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Fish Mortality

Lake County, Calif. (May 10, 2017) - What might have caused the recent appearance of dead fish along the shore of Clear Lake?

The County Water Resources Department has received several reports of dead fish washing up on the shores around Clear Lake. Upon investigation, Phil Moy, PhD, Director of Water Resources has determined that it appears the fish likely succumbed to a rapid temperature change in shallow water during spawning. The fish he observed were predominantly adult crappies, a few bass and several large carp. These are all fish that spawn in the spring in shallow water. Moy states that “Until recently, we were enjoying a cool spring then suddenly had a spate of rather hot weather. The rapid temperature change in nearshore waters where these fish spawn, in addition to the normal stresses of spawning, likely caused the mortalities.”

On May 10th, Moy observed dead crappie and carp on the north shore of the lake and an accumulation of dead crappie, carp and few bass at Austin Park in Clearlake. Moy said, “It is unlikely the fish all died in the waters off Austin Park. The fish were predominantly accumulated on north-facing shorelines with some adjacent to the water and some father up on the shore, suggesting a strong north wind and waves carried them from other parts of the lake.” The fish were in various states of decomposition suggesting that the cause of the mortalities was more chronic than acute – that is, taking place over a longer time frame than a single rapid event.

The distribution of dead fish all of similar species distributed around the lake suggests the die-off was not a singular event involving a single location, but rather a lake-wide phenomenon which, in consideration of the rapid cool to hot weather changes further suggests thermal environmental stress rather than a toxic algal bloom.

For more information you may contact Phil Moy at (707) 263-2344 or phil.moy@lakecountyca.gov.