Environment Agency (EA) investigators say they have found high levels of ammonia in the River Witham where thousands of fish have been reported dead in a pollution incident.
The EA 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.
On Monday, an agency spokesman said: “We believe this to be the cause of the fish deaths.”
Several residents in Woodhall Spa said they had seen large shoals of dead fish on a stretch of the Witham near the village.
There was speculation the deaths could have been caused by the recent cold weather.
However, the EA said they ‘believed’ the deaths were the result of pollution.
On Sunday, an EA spokesman said: “We responded to reports of pollution in the River Witham at Southrey.
“Officers used 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 EA will continue to monitor the situation.”
The investigation is still ongoing, and the agency is following up several potential sources of the pollution.
The EA 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.”
Andrew Tansley, who lives near Woodhall Spa, said: “We spotted a few dead fish on Sunday, and then a few yards away there were hundreds. It was a shocking sight. I’ve lived near the river for years but have seen nothing like this. I hope they get who was responsible and take them to court.”