically cooked in the sun. In addition, mussels are a foundation species that provides habitat for other organisms, creating the structure in which they live.

The bivalves attach themselves to the rocks in tightly packed colonies in both the intertidal zone, which lies above water at low tide, and the subtidal zone, which sits below the low-tide mark. They help to ameliorate the energy of the surf and shade out the sun so smaller creatures can find refuge.

Snails, worms, barnacles and anemones are among dozens of species found where California mussels live. So the mortality event may extend to other species, Sones suggested.

“There's like a hundred different species that you find in a typical mussel bed,” said Mark Denny, professor of marine sciences at Stanford University and director of the university's Hopkins Marine Station in Pacific Grove. “If the mussels go away, all the things that live in the mussel bed get exposed.”

Sones works out of the UC Davis Bodega Marine Laboratory, where scientists have long studied a variety of impacts related to climate change, from ocean acidification to strange marine wildlife migrations to the “warm blob,” the band of unusually warm water that has drastically altered the environment off the North Coast, which has seen a collapse in its abalone fishery and bull kelp forest.

Evidence of the latest die-off, the mussels, surfaced for Sones in the middle of last month.

She was heading out to the rocky shoreline behind the Bodega Head lab to survey for signs of recovery in the local sea star population, which has been virtually wiped out up and down the North Coast by a wasting disease since 2013.

But what caught her eye were patches of snow-white seaweed on the rocks, apparently bleached during the record high temperatures a week earlier. Nearby mussels, she soon discovered, suffered even heavier damage.

The heat spell coincided with midday low tides that left the shellfish exposed to the sun during the hottest part of the day without tidal relief or wave splash. Their dark shells would have absorbed the harsh sunlight, elevating the creatures' internal heat well above the ambient air temperatures, Sones said.

No temperature sensors are currently planted in the mussel bed, but earlier research suggests that June 10, when Santa Rosa's record high hit 101 degrees - and the Bodega Marine Lab topped out at 77 degrees - the temperature inside the mussels would have reached about 105 degrees or more, Sones said.

The high reached 81 degrees at the reserve the next day, when low tide again hit at midday. The mussels would likely have been exposed to high air temperatures for six hours or more on each of those days, Sones said.

The mussels were so physiologically stressed that they died. They were left yawning open, some still with tissue inside. Much of the tissue has since been washed away by the waves or consumed by other animals, leaving the mussel beds a gleaming surface of white and blue from the inside of the shells.

Patches of the bivalves, usually stuck to the rock by thread-like fibers, are beginning to detach and wash away.

“You can see where instead of a continuous mussel bed, you're seeing gaps of bare rock,” Sones said. “There's a change in the structure of the mussel bed.”

While rare, the mass mussel mortality is not unprecedented, and the latest occurrence resembles a widely publicized die-off in 2004 in Bodega Bay, documented by a post-doctoral student at the marine lab named Christopher Harley.

That event resulted from concurrent low tides and high spring temperatures and was similar to one a few weeks earlier affecting limpets. The shellfish on rocks angled toward the sun suffered the worst destruction.

In his paper on the subject, Harley referenced an earlier die-off of California mussels in the late 1960s, as well as an extensive mortality of two other mussel species in Tomales Bay in early May 2007 related to low tides and heat exposure. A large number of limpets on Bodega Head also died in May 1967, he wrote.

But he and others are clear that rising global temperatures are predictive of more frequent mortality events, with potentially far-reaching repercussions.

“Of course, the worry is that, as air temperatures go up with global warming, these things are going to get to be more and more common,” said Denny. “Given that mussels are sort of a dominant competitor for space on the rock, if you start losing them, then it's going to change the total mix of what's out there.”

Sones noted that the data from the 2004 die-off indicated 10% of the mussel bed was affected. This time it ranged from 5% to 70%, with an average of 30% of the shellfish lost across the surveyed bed.

Mussels, like many wildlife species, face specific physiological threats in a warming world. At temperatures above 93 degrees, they are no longer capable of synthesizing protein.

“One of the biggest questions now is, ‘What is their capacity for evolving to cope with new conditions?'?” Sones said.

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.