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Almost two weeks after thousands of carp began turning up dead in Lake Mendocino, the cause of their demise continues to elude lake officials.

The initial tests for koi herpes virus were inconclusive and other test results aren't expected for another few weeks, said J.D. Hardesty, chief of public affairs for the San Francisco office of the U.S. Army Corps of Engineers, which operates Lake Mendocino.

"We don't have a definitive answer. It could take up to 30 days" from when the fish samples were collected before other results are available, Hardesty said Thursday.

An estimated 5,000 to 10,000 carp began washing up on the shores of Lake Mendocino over the Memorial Day weekend.

State Fish and Wildlife officials believe they're common carp, a bottom feeding fish that likely was introduced by people dumping pet fish into the lake.

Fish and Wildlife biologists are not concerned about the die-off if that's the only fish in the lake that is affected, said state Fish and Wildlife spokesman Andrew Hughan.

"They're an invasive species" that competes with native fish for food, he said.

Officials suspect the koi herpes virus, but that diagnosis may never be confirmed, Hardesty said. He noted that a number of carp die-offs elsewhere in the country are believed to have been caused by the virus but tests have often been inconclusive.

Koi are a type of domesticated carp. The koi herpes virus affects only those fish and their relative, the goldfish. It is not believed to be harmful to humans or other animals. Virus infections may be triggered by people releasing their unwanted koi or goldfish into lakes, officials have said.

Until more is known about the cause of the fish deaths, corps officials are warning visitors to the lake not to swim or fish near the carcasses and to keep pets away from the dead fish, officials said.

The corps has largely removed the fish from recreational areas and is now trying to retrieve those in less accessible sections of the lake, Hardesty said.

While dead fish can provide nutrients to the lake, too many can cause excessive algae growth, he said. Hardesty said the cleanup is moving slowly due to staffing shortages.

The koi herpes virus was first detected in the United States in 1998 and outbreaks have since been identified worldwide, according to the UC Davis Veterinary Medical Teaching Hospital's website.

The disease is suspected of killing thousands of carp in Lake Ming near Bakersfield in 2010, Lake Kaweah in Tulare County in 2009, and Clear Lake in 2008, according to news reports. The Clear Lake case was among those confirmed to have been caused by the koi herpes virus, Hardesty said.

This year, an estimated 500,000 Asian carp died in the Cumberland River below Lake Barkley dam near Nashville.

Asian carp also are an invasive species and efforts have been made to keep them out of waterways, Hardesty noted. Silver carp are a jumping species that has invaded waters of the southern states. They can grow up to 100 pounds and — when disturbed by motors — have been known to leap onto boats and injure boaters.

The common carp is a smaller bottom feeder that is now prevalent in warmer California lakes.

Carp first appeared in Lake Mendocino in the late 1990s or early 2000s, Hardesty said.

(You can reach Staff Writer Glenda Anderson at 462-6473 or glenda.anderson@pressdemocrat.com)