OAKLAND – As the massive harmful algal bloom, or HAB, stretching throughout the San Francisco Bay appears to be in decline, the State Water Resources Control Board, San Francisco Regional Water Quality Control Board and California Department of Fish and Wildlife are warning that depleted oxygen levels could lead to large-scale aquatic deaths in the days ahead.

Better known as a “red tide” since it has turned much of the Bay a reddish-brown color, the HAB was first detected in Alameda in late July and has grown to become the largest in the Bay’s recorded history.

Already, the HAB has been responsible for the deaths of thousands of fish, including large sturgeon, sharks, striped bass, bat rays and anchovy.

While this type of HAB is not considered a health threat to humans, it is recommended that people avoid swimming until further notice. The cause of the HAB is still not known.

“It’s very upsetting to see the scale of harm to aquatic life and we know how disturbing this has been for the public,” said Eileen White, executive officer of the San Francisco Regional Water Board. “We are doing everything possible to monitor the situation, work with other agencies and search for solutions.”

At Lake Merritt, which is connected to San Francisco Bay, reports suggest as many as 10,000 fish died in late August.

On Aug. 29, San Francisco Bay Regional Water Board staff conducted a field investigation at Lake Merritt, where very low dissolved oxygen levels were measured in the water.

Water samples were collected for identification of algal species and toxins. Updates to this sampling event will be posted on the HAB web map.

CDFW will be conducting both boat and shore-based surveys next week at various locations around San Pablo and Suisun bays.

The goals of these surveys will be to determine the geographic extent of the fish kill, any expansion into new areas, the species affected and the numbers of dead fish on select target species such as white and green sturgeon.

CDFW is also tracking reports from partners and community scientists to determine where fish mortalities are occurring.

Due to the likelihood of increased fish mortalities through the weekend, CDFW is encouraging people who may be recreating on the nearby shorelines in affected areas to report sightings of dead fish through the iNaturalist smartphone app.

The water boards have worked with various agencies, including the City of Oakland, Alameda County, San Mateo County and East Bay Regional Park District to post caution advisory signs near affected waters (e.g., Lake Merritt, the Oakland Estuary, Coyote Point, and Crown Beach) to inform the public to avoid contact with the discolored water caused by the red tide.

For questions specifically related to fish mortalities from this HAB, please contact Jordan Traverso at CDFW at 916-212-7352.

For more information about HABs, please visit Cabs/slifornia Harmful Algal Blooms Portal.