River Tees tributary pollution kills thousands of fish
Pollution has killed thousands of fish in a tributary of the River Tees.
Dead fish were found in Aldbrough Beck, west of Darlington, as Environment Agency officials were about to release young grayling into the water course.
The agency is carrying out an investigation.
A spokesman said it was a criminal offence to dispose of waste in watercourses and those found doing so would be prosecuted.
Jon Shelley, from the agency, said: "Reports suggest that the pollutant entered the water on Tuesday evening.
"Our officers have been taking water quality samples, assessing the full impact on the environment and investigating the source.
"It appears the watercourse is now running clear, and there is no on-going pollution."
The fish restocking is now taking place in different locations, including the River Browney near Langley Park; River Gaunless, near West Auckland and the River Deerness at Esh Winning.