Environment Agency staff took river samples

Officers from the agency were called out on Tuesday after discovering the incident in the River Fowey.

At least 5 miles (8km) of the river was hit by the pollution, leaving the water and banks littered with the dead creatures, the agency said.

Officers searching the area have taken samples and are trying to trace the pollution's source.

The discovery was made by an angler, who spotted the dead fish on Tuesday morning and alerted the agency.