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Fish die-offs are not uncommon in Clear Lake. Sometimes fish suffocate when oxygen-depleting algal blooms explode. Other times, koi herpes virus attacks carp, causing their carcasses to litter the shoreline.

But two early October incidents, about 3 miles apart at the south end of the lake, are believed to have been caused by a less natural killer, capturing the attention of state Fish and Wildlife officials.

“It’s under investigation,” said Fish and Wildlife spokesman Steve Gonzalez. He did not divulge any other information.

Environmental scientists working for area tribes suspect a chemical spill, possibly petroleum based, killed the fish, estimated in the hundreds. Witnesses reported a chemical odor and oily sheen on the water, said Sarah Ryan, environmental director for the Big Valley Rancheria. Clear Lake tribes’ environmental agencies work closely with state and local government agencies in monitoring the health of the lake, she said.

“We sent (water samples) to a local lab for analysis” of petroleum components, she said. “We’re thinking it’s some sort of chemical spill.”

The results of the tests are expected later this week, she said.

Carcasses of some of the dead fish were sent to Fish and Wildlife officials, who are conducting their own analysis, Ryan said. Two otters reportedly also were found dead in the area but the person who discovered them disposed of the carcasses, so they have not been verified or examined, Ryan said.

Besides the oily sheen and odor, there are a number of other reasons to suspect a toxic spill or release into the lake.

Ryan said her counterpart who works for the Elem Pomo tribe conducted tests at one location on Oct. 2, the day the dead fish were reported. She found that oxygen levels were more than adequate to sustain fish.

“The oxygen level was fine,” Ryan said.

There also were many different species and ages of fish killed. Normally, larger fish simply swim away from oxygen-depleted or fouled areas, so mostly smaller, shore-hugging fish are found dead.

That wasn’t the case with the recent incident.

“Something overwhelmed them very quickly,” said Greg Giusti, a UC Extension ecologist who has studied the lake for 20 years. He said he has received multiple phone calls about the event.

“I’m of the opinion it was some kind of pollutant,” Giusti said.

Lake County resident Terry Knight, an environmental and outdoor writer who has been keeping close tabs on Clear Lake for 28 years, said he has never seen an event that affected so many species in such a short time. There have been no additional reports of dead fish since then, he said.

“It was not a normal die-off,” he said.

Fish and Wildlife officials suspect something was dumped into storm drains, which empty into the lake, Giusti said.

Ryan said it could be a fuel spill, possibly from a fueling dock, or chemicals dumped into storm drains. Some people are careless about fueling their boats, allowing gasoline to spill into the water, she said. Others seem unaware that anything dumped into a storm drain will end up in the lake.

Everyone involved in the case has their suspicions of what caused the fish to die, but no one will know for sure unless tests produce revelations.

Meanwhile, “it’s a big mystery,” Giusti said.