Monterey County lake to reopen, massive fish die off likely due to algae bloom

After reports surfaced of a massive die-off of small fish around July 5 when temperatures climbed to 114 degrees at the lake, and then spread to larger fish in the following days, the recreation area was closed to the public July 10.

SALINAS – Test results at Lake San Antonio in southern Monterey County suggest the culprit in a recent massive fish die-off that closed the lake may have been an algae bloom spurred by high heat that depleted the oxygen in the water and suffocated the fish.

According to officials, there is no threat to public health and the lake will reopen Wednesday.

After reports surfaced of a massive die-off of small fish around July 5 when temperatures climbed to 114 degrees at the lake, and then spread to larger fish in the following days, the recreation area was closed to the public July 10.

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The continued die-off prompted the Environmental Health Bureau to issue a “precautionary measure directive” to the Monterey County Parks Division to keep the public from boating, fishing and swimming in the lake.

“The fish die-off seemed to cease around July 12,” said Chief of Parks Bryan Flores. “Unfortunately the test data that came back and our investigations, we’re not able to have a conclusive result of why the die-off occurred but the prevailing thought is that … an algae bloom occurred.”

At a press conference on Monday, County of Monterey Parks and Lakes Division, Environmental Health Bureau and Water Resources Agency staff said data and testing results from water samples taken in the wake of the fish die-off suggest the cause was a natural algae phenomenon and conditions at the lake have now stabilized.

Flores said that through the natural biological process, “the fish die-off was related to low dissolved oxygen concentrations in the water essentially suffocating the fish.”

When the die-off first occurred, the California Department of Fish and Wildlife was immediately notified, as well as the Water Resources Agency, the State Water Resources Control Board and the Monterey County Health Bureau.

During the ensuing cleanup, that is still in progress, staff has seen other live fish swimming. Other wildlife such as turkey vultures, pelicans, wild pigs and racoons, were eating the dead fish which helped with the cleanup but there is still much that needs to be done with equipment and staff. The cause of the die-off did not affect the animals that dined on the carcasses of the fish.

Marni Flagg, assistant bureau chief of the Environmental Health Department, said health department staff participated by collecting water samples for analysis in the public health laboratory, and that “nearly all the results came back in the normal or the non-detect range.”

“This is important to note for public health that it is safe for humans to go back in the water and everyone can enjoy the lake and have a great time,” said Flagg.

Everyone should realize that the testing the Environmental Health Department does is for recreational bodies of water, she said, and when people enter the water they should realize that the water they come in contact with is not necessarily 100% clean, as there are fish and wildlife present.

“There is no conclusive evidence or definitive reason for the fish die-off,” said Flagg. “When the lake reopens, we just want everyone to know that it is safe for regular recreational activities, and if anyone has any questions they can call environmental health.”

Monterey County Supervisor District 3 Chris Lopez, whose district includes Lake San Antonio, noted that the recreational facility, which covers roughly 5,000 surface acres, is a central gathering point for families. He said the county took the right steps in closing the lake to public access until it could be determined what was happening. Lopez thanked everyone who had a hand in dealing with the die-off, and welcomes everyone to come back now that it is safe to do so.

“Whatever caused it is over for now,” said Lopez.

Flores said that the financial impact of the closure was more than $100,000 due to having to cancel two normally packed weekends for visitors.

With temperatures at Lake San Antonio forecast to be in the upper 80s to low 90s this weekend, crowds are anticipated to return.