Tens of thousands of fish that washed up on the north shore of Lake Erie over the Labour Day weekend died of natural causes, according to the Ministry of the Environment.

Water tests confirmed that the fish likely died due to causes related to a lack of oxygen in the water. The Environment Ministry says a naturally-occurring temperature inversion may have brought an oxygen-depleted layer of water from the bottom of the lake to the surface, leading to the widespread deaths.

This phenomenon, sometimes called upwelling, involves the nutrient-depleted waters at the bottom being forced to the top, replacing the warm and nutrient-dense water that often provides for fish.

A ministry spokesperson told the Toronto Star that tests did not find any evidence of manure spills or contaminants, while Chatham-Kent Medical Officer of Health Dr. David Colby said acidity tests came back negative. He added that the deaths appear to be consistent with upwelling.

A limited cleanup is underway, but rotting fish carcasses along with some dead birds continue to line the shore of Lake Erie along a nearly 40-kilometre stretch.

The beach at Rondeau Provincial Park is being cleaned but the ministry could not confirm when, or if, the province planned to proceed with the remainder of the cleanup.