Lake San Antonio fish
Dead fish lie on one of Lake San Antonio’s boat launches in South Monterey County in an undated photo. Out of caution for public health, the facility has been closed due to the large-scale fish dieoff from an unknown cause. (County of Monterey)

BRADLEY — Monterey County officials are closing Lake San Antonio to visitors Wednesday and asking all campers to vacate following a mass fish dieoff that county staff first noticed last week.

State and county experts have not yet figured out why the fish are dying. For almost a week, larger and larger dead fish have washed up on the lake’s shores. One dead bass weighed 3 to 4 pounds, county officials said in a statement. They issued a photograph that showed waves of hundreds of silver-colored fish on the lake’s beach.

Last Friday, parks employees noticed a large number of dead baitfish, mostly shad, on the shore of Lake San Antonio, roughly 30 miles northeast from Hearst Castle. Affected fish species spread to larger creatures like bass, trout, carp, crappie and catfish. 

Larger species should not have been affected by the recent heat wave, Monterey County officials said. The county called in state wildlife, water and environmental experts as the fish dieoff continued.

The county’s Environmental Health Bureau took the “precautionary measure directive” of asking parks officials to close the lake to swimming, boating and fishing, a county spokesperson said.

“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.

The same week, Fremont city officials saw a mass death of fish, primarily carp, in Lake Elizabeth. In a press release, a city spokesman blamed diminished dissolved oxygen levels in the water caused by the Bay Area’s recent heat wave. 

Monterey County staff suspected the same cause in Lake San Antonio’s dieoff, but state biologists’ tests showed normal oxygen levels, county officials said in a statement. 

Scientists have linked fish dieoffs to climate change. University of California, Davis, researchers found that heat contributed to one in five such events in Wisconsin from 2010 to 2014.

Monterey County officials asked the few campers who remain at Lake San Antonio to pack up and leave by noon Wednesday. 

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