Hundreds of dead fish found in River Nene near Peterborough
About 400 fish have been found dead in a river after a "pollution" incident, according to the Environment Agency.
Residents living in Stanground, near Peterborough, reported the sightings in the River Nene on Monday.
A spokeswoman for the Environment Agency, which is investigating, said the fish were killed by "elevated levels of ammonia and low oxygen".
She added that there was no risk to drinking water but advised people not to let their pets drink from the river.
The organisation said levels of ammonia and oxygen in the water were stabilising, but "possible enforcement action" could still be taken.
The spokeswoman added: "We have identified the source and cause of the pollution, which has been stopped and isn't posing any further risk to the environment.
"As always, we'd remind people to please report fish in distress or suspected pollutions to us as soon as possible so we can investigate."
Residents shared their concerns about the discovery on social media, with one calling it "so sad".
A spokesman for Anglian Water also confirmed it had received reports of dead fish in the Riverside Mead area.
He added: "We're working with the Environment Agency to investigate the cause and assisting with clean-up actions in the immediate area."