Tens of thousands of dead fish have washed up on a 25-mile stretch of Lake Erie's northern shore, and Ontario environmental officials say they could be victims of a natural phenomenon called a lake inversion.

The inversion brings cold water, which has lower oxygen levels, to the lake's surface and fish suffocate.

"Essentially it's a rolling over of the lake," Ontario Ministry of the Environment spokeswoman Kate Jordan told The Chatham Daily News. "Something - whether it be a storm, or cooler temperatures at night, or strong winds - triggers a temperature change in the lake."

Jordan said it was windy and choppy on the lake Friday night, according to a report in The Windsor Star. The fish kill was reported Saturday.

The U.S. Environmental Protection Agency said the central basin of Lake Erie, between Cleveland on the south and Chatham, Ontario, on the north is particularly susceptible to oxygen deprivation, with the danger peaking in late August and mid-September.

Others suspect a sewage spill may have something to do with the fish kill.

David Colby, chief medical officer of the Chatham-Kent district where dead fish litter the beaches, told The Windsor Star that residents reported a strong sewage smell the night before the fish washed ashore.

“All kinds of people were woken out of a sound sleep by a stench and it was like a septic tank was backing up,” The Windsor Star quoted Colby as saying.

But Jordan said tests of lake water taken Saturday showed no signs of what might have killed the fish. The water was tested for oxygen, PH levels, conductivity and temperature, she said.

"The ministry did not observe any evidence of a spill or pollution and water quality measurements done did not show anything unusual," Jordan told CNN.

The investigation was continuing, she said.

The dead fish included carp, sheepshead, perch, catfish and suckers, the Daily News reported, and Colby said most were of good size.

"I haven't seen anything like this in quite some time," the Daily News quoted him as saying. "The interesting thing is that most of the fish are sizable. There are very few little ones."

Jordan told the Toronto Star the cleanup of the fish has yet to begin.

“We are having discussions with Environment Canada, the health unit and natural resources about that now,” the Toronto Star quoted her as saying.

Meanwhile, residents said the smell of rotting fish is overpowering.

"I had family here (on Monday) and I didn't allow them to take the dog or the children down to the beach," Chatham-Kent resident Patricia Pook told CNN affiliate CBC News. "I knew it was bad, but the smell is just overwhelming. It would make you sick to your stomach."