Tens of thousands of rotting fish are lining a 40-kilometre stretch of shoreline along Lake Erie, reports the provincial environment ministry, which is investigating the cause.

A spokesperson for the Ontario Ministry of Environment said Tuesday the kill was reported on the weekend. So far it appears the fish may have died from the affects of a naturally occurring lake inversion rather than a spill, but cautioned the investigation is continuing.

The question now is which agency is responsible for cleaning up the rotting carcasses of thousands of yellow perch, carp, sheepshead, catfish, big head buffalo and suckers, which kept untold beachgoers from enjoying their Labour Day weekend.

“It (the water) was quite putrid really … I had never experienced anything like this,” said Neville Knowles, of London, Ont. and cottager at Rondeau Provincial Park for more than 50 years.

The dead fish stretch from west of the fishing village of Port Stanley in Elgin County to the village of Morpeth in Chatham-Kent or just east of Rondeau.

“There was a significant number of fish, tens of thousands,” the environment ministry’s Kate Jordan told the Star.

Jordan said the ministry officials took fish and water samples for analysis, “but all observations made at the site … did not show anything unusual and we did not see any evidence of … a spill to the lake or man-made pollution … so we are considering natural causes, including a lake inversion.”

She explained that an inversion happens when the surface water cools down dramatically, sinks and displaces the bottom layer, which has lower oxygen content. As the bottom layer is displaced, it rises and robs fish of oxygen needed to survive. The phenomenon is also referred to as the lake “rolling over.”

Even so, some residents are suspicious just the same that run-off from a large pig operation along the stretch may have caused the fish to die, said Knowles, who quickly added there is nothing to support that position.

“At this time of the years it is common to get lake turnover or lake inversion and you usually do get a few fish killed … but this smell smelled like a sewer … and on top of the water there was a brown kind of milky film that was at the water’s edge,” said Knowles, who first noticed the smell Friday.

He said after he took a dip Friday afternoon he quickly took a shower to wash to smell away.

The bulk of the fish washed up on shore east of the Rondeau. In some spots, the fish were piled on top of each other.

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Jordan said it has not yet been decided which agency, federal, provincial or local agency will have to foot the cleanup bill.

“We are having discussions with Environment Canada, the health unit and natural resources about that now,” she said.