Hundreds of dead fish are turning up at a lake in New Dundee, but no one seems to know why.

Residents began reporting the fish earlier this month as carp by the dozens were seen floating belly-up in Alder Lake.

"Up until two weeks ago, it was great," says Joan Jackson. Since, her backyard has been full of the dead creatures.

The result is both alarming and stinky.

Jackson says in her decades living nearby, she's never seen anything like it.

"I've lived here since 1963 in this house, and I lived a few years across the road and I've never, ever seen the fish die," she says.

The Grand River Conservation Authority estimates that several hundred of the bottom-feeding carp have died, but there's no sign of any spill or algae blooms.

Meanwhile, late last month, hundreds of dead fish were found floating in a river in the town of Clarence-Rockland, east of Ottawa. That situation, too, remains a mystery.

Clarence-Rockland announced that testing had not shown any adverse effects on the local drinking water supply, but that the water's temperature has changed.

A couple of weeks later, seagulls began turning up dead along the same stretch of river.

Amir Wafir used to fish in Alder Lake until a couple of years ago. He says the site and smell are sad.

"The water smell, I don't know. It's very bad," he says. "Doesn't look clean, something's wrong."

The Ministry of Environment was at the lake on Monday to take water samples, but no results have been made public yet.

Neighbours suggest it be anything from climate change to nearby polluters. Others say the silt in the lake runs between 2.5 and five feet deep, and that maybe its condition could be related to the fish die-off.

As for Joan Jackson?

"I don't know. It's a mystery," she says.

A mystery she hopes will be solved soon.