Preliminary tests suggest a naturally occurring bacteria may have killed hundreds of fish in the Ottawa River, but officials say people are not at risk.

One catfish tested positive for a bacteria known as columnaris, which is found in warm water or water with lower oxygen levels.

Doug Skeggs, an official with the Ministry of Natural Resources, said the bacteria poses no threat to humans, but to catfish it can be deadly.

"Please understand that this is preliminary. This is one sample from one fish," he noted.

"It is certainly causing us to focus our efforts that this is a natural event caused by a natural fish disease."

Since last week, dead catfish, bass and even some turtles have washed up on the shore of the Ottawa River along a 60-kilometre stretch west from Pembroke to Constance Bay.

As a precaution, Renfrew County health officials are still advising people to stay out of the river.

Officials say the number of fish washing up on the shore appears to be going down.

The investigation is ongoing. More test results are expected to be available by Friday.