How to resolve AdBlock issue? LAKE COUNTY, Calif. — The work to understand the cause of a September fish die-off in Clear Lake is continuing.
This week, the Board of Supervisors heard the latest on the ongoing efforts from scientists and local officials as part of a report from the Lake County Fish and Wildlife Advisory Committee.
At the start of September, a fish die-off was reported in Clear Lake. At that time, the California Department of Fish and Wildlife told Lake County News that the die-off was believed to have been related to low levels of dissolved oxygen in the water.
Luis Santana, chair of the Lake County Fish and Wildlife Advisory Committee and fish and wildlife director of Robinson Rancheria’s Danoxa Fish and Wildlife Department, presented the update to the board on Tuesday.
He explained during the discussion that he’s close to completing a full report on the die-off, and also is studying the case involving the discovery in September of a white sturgeon that had been living in the lake, which isn’t part of its natural range.
Santana said the fish kill occurred during the first couple of weeks in September. County residents shared photos with him of the incident.
In one case, a Soda Bay resident told him that they had been out on the lake the night before, “and everything seemed fine, but then the next morning, when they took a walk down the shoreline, there was a bunch of dead fish all over the place.”
Santana responded to the area on Sept. 7 and began doing water quality readings, specifically measuring dissolved oxygen levels. He took a shoreline sample and found that it was bad — but not so bad that fish couldn’t survive.
Fish like the Clear Lake hitch, Sacramento sucker and related fish should have about five milligrams per liter of dissolved oxygen in the water, but they can survive at lower dissolved oxygen levels, Santana said.
However, Santana said that once the dissolved oxygen levels drop to around 2.0 milligrams per liter, fish start to die.
When he conducted measurements at the shoreline in Soda Bay, he got a reading of 3.95 milligrams per liter, still survivable but not good for the overall fitness of fish.
Santana said a shoreline measurement is not really accurate for dissolved oxygen because wave action will provide some dissolved oxygen.
So he went out the next day to get vertical profiles of the lake. Launching from Keeling Park in Nice, Santana said as he was passing Lucerne he saw what he initially thought were waves but which he estimated were millions of threadfin shad, an introduced — not native — species to Clear Lake. He said the shad is a bait fish that the bass — also not native — love to eat.
When he saw the shad, Santana stopped at a spot in the middle of the lake and took dissolved oxygen measurements. Three feet below the lake’s surface, the dissolved oxygen measured 0.52 milligrams per liter. At 17 feet deep, it was 0.28 milligrams per liter, and at the lake’s bottom it was 0.22 milligrams per liter.
When he reached Dorn Bay, near the area where he first conducted measurements the previous day, Santana said the dissolved oxygen levels were not much better and below the 2 milligrams per liter level at which fish tend to die. At seven feet below the surface, the measurement was 1.0 milligrams per liter, falling to 0.2 milligrams at 13 feet down.
“So basically, in the entire water column, it was not suitable for fishes in the area,” Santana said.
The fish he saw dying included Sacramento blackfish, threadfin shad, largemouth bass, black crappie and bluegill.
Santana said he found more measurements that were similar when heading back across the lake toward Keeling Park, and he saw still more fish species — including bullhead catfish, channel catfish and hitch — impacted.
He said the low dissolved oxygen levels impacted the entire ecosystem as far as fish were concerned.
Supervisor Bruno Sabatier thanked Santana for his presentation. “Definitely, if you live around the lake there, you can't not know about the fish kill that occurred this year and previous years. Typically it is dissolved oxygen,” he said.
Sabatier added, “It's been a bit of a stranger year for the lake, where the lake has not appeared to have as much algae. Now, algae can be in many different stages. It doesn't have to be on the top floating, but typically what we find is the dissolved oxygen does occur when we have the mats and other things where it gets really bad during the summertime.”
He asked if there was a culprit for the dissolved oxygen levels in Clear Lake this year, adding, “The lake just looked vastly different than what I was expecting.”
Santana said algae decomposition causes low dissolved oxygen levels, explaining that when the plants decompose, they suck oxygen from the water, and as a result fish kills occur.
However, “In this instance, that definitely wasn't it. I think it was lake turnover, but I can't really prove that, because I wasn't taking samples year round to be able to say, ‘Hey, this is what's occurring,’” Santana said.
Lake turnover explains what happens when a lake’s layers mix on a seasonal basis — typically in the spring and fall — causing the temperature to be more uniform through the water column. The process also moves dissolved oxygen from the surface of the water throughout the water body, while also distributing nutrients from the lake bottom through other lake layers.
Santana called the die-off “a really weird event,” because the lake was looking really good and the bass in the lake were massive in size.
He said, however, that the situation wasn’t the result of algae. Santana cited findings from Big Valley Rancheria’s environmental department which monitors the lake, and its data concluded the same thing.
Chris Childers of Lake County Water Resources Department said they did vertical profiles on Clear Lake on Sept. 10 and found the same thing that Santana did — very, very low dissolved oxygen from the surface all the way down to the bottom and all three arms of the lake.
Childers thanked Santana for his work, noting that it’s great that “we can all collaborate and work together and make things happen.”
Angela DePalma-Dow, executive director of the Lake County Land Trust and a former Water Resources staffer, thanked Santana and Childers for their work.
DePalma-Dow, who has worked with lakes in six different states, offered an explanation for conditions that are seen in small and large lakes alike.
Fish die-offs happen in some years and not in others, and DePalma-Dow said that while there wasn’t that much cyanobacteria — or algae — in Clear Lake this year, there were other things that grew a lot, and they tend to die at the end of the season, like the plants in a vegetable garden.
“So if you have a lot of things growing in your garden at the end of the year, right now, in fall, stuff will start to die, and that can consume oxygen. So that is one factor,” she said.
DePalma-Dow said she looked at seasonal patterns going back to 2012. In that year and in 2016, Lake County had a very significant drought, which was followed by a flood year in 2017.
She said there was a significant fish die-off in the drought year, followed by high water levels. The pattern continued in 2020 to 2022, with lower water levels, followed by more water in 2023 and 2024.
“So there's some patterns where you see drought and then flood years where you have a lot of growth of vegetation, a lot of space, a lot of disturbed soils that can contribute to algae and plant growth that all then died and could create these fish kill events,” she said, adding that these “are natural processes.
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