Tens of thousands of dead fish are turning up along the northern shore of Lake Erie.
Ontario Ministry of the Environment spokeswoman Kate Jordan said the fish washed ashore last weekend. She says they likely died of natural causes, although officials are still awaiting lab analysis results.
Jordan says investigators found no evidence of a spill or pollution and water chemistry measurements turned up nothing unusual.
She says a possible explanation for the dead fish was a so-called "inversion." Inversion occurs when temperature changes send water with low oxygen levels from the bottom to the surface, causing fish to suffocate.
The fish were scattered along a 25-mile stretch between the cities of Windsor and London in Canada.
Among the dead fish were perch, bighead buffalo, sheephead and catfish.
Lake Erie is the shallowest of the Great Lakes and is especially vulnerable to fluctuating water levels.