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

Investigations continue on the red tide’s toll on aquatic life. Scientists and local officials are now looking for possible explanations as to why this happened and what they can do to prevent it from ever happening again.

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

Current estimates by the Department of California Fish and Wildlife suggest that over 11,000 fish perished. The bloom has killed approximately 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 located in Oakland connected to the bay. However, these numbers are only estimates, as it is currently impossible for wildlife officials to know the true totality of the kill.

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

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

“We need to understand the trigger because that understanding is essential for being able to forecast the likelihood of it happening again in the future,” Senn said.

Scientists and California officials first became aware of the bloom in late July, when it was detected in the waters separating the cities of Alameda and Oakland in California’s East Bay. Though 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 extending through the bay as it rapidly spread in high concentrations past the Bay Bridge, which connects San Francisco to Oakland.

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

As the alga consumed the bay, scientists worried that when the alga died, it would provoke a massive drop in the bay’s oxygen, which could potentially cause further death to aquatic life. According to Senn, oxygen plummeted to unprecedented low levels on Aug. 25 in the southern parts of the bay. These low readings continued through approximately Aug. 31.

Though the deadly effects of herterosigma akashiwo are well understood, scientists don’t understand how the organism emits its toxins. So, it’s unknown whether the fish were killed by the alga bloom itself, the resulting drop in oxygen, or a combination of the two forces.

By Sept. 1, the toxic bloom had largely subsided, and scientists and wildlife officials looked for an explanation as they examined the thousands of fish laid to waste across the bay.

“I’m glad that it is one sort of kill event and it is not sustaining,” Deputy Director of Communications at the California Department of Fish and Wildlife, Jordan Traverso, said.

Though the bloom was devastating to wildlife—and a potential harbinger of things to come—it wasn’t nearly as catastrophic as it could’ve been. Though the number of fish casualties was unprecedented, most of the documented deaths occurred in species with relatively healthy populations in the bay (with the exception of green sturgeon), and the kill is unlikely to have impacted their overall numbers.

Things could have easily been much worse, Traverso noted. Had the bloom happened just a few weeks later, it could have impacted the local salmon migration, which would have caused truly lasting environmental damage.

Scientists and local wildlife officials now fear that a bloom like this could repeat itself and are looking into causes and possible solutions to prevent this from ever happening again. There isn’t a clear culprit, though there are some usual suspects.

“I think the algal bloom is one more telling things that we have going on in the state,” Traverso said.

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

This deadly variety of alga needs light to survive. Historically the bay’s muddy waters have prevented light from penetrating its depths, stunting algae growth before it reaches harmful levels. The Bay Area also receives freshwater run-off from the Sierra Nevada mountains, which mixes into the saltwater and prevents algae from reaching the surface because of its lighter density. But something changed that caused the bay’s phosphorus and nitrogen levels to act like fertilizer for the alga.

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

The Bay Area, and California as a whole, have been afflicted by hot weather patterns and drought conditions that continue to worsen. Summers are less windy and temperatures continue to increase. Historically, the Bay Area has depended on run-off from the Sierra Nevada mountains to feed its waters, but snowfall continues to decrease. This past April, when the snowpack is typically at its deepest, it was only 38% of its annual average, according to the Sierra Nevada Conservancy. With less run-off, there is less sediment in the water, one of the key factors to maintaining the bay’s murkiness and ability to prevent large algae blooms.

“Our waters are drying up on some river streams on the North Coast,” Traverso said. “It is stressful on the fish.”

The troubling thing for the Bay Area is that these patterns aren’t one-off events and appear to be establishing themselves, which could mean that toxic alga blooms like this could become a regular occurrence 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 those conditions will set up every one year, every two years, every three years? Or will another event like this not occur for another 30 years?” Senn said.

The factors changing California’s climate are difficult to manage and unlikely to be changed in time to prevent another deadly algae bloom. However, one possible solution that local officials can control is how the bay manages its wastewater. Though the nutrient levels from wastewater have existed for decades and historically posed little environmental issues, proper regulations could curb nutrient intake into the bay—eliminating a crucial element in this last alga bloom. Currently, the San Francisco Bay Regional Water Quality Control Board is examining options for science-based regulations that could deter future blooms.

“We are using this opportunity to learn as much as we can to prevent it from happening again,” White said.

Though scientists are working to find a way to prevent this from happening again, it remains unclear if it’s a problem that can be solved or just another reality of California’s changing climate. However, thousands of dead fish floating in the waterways and rotting on the beach is all too real of an image to be ignored.

“Prior to this event occurring it really was a hypothetical,” Senn said. “It was something I would hope we would never see.”

Feature image via iNaturalist.