The city is testing the water in Red Hill Creek after reports of hundreds of dead fish in the creek in Hamilton's east end.

The city and the Ministry of Environment investigated after a nearby resident reported hundreds of dead fish in the creek Tuesday. It is now assessing what could have killed the scores of fish, which included white suckers, creek chubs, crayfish and minnows near the overpass of King Street East.

It was likely the sudden heat that killed the fish, depriving them of oxygen in the water, said Jennifer Hall, spokesperson for the regional office of the Ministry of the Environment.

"On days like yesterday and today, where there are rapid temperature increases and low shallow water, the temperature spike shocks the fish," she said.

"This is a natural phenomenon."

The fish were found in a shallow area of the creek, which meant they couldn't dive deeper for cooler temperatures, she said.

There was no odour or evidence of a spill. There was no sheen on the water, and areas where water flows into the creek were dry.

But the city's environmental monitoring and enforcement team is testing water samples to make sure, said Kelly Anderson, public works spokesperson.

The water temperature where the fish were found was 29 C, she said.

Lisa Jennings, aquatic ecologist with the Hamilton Conservation Authority, said large numbers of dead fish are not a common sight in local waters.