Around 700 fish were rescued and moved to safety after pollution in a Lincolnshire river put them at risk.
Over 500 fish and nearly 200 protected crayfish were rescued after a tractor overturned last week. It spilt fertiliser into Gunby Brook, which is a tributary of the River Witham.
Samples showed high levels of ammonia in up to 4km of watercourse meaning the fish were at risk.
The Environment Agency confirmed an investigation is now underway into the circumstances surrounding the incident, which could lead to further action in the future.
The white-clawed crayfish, which is a protective native species under threat across the country, were saved by Environment Agency ecologists.
They were transported to safe locations where their populations can re-establish out of harm’s way.
Fisheries experts also recovered wild brown trout, bullhead, and coarse fish, moving them away from the pollution and downstream to safety while clean-up efforts got underway.
The Environment Agency is now working with the landowner to contain the pollution behind a series of temporary dams, carrying out sampling and monitoring conditions, and providing specialist advice on clean-up.
The affected water is now being treated. Some of the temporary dams are being removed, where it is safe to do so, in order to restore the natural flow within the river.
The Environment Agency said the rescued 58 male and 41 female crayfish, plus juveniles and eggs, have been moved to safe locations in the nearby Cringle Brook and Wyville Brook.
Area Duty Manager at the Environment Agency Peter Reilly said: “This is a prime example of how we aim to protect wildlife and nature – undertaking this rescue meant we were able to save hundreds of creatures which otherwise could have been killed even as we were working non-stop to control the pollution.
“The spill is now contained and clean-up is ongoing, but our teams are still closely monitoring conditions and an investigation is underway.”