About 1,000 fish died this week in Lake Elizabeth in Fremont's Central Park after the acute heat wave caused low oxygen levels within the water, in accordance with a city spokesman.
With maximum temperatures of over 35 degrees Celsius in the primary half of the week, the lake became warmer and the oxygen level dropped, causing almost 1,000 fish to suffocate since Wednesday.
“The fish kills in Lake Elizabeth in the summer are not entirely unusual. A few fish die every year,” said city spokeswoman Geneva Bosques. But this 12 months's heat wave is having unexpected effects on the lake's fish population.
The city has sent a specialist from Livermore-based Applied Marine Sciences to take water samples and study oxygen levels and the potential for an algal bloom, Bosques said, but she added that there are not any “visible signs” of a dangerous algal bloom just like the one which killed 1000’s of fish in Oakland's Lake Merritt during a 2022 heat wave.
“It's so hot and the water is so shallow. Over the last few nights, the air hasn't cooled enough to lower the temperature at night,” Bosques said. “We haven't done an in-depth analysis, but we believe it's gotten shallower over time.”
The deepest parts of the lake, which were once about 1.5 meters deep, are actually about 1.2 meters deep, Bosques said, and within the shallowest parts the lake is 60 to 90 centimeters deep.
Bosques added that town's specialists are currently investigating the reason for the mass deaths, but that lack of oxygen is probably going the major reason for death.
“The consultant believes the problem is due to reduced dissolved oxygen (DO) levels, which is stressing the fish and causing them to die,” town said in a press release released Friday afternoon. “A DO level of 5.0 mg/L is the standard target and our level is currently just below 1.0.”
A small portion of the lake is owned by town, but the bulk is owned by the Alameda County Flood Control District, which regulates the flow of water into the lake. Bosques said town has an agreement with the flood control district to keep up the lake and this week, as a consequence of the warmth, deployed an automatic pump to extend the flow of fresh water.
The city stocks several hundred kilos of catfish and trout annually, Bosques said, but fish species swimming within the lake also include Sacramento suckermouth, sunfish and carp, which don't actually belong within the lake.
Mark Carr, a professor of marine ecology on the University of California, Santa Cruz, said the fish are suffocating because they’re competing with the lake's ecosystem for limited oxygen and losing out. The biggest competitors for oxygen outside of the fish are likely phytoplankton and algae, he said.
“Phytoplankton blooms and algal blooms can occur at night because the phytoplankton respire, which means they use up oxygen, which in turn lowers the oxygen levels in the water – especially in shallow water, which is more susceptible to heat,” Carr said in an interview. “When (fish) lose oxygen in the water, they simply suffocate.”
An excess of nutrients within the water, comparable to fish waste and fertilizers from the encircling park, can result in an algal bloom.
“What confuses people is the fact that algae generally use carbon dioxide for photosynthesis and they don't think that algae use oxygen, which is what algae in plants do,” Carr said.
And the fish which can be naturally introduced into the ecosystem, comparable to catfish and trout, also compete with the invasively introduced fish for oxygen.
To fix the issue, ecologists must reduce the nutrients available to algae and phytoplankton to curb their growth and make it easier for fish to breathe.
But due to the holiday, city officials and far of the parks administration are out of the office and can’t immediately address the fish kill, spokeswoman Bosques said. Even town's specialist took samples from the lake under emergency conditions and is now back on vacation, she said. The city is not going to have the option to completely address the lake's problems until early next week on the earliest.
If Fremont officials were to deepen the lake to make more room for the fish, town would hypothetically should spend hundreds of thousands of dollars draining and dredging the 83-acre site. That hasn't been done in a few years, Bosques said, and on the time it cost greater than one million dollars.
She said town's major focus straight away is to wash the lake of dead fish and assess the water condition.
“To manage the lake long-term, we definitely need to have some discussions,” Bosques said.
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