Scientists seek clues in Clear Lake fish die-off

Hundreds of fish have turned up dead in Clear Lake, a periodic occurrence likely triggered by oxygen-depleting heat and algal blooms.

State Fish and Wildlife officials have taken samples to determine the cause, which could also be disease, said Carolyn Ruttan, invasive species coordinator with the Lake County Water Resources Department.

The fish, including bass, crappie and bluegill, were first noted floating on the lake’s surface on Sunday. The die-off followed a week of above 100-degree temperatures that also fueled a large algal bloom. Both warm water and the abundance of cyanobacteria likely depleted oxygen levels in parts of the lake, killing the fish, Ruttan said. While algal blooms produce oxygen, they can also outcompete fish for oxygen when blooms are thick, she said.

Decomposing algal blooms further deplete oxygen levels, Ruttan said, and warmer water generally has lower levels of dissolved oxygen.

Making matters worse, strong winds have been pushing warm water deep into the lake, where fish normally seek refuge from the heat and low-oxygen conditions, she said.

“Now where are the fish going to go?” Ruttan asked.

While the situation looks worrisome, it’s probably not a serious problem, she said. It may even be a good thing for the fish, killing off mostly older fish that may be weak or ill, Ruttan said.

“Maybe we need to refresh the population anyway,” she said.