A harmful algal bloom known as a red tide is killing off “uncountable” numbers of fish in the San Francisco Bay Area, with residents reporting rust-colored waters, and piles of stinking fish corpses washing ashore.
The fish, first reported dead along the San Mateo County shoreline last Tuesday, are most likely being asphyxiated as a result of the algae, said Jon Rosenfield, a senior scientist with San Francisco Baykeeper, an environmental group that is tracking the fish kill.
Government scientists have identified the dominant species causing the bloom as Heterosigma akashiwo, a microscopic swimming algae that can cause red tides. The bloom is affecting the water in the San Francisco Bay, San Pablo Bay and South Bay.
While such algal blooms are not uncommon, the scope and deadliness of the one in the Bay Area is concerning, Dr. Rosenfield said. Even the hardiest of fish, like the sturgeon, an ancient creature, are dying, he said. Bat rays, striped bass, yellowfin gobies and even sharks are washing ashore dead.
“What we’re seeing is just the tip of the iceberg,” Dr. Rosenfield said, adding that many more fish were likely to have died at the bottom of the bay. It was over the weekend, he said, that the horrifying breadth of the situation became clear, when dead fish were appearing on nearly “every public shoreline” in the region.
He added, “We’re continuing to get reports of dead and dying fish.”
Though scientists can’t be certain what caused the algal bloom, experts say it is likely a combination of factors including warm water temperatures and a high concentration of phosphorus and nitrogen — the runoff from urban and agricultural sources as well as dozens of wastewater treatment plants that surround San Francisco Bay.
For decades, researchers have been waiting with bated breath, expecting a harmful bloom.
“This was always very likely to happen, and we’ve just gotten lucky,” Dr. Rosenfield said. “There’s no reason it couldn’t happen again.”
It is an unlucky time for fish in Northern California. In July, several thousand dead anchovies washed ashore in the Bolinas Lagoon, about 30 miles north of San Francisco. Earlier this month, the McKinney fire led to a mass fish kill in the Klamath River in Siskiyou County, Calif. Also this month, about 21,000 fish at an aquatic research center at the University of California, Davis, died from chlorine exposure in what the university described as a “catastrophic failure.”
But environmental groups and others say that there are solutions that could help to avoid another die-off in the San Francisco Bay, such as upgrading the city’s water treatment plants with better filtration systems.
“It is time for San Francisco and our sister cities in the nine Bay Area counties to start looking at what kinds of infrastructure investments will need to be made as this becomes, sadly, the new normal,” said Aaron Peskin, a member of the San Francisco Board of Supervisors who represents neighborhoods in the northeastern part of the city.
Eileen White, the executive officer of the San Francisco Bay Regional Water Quality Control Board, said that the board had spent years working collaboratively with stakeholders to study, monitor and model the impacts of the phosphorus and nitrogen, as well as climate change, on the San Francisco Bay.
“Our goal at the regional board is to make sure our regulations are based on sound science,” Ms. White said, adding that upgrades to the wastewater system would cost billions of dollars. “We’re on it,” she said. “We’re taking action.”
Raphael Kudela, a professor of ocean sciences at the University of California, Santa Cruz, said that while this specific bloom could not be linked directly to climate change, global warming was making such algal blooms more likely. “It’s exactly what we would expect with climate change,” he said. “We’re going to see more blooms like this.”
Scientists have been tracking the bloom since late July, when they first heard reports of reddish tea-colored water in the Oakland Estuary, between Oakland and Alameda.
On Tuesday, fish were still dying, but Dr. Rosenfield said that he expected the bloom would eventually dissipate as it burned up the nutrient supply, and the days became shorter, reducing the number of hours of sunlight.
San Francisco Baykeeper has advised against people or pets entering the water, or eating fish from the affected parts of the Bay, until the algal bloom dissipates.
Mr. Peskin, the city supervisor, who described himself as an avid swimmer, said that he first noticed the water had “assumed a rather rusty tint” during an early morning swim last week at the San Francisco Aquatic Park, just north of downtown San Francisco.
“I swam in it for three days before I stopped,” Mr. Peskin said, adding that the water smelled musty and felt different on his skin.
“If fires and plague weren’t enough, now we’ve got red tide,” he added. “You would think the end is near.”