The death of hundreds of fish in a Greater Manchester river is being investigated by the Environment Agency.
Officers were called to the River Tame in Mossley on Saturday after a member of the public saw about 250 dead and distressed fish in the water.
Simon Oldfield from the agency said the trout, pike and chubb died when a pollutant went into the water.
He said the pollution was short-lived and was confident that the wildlife in the river would recover.
Water samples have been taken from the river and the results will be analysed later this week.
"Incidents of this nature are usually the result of a spillage or a business using the wrong drain," he said.
"It is likely that a pollutant has entered the river which has been toxic to the fish."
Mr Oldfield added that although the incident came to light on Saturday afternoon, the pollution may have already been present in the drainage system for some time.
The Environment Agency has appealed to members of the public who saw anything suspicious in the area on Saturday to contact them.