The astounding mass die-off of marine life in Oakland’s Lake Merritt and other parts of the San Francisco Bay continued Monday night, and by Tuesday morning visitors to the lake saw tens of thousands of anchovies washed ashore. The small, silvery fish piled up around larger rotting striped bass, bat rays, and smelt that died over the past 72 hours. People strolling the lake stopped to stare at the morbid scene as a sulfuric stench wafted from the fetid brackish water into nearby neighborhoods.

The Oaklandside met up with Damon Tighe, a naturalist who documents wildlife at Lake Merritt, to check out the latest results of the algae bloom that is thought to be causing the fish kill.

While standing in the muck near the Lake Chalet waterfront restaurant to get a close-up view of recently deceased shrimps and clams, a random passerby stopped and called out to us, “I never realized we had that many fish in the lake!”

“It’s a sad way to find out,” said Tighe while gazing out at floating carcasses of large fish.

What’s causing the fish die-off?

A dead bat ray surrounded by sun-cooked anchovies and shrimp. Credit: Amir Aziz

The fish are probably dying because they can’t breathe.

The algae bloom has been growing in the Bay and lake since at least July, when it was first spotted in waters off Alameda Island. Called Heterosigma akashiwo, the microscopic lifeform turns the water a syrupy brown as it multiplies. At a certain point, it grows so prolific that it starts to consume all of the oxygen available, causing fish and other lifeforms to suffocate.

Tighe said that the algae conducts photosynthesis during the day and breathes at night. As a result, nighttime is when oxygen levels in the lake drop the fastest, and it’s when the fish appear to be dying in the largest numbers.

Volunteers with the group Rotary Nature Center Friends, which does education and advocacy for the lake, have been testing oxygen levels in the water for many years. Recently, the amount of dissolved oxygen in the water has plummeted. 

The group’s co-chair Katie Noonan said she tested the water on Sunday and found that it had less than one part per million of dissolved oxygen, a very low amount. “The lack of oxygen would kill many fish and other organisms with or without a toxin,” she wrote in an email. “I don’t think we know if a toxin is involved. It could be both.”

Some have speculated that another species of algae emitted toxins into the water that are killing fish, but so far no one has shown dangerous levels of any known toxins in the lake or other parts of the Bay.

Where did the algae come from?

Damon Tighe collects scum samples at Lake Merritt. Tighe and other researchers are trying to learn more about the massive fish die-off. Credit: Amir Aziz

The bloom of Heterosigma akashiwo is natural and occurs every few years, but rarely in such incredible abundance. 

Tighe explained that this organism is always present in the water, but it spends part of its life in a “cyst” form, similar to a seed that floats or rests in the sediment, waiting until the conditions are right to grow, bloom, and reproduce.

For a variety of reasons we don’t yet fully understand, the conditions are great right now for Heterosigma akashiwo to grow. We do know that the algae need nitrogen and phosphorus to feed on. Some have speculated that wastewater outflows into the Bay from the many sewer districts where human and industrial waste flow could be providing these nutrients. 

A study of a red tide that killed fish in Florida last year showed that wastewater worsened the deadly event for fish and other wildlife. Others wonder if there was excessive runoff from agricultural fields in the Central Valley this year.

What’s dying in Lake Merritt, what’s threatened, and can it all bounce back?

Young brown pelicans feed on the few remaining schools of anchovies that have so far managed to survive. Credit: Amir Aziz

The fish are the most obvious casualty of the algae bloom. On Tuesday, Tighe pointed out half a dozen species of fish that have perished over the past 48 hours. Large fish bodies were clustered in the densest groups along the shore of Lake Merritt’s western arm. Tighe counted over 500 dead striped bass and dozens of dead bat rays.

But he said he noticed something new and disturbing today: lots of dead clams and other bivalves. Numerous crabs have also been suffocated by the algae. He was also sad to find his first mud shrimp, a large crustacean that looks like a cross between a shrimp and lobster, among the dead.

Three large striped bass and hundreds of anchovies killed over the past 72 hours. Credit: Darwin BondGraham

Seeing large fish, some of which take many years to grow to maturity, dying in such large numbers made Tighe think of the 2020 CZU Lighting Complex Fire in Santa Cruz, which burned not only shrubs and grass but killed many old-growth redwoods and other trees. “Anchovies could bounce back in a year or two,” he said. “What’s really hard to see is the old stuff dying.”

He added that the collapse of small fish species, clams, crabs, and other invertebrates could get big enough to have cascading impacts on the entire food chain. Any large fish that survive the algae bloom will be left with fewer food sources. And birds will find hunting on Lake Merritt and around the Bay much more difficult.

Catherine Becker, an avid birder we met walking around the lake, said she’s worried about pelicans and other seabirds that consume anchovies, herring, and other fish.

Countless clams and other small invertebrates are dying right now also. Credit: Darwin BondGraham

“They’re behaving very strange,” she said about a big flock of brown pelicans that had massed in the lake’s western arm.

Brown pelicans often dive bomb their prey, plunging into the water above a fish school to suck them into their baggy gullets. But today, the pelicans were swimming on the surface, hurriedly chasing a few small schools of anchovies that have so far avoided dying in the night.

“I’m really freaked out,” Becker said. “I’m going to guess we’ll have to rescue a lot of birds.”

The entire San Francisco Bay, but especially Lake Merritt, is an important stopover for migratory birds, noted Tighe. If fish populations drop because of the red tide, it could interfere with this. “We might see starvation,” he theorized.

What could the city do to help stop the die-off?

A work crew lifts one of the lake’s fountains from the water so that it can be repaired. Credit: Amir Aziz

The algae bloom appears to be driven by enormous forces of nature and possibly human causes far beyond the control of Oakland. Think atmospheric and oceanic cycles, climate change, wastewater flows from San Jose to Marin County, and agricultural runoff.

But one thing that might help prevent more fish from dying is getting more oxygen in the water. With the Lake Merritt Institute’s help the city operates several large fountains, one at the tip of the lake’s western arm and one near the Pergola. Pulling water from several feet below the surface and spraying it into the air replenishes oxygen in the lake.

Both fountains have been out of service for a while now. On Tuesday, we saw workers transporting one of the fountains to the Lake Merritt Boathouse where a mechanic said he was replacing the motor so that it could be put back in place and turned on, possibly as early as tonight.

Tighe said the fountains could save fish in the immediate area. It would also help if the wind increases. Opening the tidal gates that allow Bay water to flow into and out of the lake could also help oxygenate the water.

Earlier this morning, the city posted an update on other actions it’s taking to address the situation. It’s waiting for results from tests conducted on Aug. 22 by the San Francisco Bay Regional Water Quality Control Board to understand better what’s causing the fish to die. In the meantime, the city is working with a contractor to clean up fish carcasses “to provide mitigation of the odor and public nuisance ahead of the forecasted hot weather and the long weekend,” wrote Oakland Public Works spokesperson Sean Maher in an email.

Can Oakland residents do anything to help?

People walking the lake stop to take photos of the dead fish lining the shore. Credit: Amir Aziz

The Oaklandside met several volunteers with the Lake Merritt Insitute near the amphitheater on the lake’s southern edge. They were gearing up with tools from one of the institute’s many utility boxes staged around the lake to do some clean-up on their own. 

A volunteer named Tim said he planned to remove the larger fish carcasses while leaving the smaller ones on the shore. He added that anyone can take part in cleaning up the lake. Interested people should contact the Lake Merritt Insitute.

Another thing anyone can do to help is to document living and dead organisms you might see around the lake on the iNaturalist.org app. This crowdsourced website helps researchers pinpoint where different species are turning up.

It’s also helpful to contact your local and state government representatives, as well as government agencies like the San Francisco Bay Regional Water Quality Control Board, California Department of Fish and Wildlife, and let them know that you’re concerned about what’s happening and that you want them to study the algae bloom so that we understand how it came about, and how events like this might be avoided in the future.

David Wofford, a Rotary Nature Center Friends member, told The Oaklandside he hopes the fish die-off is eye-opening for people who haven’t given Lake Merritt much thought. He said he hopes more people volunteer with groups like his because the lake’s health is a group effort that requires various skills and knowledge.

Eating lunch by the boathouse on Tuesday afternoon, Wofford said he and a friend were about to go kayak out into the center of the lake to take oxygen level readings from deeper water. “People should get involved for the long-haul,” he said.

Before joining The Oaklandside as News Editor, Darwin BondGraham worked with The Appeal, where he was an investigative reporter covering police and prosecutorial misconduct. He has reported on gun violence for The Guardian, and was an enterprise reporter for the East Bay Express. BondGraham's work has also appeared with KQED, ProPublica and other leading national and local outlets. He holds a doctorate in sociology from UC Santa Barbara and was the co-recipient of the George Polk Award for local reporting in 2017.