Massive fish die-off reported at Central Coast lake. What caused it?

A large fish die-off event hit Lake San Antonio on the Fourth of July, according to California Department of Fish and Wildlife biologist Zachary Crum.

Biologists are still investigating the incident, but they suspect that extreme heat caused algae to bloom in the lake — consuming most of the oxygen in the water and suffocating the fish.

“Algal blooms produce oxygen through photosynthesis during the day when sunlight is available, but algae will consume large amounts of oxygen at night when cellular respiration is occurring in the absence of sunlight,” Crum wrote in an email to The Tribune. “This can lead to lethally low dissolved oxygen levels in reservoirs, which can essentially cause fish to suffocate.”

All shallow lake fish species were impacted by the die-off, including largemout bass, bluegill sunfish, black crappie, channel catfish, common carp, Monterey sucker, Sacramento pikeminnow, rainbow trout and threadfin shad, he said.

Humans and pets were likely not at risk, Monterey County spokesperson Maia Carroll said.

“Lake goers can still fish, but people are advised to leave the dead fish alone,” she wrote in an email to The Tribune.

The Monterey County Parks Department manages the lake, which sits near the southern border of the county. Visitors can swim, fish, camp, boat and horseback ride at the county park.