Thousands of dead fish have been washing up on Lake Erie shores recently, but scientists suspect it may be from natural causes.

From this story in the Christian Science Monitor:
Canadian officials say tens of thousands of dead fish are littering a section of Lake Erie’s northern shoreline.

Ontario Ministry of the Environment spokeswoman Kate Jordan said Wednesday that the fish washed ashore last weekend. She says they likely died of natural causes, although officials are still awaiting lab analysis results. The fish were scattered along a 25-mile stretch between the cities of Windsor and London._
Jordan says investigators found no evidence of a spill or pollution, and water chemistry measurements turned up nothing unusual.

She says a likely explanation for the fish deaths was a so-called “inversion.” That happens when temperature changes send water with low oxygen levels from the bottom to the surface, causing fish to suffocate._