More than 15,000 coarse fish including pike have died in a pollution incident on the Dorset Stour.
The Environment Agency launched an investigation after dead and dying fish were reported at Sturminster Mill, upsteam of Hinton St Mary, on Wednesday.
Tonight the agency said it believed farm run-off was the cause of the outbreak. The pollution is believed to have entered the main river via the Caundell Brook and River Lydden.
Officers are trying to trace the source of the spill and have visited several farms on the two tributaries.
Work is under way to recover and dispose of fish which have perished in the outbreak, which include roach, chub, perch and pike.
EA spokesman Andy Martin said: "We are currently focussing on identifying the sources of the pollution and a number of agricultural farms and industrial units are under close inspection.
"Clearly this incident will be of great concern to the angling community as a significant number of fish have died. Over the coming weeks we will be assessing the full impact on the ecology of the river and considering options to enable the river to recover as soon as possible."