San Francisco Bay Experiences First Ever Massive Fish Kill from Red Tide


A toxic algal bloom swept across the California Bay Area late last month, leaving thousands of dead fish in its wake. Known as the “red tide” due to the red-brown color of the seaweed, it is the largest such environmental disaster in the bay’s recorded history.

Research into the Red Tide’s toll on aquatic life continues. Scientists and local officials are now looking into possible explanations as to why this happened and what they can do to prevent it from ever happening again.

“It’s very rare in the San Francisco Bay area, and the bloom is unprecedented,” said Eileen White, executive officer of the San Francisco Bay Regional Water Quality Control Board.

Current estimates from the Department of California Fish and Wildlife assume that over 11,000 fish have died. The bloom has killed about 10,000 yellowfin gobies, hundreds of striped bass and sturgeon – including the endangered green sturgeon – as well as sharks, bat rays and anchovies. Most of the deaths occurred in Lake Merritt, a tidal lagoon in Oakland that connects to the bay. However, these numbers are only estimates as it is currently impossible for wildlife authorities to know the true total number of kills.

“It’s a very difficult number to estimate, and more needs to be done to understand the impact on the fish population in the Bay,” said David Senn, a senior scientist at the San Francisco Estuary Institute, a scientific institute that conducts studies Bay Area waters.

The bloom was caused by the so-called phytoplankton herterosigma akashiwo. Although this specific alga is well documented in the Bay Area, it has never been concentrated in numbers high enough to pose a serious threat to aquatic life. However, something has changed, and now California officials are trying to figure out what allowed the massive bloom to have such deadly consequences.

“We need to understand the trigger because that understanding is essential to be able to predict the likelihood of it recurring in the future,” Senn said.

Scientists and California officials first drew attention to the bloom in late July when it was spotted in the waters separating the cities of Alameda and Oakland in California’s East Bay. Although the bloom was initially confined to the East Bay, scientists at the San Francisco Estuary Institute were aware of its deadly potential and began monitoring it via satellite. They noticed visible dark streaks stretching across the bay as it spread rapidly past the Bay Bridge, which connects San Francisco to Oakland, in high concentrations.

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The first report of dead fish came on August 22nd and continued all week. Scientists and other officials were unable to dispatch enough personnel to cover the entirety of the event and relied heavily on citizen reports of fish sacrifices, using wildlife identification apps like iNaturalist as well as traditional means of communication.

As the algae consumed the bay, scientists feared that the algae’s death would cause a massive drop in oxygen levels in the bay, potentially leading to further deaths of aquatic life. Oxygen dropped to unprecedented low levels in the southern parts of the bay on August 25, Senn said. These low levels lasted until around August 31st.

Although the deadly effects of herterosigma akashiwo are well understood, scientists do not understand how the organism releases its toxins. So it is not known whether the fish were killed by the algal bloom itself, the resulting lack of oxygen, or a combination of the two forces.

By September 1, the toxic bloom had largely subsided, and scientists and wildlife officials searched for an explanation as they surveyed the thousands of fish devastated across the bay.

“I’m glad this is some kind of killing event and it’s not sustainable,” said Assistant Communications Director for the California Department of Fish and Wildlife Jordan Traverso.

While the bloom was devastating to wildlife – and a potential harbinger of things to come – it was nowhere near as disastrous as it could have been. Although the number of fish sacrifices was unprecedented, most of the documented deaths have occurred among species with relatively healthy populations in the bay (with the exception of the green sturgeon) and the killing likely has not impacted their total numbers.

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Things could easily have been a lot worse, Traverso noted. Had the bloom happened just a few weeks later, it could have affected the local salmon migration, causing truly lasting environmental damage.

Scientists and local wildlife authorities now fear such a bloom could happen again and are looking into causes and possible solutions to prevent it from ever happening again. There is no clear culprit, although there are some usual suspects.

“I think the algal bloom is another indication of what we’re looking at in the state,” Traverso said.

Authorities believe the nutrient levels in the bay allowed the seaweed to thrive on such a massive scale, but that’s nothing new. Phosphorus and nitrogen from sewage nutrients have been present in the bay since the advent of modern sewage treatment plants. According to Senn, wastewater accounts for about 60-70% of the bay’s average annual nutrients. However, some of the bay’s natural features are historically hostile to algae growth, making elevated levels of phosphorus and nitrogen harmless.

This deadly species of algae needs light to survive. In the past, the bay’s muddy waters prevented light from penetrating to depths and stifled algae growth before it reached harmful levels. The Bay Area also receives freshwater runoff from the Sierra Nevada mountains, which mixes with the saltwater and, due to its lower density, prevents algae from reaching the surface. But something changed, causing the bay’s phosphorus and nitrogen levels to act like fertilizer for the algae.

“There are many subtle and not-so-subtle factors that could play a role in changes in how the system behaves,” Senn said.

The Bay Area and California as a whole have been beset by hot weather patterns and drought conditions that continue to worsen. Summers are less windy and temperatures continue to rise. In the past, the Bay Area relied on runoff from the Sierra Nevada mountains to feed its water, but snowfall continues to decrease. Last April, when snowpack was typically at its deepest, it was just 38% of its annual average, according to the Sierra Nevada Conservancy. With less runoff, there is less sediment in the water, one of the key factors in maintaining the bay’s turbidity and ability to prevent large algal blooms.

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“Our water is drying up on some river streams on the North Shore,” Traverso said. “It’s stressful for the fish.”

What’s worrying for the Bay Area is that these patterns aren’t one-off events and appear to be becoming established, which could mean that toxic algal blooms like these could be regular occurrences in the Bay Area if scientists don’t find a solution soon.

“Are we in a new regime? Are we in a different place now where these conditions will come every year, every two years, every three years? Or will such an event not happen again in the next 30 years?” Said Senn.

The factors changing California’s climate are difficult to control and unlikely to be changed in time to prevent another deadly algal bloom. However, one potential solution that local officials can control is how the bay manages its wastewater. Although nutrient levels from wastewater have existed for decades and have historically raised few environmental concerns, proper regulation could limit nutrient uptake into the bay and eliminate a crucial element in this latest algal bloom. Currently, the San Francisco Bay Regional Water Quality Control Board is evaluating options for science-based regulations that could prevent future blooms.

“We’re taking this opportunity to learn as much as we can so this doesn’t happen again,” White said.

Although scientists are working to find a way to prevent this, it remains unclear if this is a problem that can be solved or just another reality of California’s changing climate. However, thousands of dead fish swimming in the waterways and rotting on the beach are too real a picture to ignore.

“Before this event, it was really a hypothesis,” Senn said. “It was something I would hope we would never see.”

Feature image via iNaturalist.



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