Algae bloom in San Francisco Bay, Lake Merritt is killing fish, threatening swimmers

Alarmingly large numbers of fish have been sighted floating belly up all around San Francisco Bay and Lake Merritt, prompting environmental groups to suggest that people and their pets stay out of the water to avoid a rare and hazardous algae bloom known as a red tide.

“We are seeing dead fish all around the bay,” said Jon Rosenfield, senior scientist with San Francisco Baykeeper. “These are very large fish including sharks, sturgeon and large striped bass, and masses of smaller fish.”

Baykeeper, a nonprofit environmental watchdog, advises swimmers, windsurfers and kayakers to avoid long contact with the water because of the contamination of the bloom. This particular species of algae is not known to be acutely toxic to humans, but given its size and intensity, the bloom can cause skin irritation and respiratory problems, Rosenfield said.

“The red tide is more visible in the bay than I’ve ever remembered,” Dr. Jen Brokaw, a three-times-a-week bay swimmer and member of the South End Rowing Club in San Francisco since 2006. “It has made the water brown.”

Dave Dombrowski was running around Lake Merritt on Sunday at sunset when he stopped at the bridge near the amphitheater. He looked down into the murky brown, reddish water and saw the clusters of dead fish, which he said his roommate his warned him about before he took off on his run.

“I’ve never seen anything like this before,” said Dombrowski, 31, an Oakland resident.

While he was running around the lake he said at certain points the smell of dead fish was overwhelming.

At the Dolphin Club, a few swimmers have reported a skin rash over the last two weeks, said Diane Walton, a club board member. However, “we are still swimming,” said Walton, who was in the water Sunday morning.

The club has received no warning from any official authority to stay out of the water, she said. No dead fish have been spotted by club members, “but we are on alert to report,” she said. There has also not been any noticeable discoloration of the water at Aquatic Park, where the club swims.

The Baykeeper hotline first received reports of fish kills early last week in San Mateo County, from Foster City to Coyote Point. The reports then arrived from Hayward and Alameda, Fort Baker in Sausalito and north of the Richmond-San Rafael Bridge. There was also a dead sturgeon that washed up on Stinson Beach, but it is not known yet if this is a result of the same complications. People posted photos on social media of dead fish at Lake Merritt.

“This is very likely related to the red tide algae bloom that we have been tracking in the bay for a month,” Rosenfield said. “It is highly unusual to see this number of large fish on the shoreline or floating. For every fish we see, there are likely many, many more that are underneath the surface or washed up on a part of the shoreline where people can’t see them.”

A red tide is a general term for a wide range of aquatic microorganisms. The bay is over-rich in these nutrients and scientists have been wondering why these algae blooms have not appeared more commonly, like they do in other areas. One theory is the constant flush of tides in and out of the bay washes the algae out into the ocean before it can reach the constant mass necessary to form a bloom.

For unknown reasons starting in late July, a microorganism called Heterosigma akashiwo formed a bloom, first spotted in the Alameda Estuary. Now there is evidence the bloom is everywhere in San Francisco Bay, and also in San Pablo Bay.

“This size and duration of an algae bloom is unprecedented in the bay,” Rosenfield said.

An algae bloom happens when algae multiply rapidly to the point that they can be seen in the form of reddish or tea-brown water, indicating a mat of algae on the surface.

“The whole island of Alameda has been surrounded by this, and it has grown to infest the entire bay,” he said. “This is unprecedented.”

Chronicle Staff Writer Alexandria Bordas contributed to this report.

Sam Whiting is a San Francisco Chronicle staff writer. Email: swhiting@sfchronicle.com Twitter: @samwhitingsf

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