MADERA COUNTY, Calif. (KSEE/KGPE) — The Madera County Department of Public Health issued an advisory Monday after large amounts of harmful algae blooms were found in Hensley Lake near Yosemite Lakes.

The advisory comes after staff from the Central Valley Water Board reported spotting big patches of harmful algae and cyanobacteria, often referred to as blue-green algae, in the water at the lake.

In addition, the California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment (OEHHA) issued a state fish advisory for Hensley Lake on March 23, 2021. This advisory guides people on how to prepare the fish they catch for meals.

OEHHA issued a statewide fish advisory last Tuesday that applies to all lakes and reservoirs in California that do not have their own advisory and for species not included in a site-specific advisory.

Harmful algal blooms (HABs or blue-green algae) are an annual occurrence in the Central Valley. HABs can create toxins that are harmful to people and animals.

There has been renewed interest in the phenomenon after a Mariposa family was found dead in the Sierra National Forest. Mariposa County Sheriff’s deputies say they are looking at all possible causes for their death including signs of toxic algae.

According to the HAB incident reports map, the advisory level in Hensley Lake is listed as “Danger” which is the highest possible level for HABs that can be detected in a body of water. The south fork of the Merced River where the family was found is listed as “Caution” which is the third-highest level of HABs.

While the family’s toxicology report is pending, the State Water Board released a statement regarding their death.

“Representatives from the Central Valley Water Board, the county health department, and the sheriff’s department collected water samples from multiple sites along the Merced River on Thursday to test for the presence of HABs. The State Water Board expects to have findings from these tests within two weeks. It is important to note that lethal impacts to humans from cyanotoxins or cyanobacteria found in freshwater HABs are extremely rare.”

Statement from the State Water Board

According to the Centers for Disease Control and Prevention, harmful algal blooms can sicken people or animals if they:

  • swim, wade, or play in or near contaminated water
  • eat contaminated fish, shellfish or
  • use contaminated drinking water

Illnesses and symptoms can vary depending on how a person or animal was exposed or came into contact with algae, cyanobacteria, or their toxins. Additional variables include how long a person or pet was exposed, which type of toxin was present, and how much toxin was present.

However, according to the CDC, direct exposure rarely leads to a fatality.