Lake San Antonio reopens after fish die-off

Test results suggest that the incident may have been caused by a natural algae phenomenon exacerbated by high heat

– The County of Monterey has announced the reopening of Lake San Antonio following an investigation into a massive fish die-off. Officials from the County of Monterey Parks and Lakes Division, Environmental Health Bureau, and Water Resources Agency reported that water sample tests were inconclusive. However, they suggested that the incident may have been caused by a natural algae phenomenon exacerbated by high heat, which lowered dissolved oxygen concentrations and caused the fish to suffocate. These conditions have since stabilized.

Parks staff has been cleaning up the fish remains, and the Environmental Health Bureau, which had imposed a precautionary measure to restrict water access, has now lifted the directive. Parks plans to reopen the lake on Wednesday, July 24.

On July 5, parks staff observed dead baitfish, mostly shad, washing up on the shore. The die-off continued, prompting the decision to close Lake San Antonio on July 10 as a precautionary measure. The event lasted until July 12, making it the largest fish die-off in the lake’s history.

Testing and analysis could not conclusively determine the cause of the die-off. However, mammals and birds that consumed the dead fish did not die, satellite imagery indicated an algae bloom, and water quality tests showed normal conditions for a surface water body. The prevailing belief is that the low dissolved oxygen levels during a recent heatwave caused the fish to suffocate. As the algae bloom diminished, dissolved oxygen concentrations improved, and the die-off ceased.

During periods of high heat, algae growth can significantly increase, reducing dissolved oxygen levels in the water. Temperatures at Lake San Antonio exceeded 100 degrees Fahrenheit, warming the water in some areas to nearly 90 degrees Fahrenheit, which likely spurred a rapid but short-lived algae bloom.

“While the definitive cause of the Lake San Antonio fish die-off cannot be conclusively determined, based on the test data, we can affirmatively state that it was not caused by bacteria, toxins, or pollutants, which could pose a risk to human health,” said Chief of Parks Bryan Flores. “Algal blooms are part of the biological process in bodies of water. Unfortunately, at times, they can be impactful on the environment. Parks recommends that while the public is enjoying the lake, they use their best judgement, heed any cautionary signage that may be present, and be aware of their surroundings.”

The staff expressed gratitude to the public and the Board for their patience and understanding during this event and looks forward to the return of recreation at the lake.

Testing results showed nearly all parameters within normal or non-detect ranges. The testing and sampling regimen included water temperature, conductivity, and dissolved oxygen readings by the Water Resources Agency; bacterial, heavy metal, and toxin sampling by the Environmental Health Bureau; and daily temperature and dissolved oxygen readings, as well as algae and cyanobacterial toxin sampling by the California Department of Fish and Wildlife.