It is believed thousands of fish may have been harmed or killed as a result of a pollution incident on the River Witham.
The Environment Agency began receiving reports from the public on Saturday and Sunday that hundreds of fish were struggling to breathe and dying on a stretch of the Witham between Bardney bridge and Tattershall Bridge.
A spokesperson for the Environment Agency said on Sunday: “The Environment Agency has responded to reports of pollution in the River Witham at Southrey.
“Officers using a 4x4 to reach the site in difficult weather conditions yesterday to begin our investigations and a search for the source.
“A large number of fish have been found dead at Kirkstead Bridge near Woodhall Spa. The Environment Agency will continue to monitor the situation.”
The investigation is still ongoing and the agency is following up several potential sources of the pollution.
The spokesman said: “We have teams of environmental experts on site, working hard to identify the cause. Our focus is on understanding the reason behind the deaths and ensuring it’s stopped. At this stage, we believe the deaths are the result of pollution, not cold weather or salt.”
Members of the public can report environmental incidents to the Environment Agency on 0800 80 70 60.