California officials closed San Antonio Lake to visitors and campers this week after a significant fish die-off was first observed during the July Fourth holiday weekend.


Experts from state and local agencies have yet to determine the cause of the fish deaths at San Antonio Lake in southern Monterey County, located about 150 miles south of San Jose on California's Central Coast. The lake was closed by county officials on Wednesday.


For over a week, various species of dead fish have been washing up on the lake’s shores. Among the dead fish was a bass weighing 3 to 4 pounds, according to a statement from Monterey County officials. Photographs and videos released by officials show thousands of dead fish littering the lake’s beach.


“While this may be a natural occurrence, until it is confirmed that this is not caused by a biological vector or pollutant that could pose a risk to the public, we must prioritize public safety by closing the park,” wrote Bryan Flores, the county’s Chief of Parks, in the statement.


Flores told Ain San Francisco that his office, including an employee who has worked at the lake since the 1990s, had never witnessed a die-off of such scale or intensity at the lake.


"It seems like every fish species in the lake is being affected," Flores told SFGATE.


Have there been similar fish die-offs around California?


At Lake Elizabeth in Fremont, located in the San Francisco Bay Area, over 1,000 pounds of fish died between July 4 and July 9.


Lake Elizabeth, an 83-acre man-made waterway in Fremont's Central Park, is home to catfish, trout, crappie, and carp.


On Tuesday, Fremont officials reported that about 90% of the dead fish were carp, and there have been no new die-offs since July 9.


Hundreds of dead fish have continued to wash up on the shores of Lake San Antonio since the July 4 weekend. Biologists and county officials remain uncertain about the cause and have closed the lake to visitors.


Why are fish dying off in California lakes?


A Fremont city spokesman stated in a press release that the recent heat wave had reduced dissolved oxygen levels in the water, leading to the fish deaths.


"The lack of our usual afternoon and evening cooldowns is worsening the situation by not allowing the water temperature to drop and partially improve overnight," city officials explained.


Initially, Monterey County staff suspected that the same cause might be responsible for the die-off at Lake San Antonio. However, state biologists found normal oxygen levels during their tests, according to county officials.


"We're starting to see fish species affected that shouldn't be impacted by the heat ... that's when we realized something else might be going on," Flores said.


Water samples from Lake San Antonio have been sent for testing to check for chemicals or toxic algae. The results, which may take several weeks, will determine whether the lake remains closed until more information is available.