By Laura Zuckerman

(Reuters) - Biologists scrambled on Thursday to unravel the mystery behind a massive fish kill in a popular manmade lake in Nevada that draws thousands of anglers each winter for prized game fish such as trout.

More than 100,000 stocked fish such as bass and catfish are estimated to have perished in the lake in the northern city of Sparks in recent weeks in an unprecedented die-off that has destroyed the entire fishery, said Chris Healy, spokesman for the state Department of Wildlife.

Testing of water at the Sparks Marina, a city recreation complex whose chief attraction is a 77-acre (31-hectare) reservoir, has revealed that it is nearly devoid of oxygen necessary to support aquatic creatures such as fish, he said.

"We're seeing oxygen levels that are totally lethal to fish," Healy said.

The depleted oxygen levels pose no threat to human or pet health, and sampling of the water shows no evidence of pollution or contaminants, said Adam Mayberry, spokesman for the city of Sparks.

Thousands of dead fish began washing ashore in December in a precursor to what has become a catastrophic fish kill in a reservoir that opened to the public in 2000 after being commercially mined for sand and gravel, officials said.

State biologists believe the steep drop in oxygen may be tied to a recent cold snap that may have triggered a rapid cooling of the surface water, increasing its density and forcing it to sink even as deep water that is depleted of oxygen rises to upper levels where fish swim, Healy said.

Fish gain oxygen from water cycling across their gills and suffocate when oxygen concentrations dip below a certain level. Many types of fish struggle or die when oxygen is below 5 parts per million. Sampling of water across the lake has shown so-called dissolved oxygen as low as 1.1 parts per million, Healy said.

Mayberry said city officials were determined to get to the bottom of the mystery.

"We are monitoring the situation very closely. In the weeks to come, we will be seeking all the answers we need to hopefully prevent this from happening again," he said.

Healy said the state does not intend to stock fish in the reservoir until oxygen levels rise.

(Reporting by Laura Zuckerman in Salmon, Idaho; Editing by Cynthia Johnston and Lisa Shumaker)