It is believed that as many as 50,000 fish were poisoned to death when farm slurry seeped into the river Cale and the river Stour in the Gillingham and Sturminster Newton area of north Dorset in July of this year.
The oxygen was destroyed when the high ammonia pollutant seeped into the rivers from a local farm.
Agency staff are now restocking the rivers with roach, chub, dace, bream, perch, gudgeon and pike. The operation is likely to last several weeks.
Matthew Carter, for the Environment Agency, said: “This devastating pollution left much of the River Cale devoid of fish and the Stour stocks were severely damaged.
“We are keen to see the rivers recover and hope the restocking will help speed up the process.”
A case is being prepared with a view to prosecuting the farmer involved.
