The three-day heat wave in early June cooked tens of thousands of mussels in their shells in the intertidal zones along the Marin and Sonoma coasts.
Preliminary data from the Bodega Marine Laboratory, where one researcher made the initial finding, shows an average 30 percent loss throughout the Bodega Marine Reserve and up to 70 percent losses in some areas.
The dense heat coincided with daytime low tides and topped average temperatures for the time of year by nearly 20 degrees. A week later, Jacqueline Sones, a research coordinator for the reserve, was walking on the beach near Mussel Point to conduct a survey of sea stars. She found something entirely different: thousands of dead mussels.
“The greatest loss was [in] the low intertidal zone, though even further out, in deeper water, we also saw empty shells,” she said. In some shells, tissue was still inside, meaning the mussels may have “suffered stress in the heat event but didn’t die right away,” she added.
Researchers believe the event could weaken the mussels’ resilience, making them more susceptible to disease.
Dr. Sones explained that tides are lowest around the solstices, but it was an unusual combination of events that the low tides came around midday during the three-day heat. “On the Northern California coast, the tides are the highest in the late summer and early fall, which is when we usually get our heat,” she said. “We are worried that, with climate change, there are predictions for more frequent extreme heat days.”
The chances of this combination of events taking place will increase over time, Dr. Sones said.
The Bodega Bay team has explored areas to the north and south of the reserve, including Dillon Beach and Sea Ranch, finding die-offs, though not in the same numbers. In the Point Reyes National Seashore, local biologists reported the heaviest damage to mussel beds near Coast Camp and Sculptured Beach, south of Limantour Beach.
Dr. Ben Becker agreed there was evidence that the die-offs were directly related to the high temperatures. As seen from the beach near the inlet to Coast Camp, “the large bar patches contained mussels recently—their byssal threads that they use to attach to the rock are still visible—and there were large piles of thousands of dead shells piled up along the high tide line,” he wrote to the Light.