TORONTO - Thousands of dead carp washing ashore in Ontario cottage country is cause for alarm, given the hearty fish are usually resistant to pollution and drastic temperature changes, Ontario Environmental Commissioner Gord Miller said Wednesday.

Although tests have found a bacteria called columnaris in the dead fish, biologists say it may not be the root cause for the massive die-off. They say the fish may be weakened by another environmental stress, leaving the carp more vulnerable to the bacteria, which thrive when the water warms up.

If that's the case, Miller said it's worrisome because carp often survive difficult conditions that prove fatal for their fellow fish.

"They can live in highly polluted water," Miller said in an interview. "They are some of the toughest fish . . . So when you see carp die off in large numbers, my antenna goes up. That's cause for alarm."

Thousands of dead carp have been washing up on the shores of the Kawartha Lakes region at the height of cottage season, prompting local municipalities to set up special curbside fish pickups.

Although it's the first time Kawartha Lakes has seen this kind of death toll, columnaris was held responsible for the death of catfish, bass and some turtles along the Ottawa River last summer.

Scott Watson, provincial fish culture policy and program co-ordinator with the ministry of natural resources in Peterborough, Ont., said columnaris is a relatively common bacteria which thrives when the water reaches a temperature of between 13 and 18 degrees Celsius.

But he said columnaris alone isn't usually fatal.

"It's not typically a problem unless fish become stressed out and susceptible to it," Watson said. "Usually, something else is happening which has made the fish susceptible and the pathogen is just being opportunistic."

But Watson said it may be weeks and even months before scientists can figure out what's killing the robust fish, let alone guess how long the carp will continue to die.

Whatever is proving fatal for the carp may also be affecting other species but those effects might be overshadowed by the large carp washing up on shore, he said.

Although the dead fish aren't pleasant for cottage-goers at the height of the summer season, Watson said the bacteria linked to their death isn't a threat to human health.

"It can be very disturbing to people," he said. "Carp are large-bodied fish. When they die and roll up on shore in warm weather, it gets to be a bit of a stinky mess."

That stinky mess is quickly becoming the talk of Riverview Marina on Pigeon Lake. Graham Smith, a 19-year-old employee at the marina on Scugog Lake, said people are asking a lot of questions and rumours are rampant about what's causing the carp massacre.

Some say the bottom-feeding carp must have been killed by something dumped in the lake water while others speculate the fish have been deprived of oxygen, he said.

"All the other fish are still alive," Smith said. "I don't know what to believe."

All he knows about the nine carp, each about a third of a metre long, that washed up on shore Tuesday is that they "stink really bad" when they bake in the hot July sunshine. They also don't make the water that inviting, even on a hot humid day, he said.

"I personally wouldn't go swimming right now," Smith said.

New Democrat Peter Tabuns said such disturbing phenomena is becoming more and more common as climate change sets in. The former head of Greenpeace said Ontario is going to see new diseases surface as the world's temperature increases and different bacteria start to thrive.

"It's pretty disturbing," Tabuns said.

"There is a real need for us to start thinking about how we're going to adapt to these hotter temperatures and what we're going to do to protect our wild spaces and our wildlife."