San Francisco Bay suffers first-ever mass fish kill from red tide


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

Investigations continue into the toll of the red tide on aquatic life. Scientists and local officials are now researching possible explanations as to why it happened and what they can do to prevent it from happening again.

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

Current estimates from the Department of California Fish and Wildlife suggest that more than 11,000 fish perished. The bloom killed an estimated 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 connected to the bay. However, these numbers are only estimates, as it is currently impossible for wildlife officials to know the true total of the catch.

“It’s a very difficult number to estimate, and there’s still work to be done to understand the impact on the fish population in the bay,” said David Senn, senior researcher at the San Francisco Estuary Institute, a scientific institute who studies Bay Area Waters.

The bloom was caused by phytoplankton known as herterosigma akashiwo. Although this specific algae is well documented in the Bay Area, it has never concentrated in high enough numbers 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 key to being able to predict the likelihood of it happening again in the future,” Senn said.

California scientists and officials first became aware of the bloom in late July, when it was detected in the waters between the towns of Alameda and Oakland in California’s East Bay. Although the bloom was initially contained 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 in high concentrations past the Bay Bridge, which connects San Francisco to Oakland.

The first report of dead fish came on August 22 and continued throughout the week. Unable to dispatch enough equipped personnel to deal with the entire event, scientists and other officials relied heavily on citizen reports of fish casualties, using wildlife identification apps like iNaturalist, as well as traditional modes of communication.

As the algae consumed the berry, scientists feared that the death of the algae would cause a massive drop in oxygen to the berry, which could potentially lead to the death of aquatic life. According to Senn, oxygen dropped to unprecedented levels on August 25 in southern parts of the bay. These low readings continued until around August 31st.

Although the deadly effects of herterosigma akashiwo are well understood, scientists do not understand how the body emits its toxins. Thus, it is not known whether the fish were killed by the algal bloom itself, the resulting drop in 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 by examining the thousands of devastated fish in the bay.

“I’m glad this is a deadly type of event and it’s not long lasting,” California Department of Fish and Wildlife Assistant Director of Communications Jordan Traverso said.

Although the bloom was devastating to wildlife – and a potential harbinger of things to come – it was not as disastrous as it could have been. Although the number of fish casualties is unprecedented, most documented deaths have occurred in species with relatively healthy populations in the bay (with the exception of green sturgeon), and it is unlikely that the killing had an impact on their total number.

Things could easily have been much worse, Traverso noted. Had the bloom occurred a few weeks later, it could have impacted local salmon migration, causing truly lasting environmental damage.

Scientists and local wildlife officials are now concerned that such a bloom could happen again and are studying possible causes and solutions to prevent it from happening again. There’s no clear culprit, although there are a few usual suspects.

“I think the algal bloom is another telling thing that we have in the state,” Traverso said.

Authorities believe the nutrient levels in the bay allowed the algae to thrive on such a scale, but that’s nothing new. Phosphorus and nitrogen from sewage nutrients have existed in the bay since the advent of modern sewage treatment facilities. According to Senn, sewage accounts for about 60 to 70 percent of the annual average nutrients in the bay. However, some of the bay’s natural properties are historically hostile to algae growth, making increased phosphorus and nitrogen levels non-hazardous.

This deadly variety of algae needs light to survive. Historically, the bay’s muddy waters have prevented light from penetrating its depths, curbing algae growth before it reaches harmful levels. The Bay Area also receives freshwater runoff from the Sierra Nevada Mountains, which mixes with salt water and prevents algae from reaching the surface due to its lower density. But something changed that caused the berry’s phosphorus and nitrogen levels to act as a fertilizer for the algae.

“There are many subtle and less subtle factors that could play a role in changes in system behavior,” Senn said.

The Bay Area and California as a whole have been impacted by hot weather and drought conditions that continue to worsen. Summers are less windy and temperatures continue to rise. Historically, the Bay Area relied on runoff from the Sierra Nevada Mountains to supply its waters, but snowfall continues to decline. Last April, when the snowpack is typically deepest, it was just 38 percent 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 keeping the bay dark and its ability to prevent algae blooms.

“Our waters are drying up on some north coast streams,” Traverso said. “It’s stressful for the fish.”

The troubling thing for the Bay Area is that these patterns are not one-time events and seem to be establishing themselves, which could mean that toxic algal blooms like this could become a regular occurrence in the Bay Area. bay 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 set in every year, every two years, every three years? Or will another event like this not happen for another 30 years? Senn said.

California’s climate-changing factors are difficult to manage and unlikely to be changed in time to prevent another deadly algal bloom. However, one possible solution that local authorities can control is how the bay manages its wastewater. Although sewage nutrient levels have been around for decades and have historically posed few environmental problems, proper regulation could limit the input of nutrients into the bay, eliminating a crucial component of this latest algal bloom. Currently, the San Francisco Bay Area Water Quality Board is reviewing science-based regulatory options that could deter future blooms.

“We’re taking this opportunity to learn as much as we can to prevent this from happening again,” White said.

Although scientists are working to find a way to prevent this from happening again, it is still unclear whether this is a problem that can be solved or just another reality of climate change from California. However, thousands of dead fish floating in the waterways and rotting on the beach are too real a picture to ignore.

“Before this event happened, it was really hypothetical,” Senn said. “It was something I hoped we would never see.”

Feature image via iNaturalist.

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