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's Chief of Parks Bryan Flores said fish who live in all depths have been affected, including catfish whose habitat is hundreds of feet deep.
MORE: Extreme heat has killed up to 1,000 fish at Fremont lake, city says
"We just don't know what's causing it," Flores said. "We hope it stops soon."
Flores said neither he nor his staff had seen a dieoff of this scale or intensity at the lake before, including one employee who has served at the lake since the 1990s.
Flores explained that they could be seeing the delayed effects of one spontaneous event that took time to ripple through fish populations. He said he didn't suspect someone dumped something deadly off their boat, in part because the dieoff has not extended to area birds or raccoons.
"If it was a poison, you'd think it would be impacting the other species," Flores said. "More than likely, it is some kind of biological process that's occurring, whether or not it's algae or protozoa."
MORE: CA officials warn of possible toxic algae bloom at South Lake Tahoe beach
The county called in state wildlife, water and environmental experts as the fish dieoff continued, spreading to the bigger and deeper water fish as the days progressed.
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.
"We must keep the public safe by closing the park," 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. Flores said Lake San Antonio reaches close to 300 feet in depth, which should have given some fish places to escape heat's effects.
MORE: Improvements eyed for San Francisco Bay in wake of algae bloom
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.
Flores said that he hoped to have water testing results back by the beginning of next week. A state fish and wildlife biologist has visited the lake every day to investigate, according to Flores.
If they can determine the dieoff's cause does not endanger humans, Flores said he hopes to reopen the lake after an initial cleanup. For the full cleanup, he estimated his department will need a few weeks.