Fremont blames heat for massive Lake Elizabeth fish die-off

Oxygen levels in the lake are less than one-fifth the standard target level, according to the city.

About 1,000 fish at Lake Elizabeth in Fremont’s Central Park have died this week after the excessive heat wave caused low oxygen levels in the water, according to a city spokesperson.

With city high temperatures in the mid-to-upper 90s the first half of the week, the lake got hotter and oxygen levels dropped, suffocating close to 1,000 fish since Wednesday.

“Fish dying off in Lake Elizabeth in the summer is not completely unusual. We do get a small number of fish that die off every year,” city spokesperson Geneva Bosques said. But this year’s heat wave is doing unexpected numbers on the lake’s fish population.

The city sent a specialist with Livermore-headquartered Applied Marine Sciences to take test samples of the water to inspect oxygen levels and the possibility of an algae bloom, Bosques said. But she added there is no “visible indication” of a dangerous algae bloom, such as the one that killed off thousands of fish in Oakland’s Lake Merritt during a 2022 heat wave.

“It’s so hot and the water is so shallow, in the last couple of nights, the air hasn’t cooled off enough to lower the temperature at night,” Bosques said. “We haven’t gone in to do an in-depth analysis but we believe it’s gotten shallower over time.”

The deepest parts of the lake, once around 5 feet, are now about 4 feet, Bosques said, and 2-3 feet deep where the lake is shallowest.

Bosques added that the city’s specialist is evaluating the cause of the die-off, but the lack of oxygen is likely the main contributor to the deaths.

“The consultant estimates that the issue is due to a diminished Dissolved Oxygen (DO) level that is stressing the fish and causing the die-off,” the city said in a statement released Friday afternoon. “A DO level of 5.0 mg/L concentration is the standard target and ours is currently just below 1.0.”

The city owns a small part of the lake but it is mostly owned by the Alameda County Flood Control District, which regulates the water going in and out of the lake. The city has an agreement with the flood control district to maintain the lake, according to Bosques, and used an automated pump to increase the flow of fresh water this week due to the heat.

The city stocks the fish annually with a few hundred pounds of both catfish and trout, Bosques said, but the species of fish found floating in the lake also include Sacramento sucker, crappie and carp, which are not supposed to be in the lake.

Mark Carr, a professor of marine ecology at the University of California, Santa Cruz, said the fish are suffocating because they are competing with the lake’s ecosystem over the limited oxygen and losing. The top competitors for the oxygen outside of the fish are probably phytoplankton and algae, he said.

“You can get phytoplankton blooms, algal blooms, at night that the phytoplankton respires, which means it consumes oxygen, so then it too will reduce the oxygen levels in the water – especially in the shallow water that is more vulnerable to the heat,” Carr said in an interview. “When (fish) lose oxygen in the water, they simply suffocate.”

An algal bloom can be spurred by an overabundance of nutrients in the water, such as fish feces and fertilizers in the surrounding park.

“What people get confused about is that the algae generally use carbon dioxide for photosynthesis and they don’t think that algae consumes oxygen, but algae in plants do,” Carr said.

And the fish naturally introduced into the ecosystem, the catfish and trout, are also competing for oxygen over the invasively introduced fish.

To remediate the issue, ecologists have to reduce the nutrients available to the algae and phytoplankton in order to curb their growth and allow fish to breathe easier.

But because of the holiday, city officials and much of the city’s parks department are out of office and unable to immediately address the fish die-off, according to spokeswoman Bosques. Even the city’s specialist sampled the lake under emergency circumstances and is back on vacation, she said. The earliest the city could wholly address the lake’s problems is early next week.

Hypothetically, if Fremont officials were to deepen the lake to make more room for the fish, the city would have to spend millions of dollars to drain and dredge the 83-acre plot. This hasn’t been done in many years, Bosques said, and back then it cost over $1 million.

She said the city’s main focus now is cleaning the lake of dead fish and evaluating the water conditions.

“For long-term management of the lake, we’re definitely going to have to have some conversations,” Bosques said.