The dead fish were found in a river in Norfolk where a large patch of white foam had appeared. Environmental Protection Agency confirmed.
Images released by the agency on Saturday showed foam covering the Teth River area.
The Environmental Protection Agency said its staff “saw small dead fish and juveniles.” The department said it was investigating the incident and that “the contaminant is an unknown substance.”
On Saturday teams were sent to collect samples from the river that runs through Thetford in a bid to find the source of the pollution and prevent further discharge.
The source of the foam, which remains visible, was located and further release ceased.
Firefighters were also present. Norfolk The fire service warned people not to enter the water or allow their pets to swim or drink from the river.
Anglian Water confirmed the incident involved a “third party” and not any of its “equipment or assets in the area”.
An EPA spokesman said more foam could form but said it would have “little environmental impact as it dissipates.”
They said: “Environment Agency officers are continuing to investigate the cause of a large amount of foam found in the River Teth in the center of Thetford yesterday.
“Our teams have collected samples, identified the possible source and prevented any further releases. An investigation will be carried out to try to ensure this does not happen again.
“It is possible that we will see a second plume of foam today and the foam will continue to spread down the river. It is still visible but will have little impact on the environment as it dissipates.”
The agency said Sunday it will wait until the contaminant disappears to assess long-term effects.
They added that their officers were present at the Teth River foam incident on December 23, 2024, but were unable to determine the source at that time.
In 2023, the Guardian and Watershed investigation discovered that toxic substances called Pfaswhich contain fire-fighting foams have been found to be polluting the protected River Wyre in Lancashire at “extremely high levels”.
