HUNDREDS of fish have been killed in a suspected pollution incident on the River Tees, which experts fear could damage the health of the river for years.

The discovery of the fish in Aldbrough Beck, a tributary of the River Tees at Aldbrough St John and Eppleby, about five miles from Darlington, has been described by the Environment Agency as a “significant incident”.

Staff from the agency had visited the beck with the intention of releasing hundreds of young grayling into the watercourse as part of a region-wide effort to boost fish stocks.

Instead, they were forced to take the grayling to other rivers for safety, including the River Browney near Langley Park; River Gaunless, near West Auckland and the River Deerness at Esh Winning.

An investigation has been launched into the cause of the pollution, which is thought to have entered the water on Tuesday night, killing hundreds of fish, invertebrates and other river life.

The pollution incident has come as a huge blow to the River Tees and Aldborough Beck after years of work to clean up the watercourse and bring wildlife back to the area.

Jon Shelley, environment management team leader at the Environment Agency, said: “Our officers are assessing the full impact on the environment and investigating the source of the pollution.

“Reports suggest that the pollutant entered the water on Tuesday evening.

“Many years ago, disposing of waste in watercourses was common, and this saw many watercourses totally devoid of fish in the mid-20th Century, but doing so is now a criminal offence.

“A lot of work has been done over recent decades to improve water quality in these catchments and incidents like this are now seen less frequently. But from time to time they do still occur, and those who are found breaking environmental laws will be prosecuted.”

A member of the public initially contacted the Environment Agency on Tuesday evening with concerns that the watercourse had been contaminated, but it was only when staff arrived on Wednesday that they saw the scale of the damage.

An Environment Agency spokesman thanked the member of the public for highlighting the incident and asked anyone with information to come forward to aid their investigations.

Any further information on this incident or any other pollution event can be reported on the agency’s 24-hour incident hotline on 0800-807060.