Ontario Ministry of Environment

These fish are among the tens of thousands found dead on 25 miles of Lake Erie beaches in Canada's Ontario Province.

Tens of thousands of dead fish that washed up on Lake Erie beaches in Ontario, Canada -- and had locals wondering if something or someone had poisoned the water -- were likely killed by a lack of oxygen caused when lake sediment was stirred up, the province reported Friday.

Water samples "do not show evidence of a manure spill or anything unusual in terms of contaminants," Ministry of Environment spokeswoman Kate Jordan told NBC News.

Jordan said it wasn't known if the die-off was unprecedented, but that "it was a significant number -- tens of thousands."

The fish were found along 25 miles of beach, with locals first coming across them on Monday.

But three days earlier, residents had complained of a manure-like smell from the water, the Chatham Daily News reported.

"It was rank, so profoundly rank, that it was difficult to stay down there and the next morning we woke up to the smell," Neville Knowles said of his family's weekend trip to Rondeau Provincial Park.

Another park visitor, Frank van den Boorn, said he and his family were at the beach when he noticed the darkened water and smelled something wrong.

"I said to the kids 'We've got to get out of here, there is something wrong with the water'," Van den Boorn recalled. "I scooped up a handful of water and ... you could still smell the body stench on it."

"I just couldn't believe people were letting their kids swim in it," he added.

Jordan said the smell and darkened water were consistent with the natural phenomenon known as "lake inversion" -- where a change in wind can kick up waves that stir up sediment and reduce the oxygen levels for fish.

The wind did change directions last week, she noted, and a local water temperature gauge showed colder water, suggesting it had been churned up from the depths.

The province is also testing some of the dead fish and those results should provide conclusive evidence, Jordan said. The results should be ready next week.

The dead fish included catfish, carp and perch.

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