An estimated 100,000 trout, bass and catfish have died over the past month in the Sparks Marina along U.S. Interstate 80 east of Reno, apparently the result of a dramatic, unexplained drop in dissolved oxygen levels, Nevada Department of Wildlife spokesman Chris Healy said Wednesday.
Scientists say a bitter cold snap could have caused oxygen-poor waters to rise from the old rock quarry's bottom to the surface, but they don't understand what sparked the massive die-off.
Fish biologists confirmed low oxygen levels caused the death of an estimated 3,000 fish in one corner of the lake in mid-December, but Healy said they thought at the time that the event was localized and of limited impact. Since then, they've been unable to detect any live fish in the 77-acre lake. Numerous dead fish have been removed from the lake's shoreline, and Healy said it's likely the rest sank to the bottom.
"The 100,000 dead fish figure is something that is probably a pretty conservative guess," said Healy, who estimates they've stocked close to 1 million adult fish in the lake since they started in 1998.
"We don't know if any small fish have survived, but for all intents and purposes, the fishery doesn't exist anymore," he told The Associated Press.
The Reno Gazette-Journal first reported scientists determined the problem was much more serious than they realized after a boat survey on Monday found dissolved oxygen levels far too low to support the fish at 11 different sampling locations. Readings from an electronic fish-finder also revealed no fish swimming in the lake's depths.
Lakes like the marina consist of different layers of water temperatures, with the warmest water on top holding the highest oxygen content, Healy said. He said one theory is that the surface water may have chilled very quickly, sank toward the bottom of the lake and stirred up material on its floor, causing a "violent turnover" that could have sucked up additional oxygen.
"Everything is a theory right now," Healy said.
Sparks city spokesman Adam Mayberry emphasized there's no health or safety threat at the marina. He said the water typically is of good quality and no similar problems have occurred before.
"Even with the biological anomaly we are seeing, it's still a very safe body of water," he said. "You just can't fish in the marina right now because there aren't any fish there, and we are trying to figure out why."
The Sparks Marina opened in 2000, with a 2-mile walking and bike trail, beaches, playgrounds, picnic areas and a fishing pier. The former aggregate pit operated by Helms Construction Co. had been found to be contaminated in 1988 by pollutants leaking from an adjacent tank farm, but state environmental officials said all the pollution had been cleaned up before a 1997 New Year's Day flood sent Truckee River waters into the pit.
Michael Drinkwater, manager of the Truckee Meadows Wastewater Reclamation Facility which collects water from the lake, is awaiting results of new toxicity tests conducted last week but said routine testing has revealed no problems before.
"There's no obvious indication hydrocarbon pollution could be associated with the die-off," he told the Gazette-Journal.
Healy said testing earlier this week found dissolved oxygen levels in the range of 1.1 to 1.9 parts per million. Fish do best with levels in the range of 7 to 9 parts per million and typically can't survive when it drops below 5 parts per million, he said.
The department annually stocks the marina in late February or early March 1, but he said they won't be doing that this year unless the dissolved oxygen level "makes a big recovery."