Ontario environmental officials are blaming a recent mass fish kill in Lake Erie on oxygen deprivation due to a temperature inversion that caused carcasses to wash up on a 20-mile stretch of the lake’s coast in early September.

Meanwhile, jurisdiction on cleanup was in question as the rotting fish—carp, sheepshead, perch, catfish, suckers, big head buffalo and others—lay on the lake’s beaches and stunk up the region for more than a week.

Initially, testing of the waters had yielded no clues, the Huffington Post reported on September 6. No altered pH or oxygen levels, spills or pollution that could cause such an event. Test results on the fish themselves were still pending. Seagulls feasting on the remains died as well, some residents told the Toronto Star.

The washup was dramatic and quick.

“All kinds of people were woken out of a sound sleep by a stench, and it was like a septic tank was backing up,” said Dr. David Colby, the Chatham-Kent medical health officer, to The Windsor Star, describing the sudden appearance of the fish. He added that he had never smelled or seen anything like it.

Colby at first suspected the deaths could have been caused by viral hemorrhagic septicemia and Type E botulism, the Huffington Post said, but test results ended up confirming, as the Toronto Star put it, that “a naturally occurring temperature inversion brought an oxygen-depleted layer of water from the bottom of the lake close to the surface, causing the widespread kill.”