May 27, 2008 — Fish may not be as plentiful at Walker Lake near Westwood as in years past, according to the California Department of Fish & Game. About 300-plus fish were killed in early April due to a depletion of oxygen in the lake, said Lieutenant Lisa Stone, who covers Lassen, Plumas and Butte counties for the Department of Fish and Game. The oxygen depletion occurs in shallow lakes when very cold temperatures cause the water to freeze and then warmer temperatures permit thawing. This flip/flop in temperatures results in a process of thawing and refreezing over a period of time, explained Paul Divine, a fisheries biologist for Lassen and Modoc Counties. When the lake begins to thaw, aquatic plants, algae and plankton start to grow and produce oxygen, but when ice covers the water, blocking sunlight, the plants consume oxygen.

“They deplete the lake of oxygen and that is when you see a wintertime die off of fish. It is usually in lakes and reservoirs that have a large amount of shallow water because they tend to freeze and thaw faster and refreeze faster,” said Divine.

Stone said this type of fish kill is a totally natural phenomenon. To be sure the fish were not diseased and there were no toxins in the lake, fish and water samples were sent to a lab in Rancho Cordova. There was nothing unusual about the water samples or fish tissue. Scavengers that included bald eagles, osprey, ravens and magpies ate the dead fish, which is the natural way such occurrences are cleanedup.

A dissolved oxygen problem has not occurred in Walker Lake in a very long time, probably 9-10 years, said Stone. It does occur regularly in McCoy Flat Reservoir off Highway 44.

“When it warms up, the fish want to get active and if there is ice on the lake you can have a dissolved-oxygen problem. They are not getting enough oxygen to breath,” said Stone.

Fish absorb oxygen gas that is dissolved in water through their gills. The main source of oxygen in water is through plant photosynthesis.

According to Divine, he had other reports this year of fish found dead in shallow Lassen County lakes. One report involved a lake in the Northeastern corner of the county, Dodge Reservoir. He said it most likely occurred in many places due to the prolonged cold spell this spring, mixed with a few warm days in between freezing temperatures.