County of Monterey Shares Fish Die Off Testing Results; Announces Reopening of Lake San Antonio is Expected to be on July 24

 

County of Monterey Parks and Lakes Division (Parks), Environmental Health Bureau (EHB) and

Water Resources Agency (WRA) staff say data and testing results from water samples taken in

the wake of a massive fish die off at Lake San Antonio were inconclusive but that a possible

cause of the incident could be due to a natural algae phenomenon spurred on by high heat, which

lowered the dissolved oxygen concentrations causing the fish to suffocate. At this point in time,

those conditions have stabilized.

Parks staff has been busy cleaning up the fish remains and the EHB, which placed a

precautionary measure directive to restrict access to the water, leading Parks to make the difficult

decision to close the facility, has now lifted that measure and Parks is planning to reopen the lake

on Wednesday, July 24, 2024.

Background

On July 5, 2024, Parks staff noticed that dead baitfish, mostly shad (a small fish species), began

washing up on the shore. The die off continued to grow and in an abundance of caution, Parks

made the decision to close Lake San Antonio on July 10th, until the situation was researched and

better understood.

The die off appeared to last until July 12, 2024, as reported by Parks staff and the California

Department of Fish and Wildlife (CDFW) biologist. This die off appears to be the largest fish die

off in Lake San Antonio history.

Determining a Cause

Based on the testing analysis and lack of conclusive evidence, the definitive reason for the fish

die off cannot be specifically determined. However, due to the fact that mammals and birds, such

as wild pigs and turkey vultures, which consumed the dead fish did not die, that satellite imagery

indicated the presence of an algae bloom, and water quality testing shows that the lake is normal

for a surface water body, the prevailing belief is that dissolved oxygen levels in the water were

low enough that the fish suffocated during the recent high heat wave event. As the algal bloom

diminished, dissolved oxygen concentrations improved, and the fish die off ceased.

During periods of high heat, algae growth can dramatically increase which can lower the

dissolved oxygen levels in the water. Prior to and through the duration of the fish die off,

temperatures at Lake San Antonio were well above 100 degrees Fahrenheit, warming the water

in some areas of the lake to near 90 degrees Fahrenheit, spurring on a rampant but short-lived

algal bloom.

Chief of Parks Bryan Flores stated, “While the definitive cause of the Lake San Antonio fish die

off is not able to 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.

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, to use their best judgement, heed any cautionary signage that may be present and be aware

of their surroundings.”

Staff thanks the public and the Board for their patience and understanding through this event and

looks forward to the return of recreation at the lake.

Testing Results

Nearly all test results came back in the normal or non-detect range. Testing and sampling

regimen included the following and are attached:

• WRA took readings for water temperature, conductivity, and dissolved oxygen.

• EHB conducted bacterial, heavy metal and toxin sampling.

• CDFW took daily temperature and dissolved oxygen readings and sampled for the

presence of algae, cyanobacterial toxins (microcystins/nodularins,

cylindrospermopsins, anatoxins, and saxitoxins) and golden algae