Dead and dying fish of all species continue to show up along the shoreline at Clear Lake. One of the worst areas is from Glenhaven to Clearlake Oaks. Actually, this doesn’t come as a surprise. The searing daytime temperatures, which have been in the 100-degree-plus range the past three weeks, have created massive blue-green algae blooms. This is especially true in the coves and bays in the Clearlake Oaks arm.

The blue-green algae blooms actually create dissolved oxygen (DO) during the daylight hours, but at night the algae dies and uses up more DO than it creates during the daytime. For example, the DO may rise to 8 or more during the daylight hours but then drop to as low as 1 at night in the same cove.

Oxygen is a vital component of life, be it human, animal or fish. Animals receive oxygen through their lungs. However, fish receive their oxygen through their gills. The gills are actually made up of a series of small membranes that allow blood to flow through them. To breathe, the fish opens its mouth and takes in water. The water passes out through the gills and contains oxygen molecules. It’s called dissolved oxygen or DO. These membranes extract the oxygen from the water and transfer it to the blood, the same as lungs do for mammals. DO consists of microscopic gaseous bubbles suspended in water. This is the actual oxygen a fish uses to breathe.

Dissolved oxygen is formed by wind and wave action that stirs up the water and allows oxygen to mix with the water. Aquatic plants, such as algae and plants, also furnish oxygen through a photosynthesis process.

DO is rated in milligrams per liter. The higher the number the more oxygen is in the water. Trout and other cold-water fish require a DO reading of at least 8. Bass need no less than 5 to be comfortable. Late in the summer in some of the channels on Clear Lake the DO readings can be as low as 2 or 3. When DO levels drop, the fish respond by either moving out into water with higher DO levels or sometimes they just become lethargic and don’t feed or do very little swimming. They will often die, especially the smaller ones.

A number of factors affect the amount of DO in a body of water at any given time. DO levels are lowest at night because the plants don’t grow in the dark and don’t produce oxygen through photosynthesis. In fact, at night these plants actually use up the oxygen. Water temperature also plays a role. When the water is cold it holds more DO than when it’s warm. Recently water temperatures have been as high was 88 degrees by mid-afternoon. This hot water tends to deplete the DO.

During the late summer and fall months, the decaying plants also use up a lot of oxygen. As the plants die and decompose, the water often turns a dark brown or even black color. Most experienced fishermen look for water that’s a green color because they know it has a higher oxygen level.

There is no question that DO readings are important to fishermen. If you know the DO in an area, you would know if fish are there. If you were fishing in one of the bays and the DO was low, chances are the bass have temporarily moved out of the area and you would be wasting your time. A few fishermen even carry DO meters that measure the oxygen levels in the water. A DO meter can be purchased online for less than $200.

The optimum DO level for bass is 7-9, but in Clear Lake it’s rare to find a DO level higher than 8 during the summer and fall months. In some areas, such as coves or bays, the DO has been registered as low as 1-3. A bass can only live a few minutes when the DO is that low.

DO levels also vary from the bottom to the top of the lake. Normally DO levels are higher near the surface because of wave action stirring up the water and creating oxygen, much as the aerator does in your aquarium at home.

With the hot month of August now here, we can expect more fish die-offs. By October most of the algae and aquatic weeds should be gone and the water will begin to clear up. By then the DO readings in the lake should be back up to 7 or even 9.

Even if thousands of bass and other fish die from a lack of oxygen there are enough fish to quickly repopulate the lake. Clear Lake has one of the highest densities of fish per acre of any lake in the nation.