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Second Fish Kill

Lake County, Calif. (August 9, 2017) - Sadly, Clear Lake is suffering the effects of a second fish kill, this time off the shores of Lakeport. This is most likely the consequence of nutrients, cyanobacteria and warm water temperatures adversely affecting water quality, primarily dissolved oxygen. Just like people, fish rely on oxygen to breathe and process nutrients in their bodies. Warm water fish such as those living in Clear Lake need to have about 5 parts per million (ppm) dissolved oxygen to thrive.  

Like many other lakes in California, nationwide and around the world, Clear Lake has this summer, been host to a large, dense, persistent cyanobacteria bloom (sometimes called blue-green algae). The bloom has been lake-wide with various intensities due to wind, water depth and nutrient cycling (phosphorus). These dense blooms produce ample oxygen during the day, but consume oxygen during the dark hours at night.

The very warm water conditions also contribute to the problem because warm water holds less dissolved gases (including oxygen) than cold water but warm water also increases the fish’s metabolic rate. So the fish need more oxygen to digest and thrive, but the water cannot hold as much oxygen as it can in the spring, winter and fall months.

At the current water temperatures of Clear Lake (80-84oF), the dissolved oxygen saturation point is about 7.9 ppm. Recent monitoring by the California Department of Food and Agriculture has shown the dissolved oxygen concentration off the shores of Lakeport during the day have been as high as 30 ppm or more. This is a situation of oxygen supersaturation.  At night, when the bacteria begin to respire or use oxygen, the dissolved oxygen concentrations plummet, bottoming out at 1 ppm or less. In the same area off Lakeport, the dissolved oxygen concentration dropped to near zero 17 nights between July 5th and August 1st.

These factors have come together in a perfect storm of sorts on Clear Lake – very warm water, dense cyanobacteria blooms, plenty of nutrients for the bacteria in the form of phosphorus from the landscape, wildly vacillating dissolved oxygen concentrations and fish that are already under stress in the warm water. Unfortunately, at this point there is little that can be done quickly to address the cyanobacteria, the phosphorus or the warm water; we simply must let nature take its course.

We must also keep in mind that the cyanobacteria blooms may at times, produce toxins that can adversely affect pets and people. For this reason caution should be exercised when considering contact with the water. Avoid heavy algal scums in the water. Pets should not be allowed to swim in the lake as they may ingest the algae or scum. Caution should be exercised when young children interact with the water because they too may ingest the water and be more prone to adverse health effects due to their smaller body size. Rinse off pets and people thoroughly with clean water after swimming and do not use raw lake water for cooking – boiling does not remove the toxins. Before eating fish from the lake, throw away the guts and thoroughly rinse the filets with tap or bottled water.

In the long term we can all be more careful about fertilizer use, sediment runoff, and off highway vehicle use and the County and cities will work to better control sediment and its associated nutrient inputs off the landscape. By working together we can in the long run, have a significant impact on improving the water quality of the lake, the reduction of cyanobacteria blooms and the improvement of the health of our fish.

For more information contact Phil Moy, Director of Water Resources at (707) 263-2344 or