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Frequently Asked Questions about the
San Francisco Bay Harmful Algal Bloom

First reported in late July, a specific kind of harmful algal bloom known as a red tide has spread throughout much of the San Francisco Bay, alarming onlookers who’ve seen the heavy toll it has already taken on aquatic life. While teams of scientists continue to investigate the cause, many are left to wonder about the ongoing impact. Here are answers to some of the most common questions we are getting.

Why is the water in San Francisco Bay reddish brown? 
The San Francisco Bay is experiencing a type of harmful algal bloom (HAB) known as a red tide. The species associated with this bloom, Heterosigma akashiwo, can cause water to take on a reddish-brown color. The red tide was first observed near Alameda. This bloom now extends throughout the open-bay regions of the South Bay, the Central Bay and into San Pablo Bay.

Is this normal? 
Red tides are known to occur off the coast of California, usually from early spring through late summer. They can be bioluminescent at night, often looking like glowing waves from the beach. They can last days to months and it is difficult to predict how long they will last due to sunlight, temperature, tidal flow, stratified water columns and nutrients. Red tides are often attributed to depletion of oxygen in the water that marine life needs to survive. It is not as common for red tides to enter the estuarine environment like the one in the San Francisco Bay.  

Is it safe eat the shellfish or swim? 
This type of red tide is not known to cause human illness, but we always encourage people and pets to avoid contact with any discolored water that may be caused by the red tide and to practice healthy water habits.  

The San Francisco Bay Regional Water Quality Control Board has 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), so the public knows to avoid contact with the discolored water caused by the red tide. We encourage other water-based facilities that observe discolored water to post similar signs to keep the public informed.   

On May 1, the Department of Public Health (CDPH) issued a prohibition against harvesting lor eating mussels along the entire California coast, including bays. The prohibition is still in place. For updates or questions regarding the prohibition, contact the CDPH marine biotoxin monitoring program or

Why are there so many dead fish? 
We are getting reports of fish deaths across San Francisco Bay that appear to be linked to the red tide, including sturgeon, striped bass, sharks, bat rays, smelt and anchovy. We are coordinating with the Office of Environmental Health Hazard Assessment (OEHHA), the California Department of Public Health (CDPH), and the California Department of Fish and Wildlife (CDFW) to investigate the potential link between this bloom and fish deaths. Algal blooms, such as this one, can create toxins that are detrimental to fish and other animals, and as algal matter dies off during and after a bloom, the decomposition process uses up oxygen in the surrounding water that can cause organisms and fish to suffocate.

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.   

What do I do if I see the red tide or dead fish? 
Suspected HABs, including red tides and potential HAB-related illness or death, should be reported to the Water Boards by filling out the online HAB Report form or contact the HAB Hotline: Email:; Phone: 1-844-729-6466 (toll free). Observations of dead fish should also be reported to Harmful Algal Bloom Fish Mortality | San Francisco Estuary Institute.

What are the Water Boards doing about this HAB? 
The Water Boards are tracking this red tide, coordinating with agency partners and sharing this information with interested parties.  

The Water Boards continue to work closely with San Francisco Estuary Institute and teams of researchers, including the U.S. Geological Survey, to understand the spatial extent of the bloom and what impacts it has on fish and wildlife. This fact sheet explains how the Water Boards are using satellite imagery to track red tide conditions. Staff also is monitoring Lake Merritt for HAB-related toxins and low dissolved oxygen.  

Ongoing field work will track this bloom’s progression, including related water quality impacts; characterize the bloom’s response to biological, geological, and chemical factors; and attempt to identify the factors that contribute most strongly to the bloom’s expansion and its eventual end. Understanding these factors will be increasingly important as the climate changes because warming waters could lead to more frequented tides in the future.  

What are the Water Boards doing to address nutrient inputs into San Francisco Bay? 
The San Francisco Bay Water Board has an ongoing effort to determine how best to manage nutrient inputs to the Bay. This Nutrient Management Strategy includes collaboration with researchers, dischargers, and other agencies to study potential impacts of nutrients on San Francisco Bay and to evaluate potential management actions to control nutrient inputs to avoid or respond to adverse impacts. The Nutrient Management Strategy is funding the San Francisco Estuary Institute to study what causes HABs and to characterize this specific bloom.  

As required by the Nutrients Watershed National Pollutant Discharge Elimination System Permit issued by the San Francisco Bay Regional Water Board in 2019, municipal wastewater treatment plants that discharge into San Francisco Bay collectively contribute $2.2 million dollars per year to fund about 80 percent of the Nutrient Management Strategy budget.  

Over the past several years, major investments augmenting our capacity to monitor HABs include the following: 

  • Systematic collection of algal toxin data during recurring channel cruises 
  • Development of DNA-sequencing techniques for specific algal species 
  • Sampling for algal species of concern and their toxins during lateral mapping surveys 
  • Measurement of algal toxins in bivalves at several locations 

Where can I find more information?