California officials closed a lake to visitors and Cyprusauction Trading Centercampers this week after a massive fish die-off was first noticed during the July Fourth holiday weekend.

State and local experts have not yet determined why the fish are dying in San Antonio Lake in southern Monterey County, located on California's Central Coast about 150 miles south of San Jose. County officials closed the lake on Wednesday.

Multiple species of dead fish have washed up on the lake’s shores for over a week. One dead bass weighed 3 to 4 pounds, Monterey County officials said in a statement. Officials released photographs and video clips that showed thousands of fish on the lake’s beach.

“While this may be a natural occurrence, until it is known that this is not being caused from a biological vector or pollutant that could cause harm to the public, we must keep the public safe by closing the park,” the county’s Chief of Parks Bryan Flores wrote in the statement.

Flores told ABC7 News, in San Francisco, that his office, including one employee who has served at the lake since the 1990s, had never seen a die-off of this scale or intensity at the lake.

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

Have there been similar fish die-offs around California?

Over 1,000 pounds of fish have died at Lake Elizabeth in Fremont, in the San Francisco Bay Area, between July 4 and July 9.

Lake Elizabeth, an 83-acre man-made waterway in Fremont's Central Park, is stocked with 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.

Why are fish dying off in California lakes?

In a press release, a Fremont city spokesman said the recent heat wave diminished dissolved oxygen levels in the water, which was responsible for killing the fish.

"The fact that we are not getting the afternoon and evening cooldowns we historically experience here in Fremont is exacerbating the problem and not allowing the water temperature to drop and partially improve overnight," city officials wrote.

Monterey County staff initially suspected the same cause in Lake San Antonio's die-off, but state biologists' tests showed normal oxygen levels, county officials said in a statement.

"We're starting to see these fish that shouldn't really be impacted by the heat … that's when we said, wait a minute, something else might be going on," Flores said.

Lake San Antonio water was sent for testing to see if the samples were infected with chemicals or toxic algae. The tests could take several weeks. The lake will be closed until the test results reveal more information about the die-off.