Anglers are being urged not to panic if they see dead fish in waterways around Spaldiing.
George Croft said he contacted the Environment Agency after seeing eight dead fish where the River Welland meets Coronation Channel.
Mr Croft (74), of Priory Road, said: “My concern is the water is polluted or someone is killing them.”
A spokesman for the Environment Agency said: “We have investigated, but found no evidence of pollution or poor water quality.
“One good sign is that the fry and juvenile fish are doing fine; these would be the first to die if any pollution was present.”