Fish Kill at Clear Lake Reveals a Seven-Foot Sturgeon Surprise
A problem lake was doing pretty well this year. Then came a series of unfortunate water-quality events.
The post Fish Kill at Clear Lake Reveals a Seven-Foot Sturgeon Surprise appeared first on Bay Nature.
Tiny silver fish float up at Clear Lake in August. Big Valley Band of Pomo Indians records indicate this was the biggest fish kill since 2017. (Courtesy of Luis Santana)As Luis Santana motored out onto Clear Lake this August, it seemed at first like a normal summer day out on the water: warm air, cloudy skies, and the wide lake waters full of what seemed like bubbles from the waves.
“Then I stopped, and I was like, Oh my god,” Santana, a fisheries biologist with the Robinson Rancheria tribe, recalls. Those weren’t bubbles; they were millions of dead threadfin shad, and others. “I saw literally every species of fish found in the lake,” except for the Clear Lake tule perch, Santana says. The measurements he took that day revealed what likely killed them: a near-total lack of oxygen in the lake. The fish had, essentially, suffocated.
Amid the silver-lined shores, one fish washed up that no one had known to be a resident: a dead seven-foot-long white sturgeon. It was Clear Lake’s first on record. No one knows for sure how it got into the waters, but Santana thinks it died with the shad. White sturgeon (Acipenser transmontanus), the biggest freshwater fish in North America, live in the Bay-Delta. They became a candidate for listing as a threatened species under the California Endangered Species Act after a 2022 harmful algal bloom that killed hundreds of them.
Big ’un: A white sturgeon—in Clear Lake? CDFW says the average sturgeon caught in the Delta these days is about 3.6 feet long, and it is rare to encounter fish larger than 6.5 feet long in California. This one was seven feet. (Courtesy of Luis Santana)This fall’s fish die-off is the lake’s largest since at least 2017, according to records from the Big Valley Band of Pomo Indians. And it is yet another environmental black mark for a lake—California’s largest freshwater body—that has been consistently troubled by poor water quality. Now, scientists are uncovering the exact cause of the die-off—and analyzing the sturgeon for more answers. For Angela DePalma-Dow, a lake scientist and executive director of the Lake County Land Trust, the event reminds her: “There’s so much we can learn from Clear Lake.”
As a five-year-old, Santana spent every summer day swimming in Clear Lake. That’s a distant dream now. The summer lake—despite the name—is rarely clear; more often, it’s clouded dirty green as harmful algal blooms take over the waters. Sometimes, Santana thinks the water smells like sewage. “I don’t think my kids have ever swam in Clear Lake,” Santana says.
Fish die-offs and fish kills are a consequence of these impaired conditions, especially the frequent harmful algal blooms (HABs), during which algae decompose and strip the water of oxygen (while also filling the water with cyanotoxins). The Big Valley Band of Pomo Indians has tracked harmful algal blooms in Clear Lake since 2014. The program started after five years of “thick, noxious blooms covering [Clear Lake’s] surface” (as the tribe writes in a history of the program) and no regular monitoring from the state, despite recommendations from the California Office of Environmental Health Hazard Assessment. “We just needed to have more data,” says Sarah Ryan, the environmental director at the Big Valley Band of Pomo Indians.
“It seems like they have fish kills every year,” says Ben Ewing, who studies the endemic Clear Lake hitch, a large minnow, at the California Department of Fish and Wildlife. “I lost track with how many.” In 2017, the state Legislature formed a committee to restore the lake, citing high mercury levels, dangerous contaminants in fish, and the regular HABs; to date, it has led to tens of millions in state funding for research, restoration, and education projects on Clear Lake, including helping sustain water quality monitoring cut by the state during the Covid pandemic.
Cyanobacteria bloom at Redbud Park, in Clear Lake’s southeast arm, in July 2020. Big Valley Pomo EPA’s sampling found toxins at a “warning” level. The lake is frequently beset by harmful algal blooms. (Courtesy of Big Valley Band of Pomo Indians)This die-off, Ewing says, caught lake-watchers off guard because 2025 seemed like the year Clear Lake might escape a fish kill. The characteristic pea soup of harmful algal blooms had been noticeably absent. Instead, the cause was likely a perfect storm of other conditions, says Ewing. “Everything had to line up correctly for this to happen,” he says.
Two bountiful water seasons laid the ground for it, DePalma-Dow explains. Fish populations—especially nonnative bait fish like shad—boomed with the increased water, which also meant some fish naturally died. She speculates that as their bodies decomposed, they stripped oxygen from the water column. Then, this fall, heavy winds came and distributed the low-oxygen water throughout the water column. A series of cloudy mornings arrived, during which the lake’s aquatic plants couldn’t respire oxygen back into the water. So more fish likely died, triggering oxygen levels to further plummet. Eventually, conditions became fatal for all species of fish. Santana says he measured “basically zeroes on every level” for dissolved oxygen through the water column.
DePalma-Dow says this process is just the lake self-regulating, as fish populations outstrip the oxygen available. “This is totally not surprising for a lake cycle event,” she says. “This is a big, huge, natural system.”
Santana blames human disturbance for the die-off. “We took away all the habitat that could potentially negate any of these effects,” he says. Clear Lake has lost up to 90 percent of its wetlands, he says, and creeks that might once have provided an infusion of oxygen-rich water into the lake now run dry in May and June. “There’s just so many things we’re taking and taking and not giving back,” he says.
A satellite image of Clear Lake during a May 2024 algal bloom. The emerald color doesn’t tell you whether toxins are present, though. That requires water sampling, which the Big Valley Band of Pomo Indians EPA has been doing since 2014. (Sentinel-2 satellite, via the Copernicus browser)In lieu of those natural processes, technological solutions are being considered: Researchers from UC Davis are exploring installing oxygenators in Clear Lake that could trap nutrients in the sediment under a thin layer of oxygen, theoretically reducing the number of harmful algal blooms—and, possibly, keeping oxygen levels higher so more fish can breathe. “That would be one of the hopeful outcomes,” says DePalma-Dow.
Neither the state nor county put out a press release about the die-off, Ryan notes. “It’s always better if you can anticipate the questions and try to get information out.” For now, those living by the lake watch (and smell) the dead fish decompose. “There’s really no post support,” she says.
The August die-off on Clear Lake silvered the shoreline. It claimed fish of “literally every species,” says Luis Santana, a Robinson Rancheria fisheries biologist. (Larger fish on shore courtesy of Luis Santana; silvery shoreline by Shawna McEwan; closeup by Sophia Zesati)
Fish populations will likely recover, scientists say. Many fish probably survived, in nooks and crannies. With good winter rains, they can breed and repopulate the waters by spring. This die-off is just another challenge for a beleaguered lake.
As for the sturgeon? USGS scientists were trying to figure out how old it was, and hoping to answer when it got to the lake, but the government shutdown has since paused their work. And they cannot answer questions about their research until the shutdown ends. Santana’s observations of the sturgeon showed it was a female with eggs. For now, the giant fish is a reminder of the treasures that may hide in Clear Lake’s murky waters. “Every year is a mystery and surprise,” DePalma-Dow says.
